michlet/rocker

[ediestel]

I want to change the rocker of a monohull; I assume that you have to set the f3 restriction, which relates to the horizontal view of the ship's forward.

I want to set a forward rocker of 2.5 inches; do I have to set:
2.5, 2.5, 2.5 for f3 ?

can anybody help me out with an example ?
I posted this under 'design', I guess it is better asked here.

[Maxim Projet]

I want to change the rocker of a monohull; I assume that you have to set the f3 restriction, which relates to the horizontal view of the ship's forward.

I want to set a forward rocker of 2.5 inches; do I have to set:
2.5, 2.5, 2.5 for f3 ?

can anybody help me out with an example ?
I posted this under 'design', I guess it is better asked here.

The Michlet parameters for shape functions are not dimensions. They just change the shape of a part of the hull, like the waterplane or the section shapes or the rocker.

[ediestel]

Thank you.

What does the allowed range relate to ? Is it the radius of an arc, is it a fraction ?

Example: What does it mean if you put f3 at 0.75 ?

[terhohalme]

Yet an other too lazy to read the manual...

[ediestel]

terhohalme

The time it takes for you to post, you can also use to share your expertise.


Do you have to export the offset to a design program and measure the rocker in inches/metric there or can
you approximate the rocker ( in/cm ) while still in Michlet ?

[ediestel]

Series 1

This hull series takes three shape parameters. All must lie between 0.0 and 1.0, inclusive. The series is such that if a parameter has value 0.0, then the shape is rectangular, 0.5 corresponds to an elliptical shape, and 1.0 creates a parabolic shape. Intermediate values will produce shapes intermediate between these familiar shapes.


Series 7

f0: Forebody waterplane shape (min. 0.0, max. 3.0)
f1: Forebody cross-section shape (min. 0.0, max. 3.0)
f2: Forebody sideview shape (min. 0.0, max. 3.0)

The first three parameters (f0, f1 and f2) are the same as for Series 1, but are relevant to the forebody only;

The maximum value of the first five parameters is 3.0; cusped shapes are possible with this series.



The forebody sideview shape at f2 = 0 is a straight line, at f2 = 3 spheroid. So far so good.



Is there a way to relate this to a forebody rocker expressed in inches, taking into acccount the length of the forebody, the draft and f2 ?

[Rick Willoughby]

Eckhart
You are looking at reasonably minor change in rocker. For such a minor change, I would allow the hull parameters to have their full range and optimise for a certain speed.

Once Godzilla has the hull shape I import the offsets produced into Delftship and then modify slightly to get closer to the desired result. I then retest the shape in Michlet to see what my modification costs in terms of performance. I have once or twice get a lower drag result than Godzilla produced but normally the drag is higher. This sounds tedious but if you have set up the right folders it only takes a few minutes.

I do force a hard chine in Godzilla if I want a very simple hull for plate development but I have only forced rocker once. You could do it by trial and error just by setting the maximum limit and ensuring the initial setting was within the range.

You do need to know what power level you aim to operate at. Even with Godzilla you will still need to iterate as you want the optimised speed to coincide with the target power level. Hence it is important to know what power level you are going to operate at.

Rick W.

[ediestel]

Thank you, Rick

I have read this advice from you here on the forum before and have been trying to go back and forth between Godzilla/Michlet Freeship as you discribe.

I quite often get an error stating that certain coordinates ' 0.018, ..., are not submerged' and FreeShip does not calculate the hydrostatics under those circumstances.
The manual does not comment on this. What can I do to avoid that ?

[Rick Willoughby]

I will play with parameters for a design and send you the input file. You can then run Godzilla and watch a hull evolve.

Rick W.

[ediestel]

Thank you very much.

I suppose it is the formula in the following article that can be used:

http://en.wikipedia.org/wiki/Air_resistance


related reading :

http://www.keelhauler.org/khcc/seakayak.htm

Eckhart

[Rick Willoughby]

I have attached the No-Compromise in.mlt file for a hull to do 3.5m/s with displacement of 110kg.

You can use this as a starter file to see what Godzilla optimises for the requirements. If you do not know how to start Godzilla then I will provide more detail.

The result is the underwater shape for a no compromise hull and performance prediction. You then have to decide if this will do the intended job.

My experience with OC1s suggest that you need a little stability in the main hull otherwise you have to load too much onto the outrigger or you have a boat that is too twitchy to handle. Loading weight onto the outrigger kills performance. That is why I use balance outriggers on my pedal boats. The aim is to not load the outriggers. They are insurance primarily.

So the next step is to adjust the minimum GMT so you get stability. If the GMT is higher than 0.3m you would not require the outrigger if you sit low. This is probably more stability than needed and will cost in performance. I have already looked at GMT of 0.2m and it was wider than most OCs I have seen. It costs a lot in performance. I think GMT of 0.1 might give a nice result.

As for rocker I have mixed view. I do not have enough experience in large waves with slender hulls to know what works best. I think most seakeeping qualities can be built into what is above the waterline. You will see the optimum hull has reasonable amount of rocker.

I have done a design for an ocean pedal boat and I have increased the optimised speed above the design power level to take advantage of mostly following sea. So there are circumstances where you might benefit from a longer hull. If you get Godzilla going you will see that the NC hull is just under 8m long. This aligns well with a heavy weight rowing scull. Are you able to transport an 8m boat!

Rick W.

[ediestel]

Rick,

thank you for the file and suggestions.

Indeed, we are trying to paddle the OC like a surfski, rigging the ama as light as possible. The use of balance outriggers would be within the rules for OC 1.

If length improves performance, than it should be tried; unless increase of weight and wetted surface area negate the benefits.

The idea for open ocean racing is of course to take advantage of the waves and the speeds while surfing. Important is further that the boat has the ability to 'connect' the bumps, that is, maintains speed as much as possible without loss of energy due to pitching or rolling.


In my experience, rolling and trim changes are very expensive, so I'd like to avoid them.



I will run your file in Godzilla and see what happens.

[Rick Willoughby]

Eckhart
Let me know if it does not get the expected result. When you start Godzilla you should see the hull changing shape. The NC optimum is almost 8m and 250mm beam (say 10"). Michlet works in metric so it is easier to do all the files in metric.

If you are allowed twin outriggers it may be worth a try. The only disadvantage I can see is slightly more difficult to right if you do roll it.

The NC hull is very easy to make light and strong because it is so narrow and almost cylindrical so is inherently strong.

Rick W.

[ediestel]

The current design - 7.90 m, beam 0.25, draft 0.11 - under water body requires about 12 % more power at speeds up to 5 m/s.
About 22 N per KAPER.

These speeds are reached while surfing though, so the paddler does not have to generate the power continuously.

This is compared to designs that I tried earlier with waterline length closer to the currently available OC 1 of about 6.00 - 6.50 m, max body width minimum 14",
beam at waterline is mostly unknown, but I don't see 2 inches of flare on each side on my boat, at the most 1, likely a little less. That would put the beam at waterline for
a OC 1 Hurricane at 12 ".

Rocker is likely more in other designs. So I would be inclined to increase the rocker a bit. This decision will be the most difficult one as nobody seems to have specific knowledge for this scenario about this.

How responsive would a 7.90 m boat be to rudder action ?

I have no experience with seakeeping features above the waterline:

I think volume in the bow in a predominantly vertical distribution is one thing.
The Epic V 10 has a lot of volume in the foreship, too much it seems as they built the Epic L with a flat foreship. Pearling can mostly be avoided once you know how to paddle a certain boat.

Flare may be needed in the midsection where the paddler is sitting ? My fear would be that it increases pitching ?

The stern should be a canoe stern, but not be too narrow, I guess. I actually would like the wave to pick up the boat early and easily.


I'd like to upload the linesplan for anybody interested, but I am not sure about how to do it.

[ediestel]

Some specs for those that are following this thread:

Hull:
from outside to inside:
carbon - thin core - carbon - thin core - carbon or glass. Duratec primer. By preference urethane paint.

Weight:
should be as light as possible, but rigidity and strength have priority.
target < 30 lbs., better 25 lbs.

Seat:
molded into the deck, height 5 - 7 " above heel.

Sit-on-top

Footwell:
Single footwell with a 'chambered' Venturi drain. The Venturi drain is at the bottom of a separate compartment; the compartment is open at the top. Overall this prevents water to flood the footwell retrogradely through the drain.

Steering:
T-bar

Rudder:
usually about 30" from the stern, minimum about 10" with 4 " chord, mostly eliptical.

Freeboard:
I would love to have the freeboard as low as possible in the seat area. Higher in the aft. I prefer a 'wet' design as long as it does not impact performance too much.

Race duration ranges between 60 min and 5 hours; the seat and footwell have to comfortable. Stability must also be good enought to alloow relaxed paddling over that time in following seas that can be quite irrregular.

While this boat is supposed to be fast, it is supposed to be fast specifically in the following sea.

[Rick Willoughby]

A boat with rocker will certainly turn easier but it will be less efficient going straight.

I find that V11 is very responsive to rudder for stability. It will turn 5 to 10 degrees easily. After this it is like turning the Queen Mary. So it is easy to use the rudder for dynamic stability once at speed. A long hull tracks very well so if it lifts the nose when surfing down into the back of a wave it should be very controllable. I think flare and buoyancy in the bow will help this.

If you change the optimising speed to 5m/s in the in.mlt file you will see that the hull length is reduced while the ends get fuller. There is not much difference in power level at any speed compared with the hull optimised for 3.5m/s.

So this might be a better hull. The 8m long hull will have very thin ends below the water and I often cut these off as they offer little for the extra length.

I also fair the hull in Delftship before moving forward and then check the result in Michlet.

There was no linesplan attached. You need to choose the file and then upload to get the attachment.

Try the 5m/s hull and post the lineplan and we can discuss the merit of various changes. Godzilla will certainly produce the best hull for calm water performance. This translates to good performance in small waves but I do not have experience in ocean conditions on one of these boats. I would be tempted to leave the underwater shape and just work with shape above the waterline. With models I have made, I found that if the hull is easily driven it tends to ride waves very easily.

Rick W.

[ediestel]

The linesplan at 5 m/s has less rocker.

Waterline length 7.28, beam 0.255, draft 0.105.

KAPER did not work in Freeship, not sure why.

Cb 0.56, Cp 0.69

LCB positive 0.014; shouldn't it be negative ?

S slightly improved with 2.21 m square

[Rick Willoughby]

Now all that has to be done is work out the 'minor' detail of what sits above the waterline.

By the way I have found drag data from Freeship next to useless. This is partially recognised by the software in that it bombs out if Cp is higher than 0.64 (from memory).

If you think about what has evolved it makes sense. The wave drag is very low in any case with these hulls so as the design speed increases you find they tend to get higher Cp. The ends are fuller resulting in lower wetted area.

I would not worry about detail in the stern too much. Buoyancy in the stern can be achieved with placement of the outriggers. I am assuming you will go for two outriggers as this is by far the best if rules permit.

In a pedal boat you do not have the use of a paddle to help stabilise the boat. Hence you are totally reliant on the effectiveness of the outriggers. I find that a single outrigger induces a lot more roll than having two. If you are powering down wind in a chop then the boat rolls badly each time you cross a wave. This does not happen with two. In a beam sea you constantly have to lean into the roll if only one outrigger. With two you get snap roll but no risk of capsizing.

I also place the outriggers so they ride the bow wave of the main hull when at design speed. The back end of each just sits on the bow wave - although it is not very big. This means you can sit the outriggers above the water level and they will just sit on the wave once moving. Same thing happens in a chop. They just ride on crests. Not sure if you will be able to make use of the bow wave at 5m/s as they will be close to the main hull and a long way back. You might choose to set them to ride at say 3.5m/s - roughly sustainable calm water speed.

Taking the hull above WL, I would now provide a gently flare in the bow with a nice wave piercing shape and rising buoyancy in the first few metres to the footwell. This can also provide some wind fairing.

In my view there is not a lot of value in using carbon fibre for these hulls. Glass will provide a harder wearing finish and the shape is inherently strong. If you do the numbers, a single layer of 200gsm CF will handle the bending loads but is hopeless for puncture resistance. I find a total layup of around 700gsm is adequate. I have a flat deckline and this adds to the strength. The detail around the footwell and seat on yours needs careful consideration as this is where bending stress is highest. This area is a candidate for adding some sandwich core inside the initial layup.

You may find that this hull is too narrow at the footwell to comfortably fit your feet for good leverage. I have seen wells that provide support up the sides of the legs.

Unless you have proven reasons to move from this design I would recommend you try it out and aim to correct any disappointing aspects in version 2. Evolution, as seen in current boats, usually gets close to best after many years but remember Godzilla did maybe 30,000 iterations to get that shape. You would want to be really confident in your knowledge of what is good and bad to vary the underwater bit.

To make the deck you just keep extending the top edge and shaping to suit. It will get complicated around the footwell but is quite doable in Freeship. I could do it in a few minutes so if you are having difficulty just post the fbm file and I will modify. Keep posting the evolution of the deck and any changes you make below the waterline. Once you are happy with what is below the water it pays to export a mlt file from Freship and check the numbers.

Rick W.

[Rick Willoughby]

Eckhart
There is one option worth considering. It is hard to beat a hard chine for ease of construction using a flat panel method. With this method you save a huge amount of time for a one-off construction and it is easy to get a nicely faired hull that requires little work to finish.

If you look down the in.mlt file you will see the parameter that sets the chine is noted. If the maximum is set low - (even zero) and the starter zero as well, you will get the optimum hard chine hull. I usually set it al little above zero and then make a hard chine in Delftship with sides splayed rather than vertical to approximate the shape. You will find the resulting hull will not have much higher drag than the NC hull. It will be much easier to make though using thin sandwich panels.

Rick W.

[ediestel]

Rick, thank you very much.

To have the ama ride on the bow wave is a very good consideration, I never heard about it. Great.

Buoyancy in the stern with the help of the ama makes a lot of sense to me.

Do the amas have to be symmetrical or would a main ama on the left and a smaller ama on the right, such in a trimaran, be feasible ?
This would allow to 'fly' the main ama while surfing, reducing drag.
The rules require that the main ama has two iakus and that the paddle is being used between those two iakus. That is diffferent from your V11 with only one iaku. Not a big issue, the forward iaku could be rudimentary.


Using glass for the hull would be much easier; if a core would only be needed in the midsection - so much better as core is always susceptible to being soaked.

To make an initial version with hard chines is definitely a good idea. The chines would help 'carving' the turns on the wave face.
According to the study from Lazauskas it does not make much of a difference re drag.

Today I talked to one of our main boat builders. The OC started with 27 feet, went down to 19, went up to 23 and are now in the 21 - 22 ft range. All this over the last 20 years here in Hawaii. The argument is that they do not fit well into the trough when to long.
On the other hand they are not built to a displacement of 0.11 tons.

At this point I hope that the 7.28 m = 24 ft may work.

I have experience with the footwell and would like to improve on my design.
The gunnels at the feet would be narrow and have little volume, if needed I would flare the shape of the deck outside to accomodate the forefoot in a transverse direction.

I need about 0.28 m inner diameter for the forefoot, 0.27 m would do.
At the heels the transverse diameter needs to be 0.24 m, 0.23 m would also do well. The shape of the bottom of the foot well would follow the outside hull, at the very bottom it would follow the heel contour.

Currently I have a small midsection at the bottom for extra strength, it contains a stringer.

The foot plate allows full contact with the entire foot up to the palms, the angle is about 60 degrees. Higher up the angle is lower, the toes extend over that edge and operate the t-bar steering.

The cables for the steering run inside the gunnels.

The surface for the seat should be 0.12 m above the level of the heels. I asume it would end up about 0.18 - 0.20 m above the bottom of the boat.
The lateral aspects of the hip will extend over the gunnels, I was planning to create a recess in the gunnel to allow for that.


Currently I am trying to fair the hull. How many lines and sections should I import to make this effective for this length ? I have designed in Blender and in Cinema 4 D before but not much in FreeShip. So that may take a while. If you find time for a rough scetch of the deck it would definitely speed up things.

Your help is very encouraging.

[Rick Willoughby]

If you are going to consider the hard chine you should look at that now with Godzilla before you put too much effort into the NC hull with fairing and the deck.

If you choose the NC hull with round chine then you need to be thinking about a foam plug to mould it on. I expect the hard chine hull will be narrower than the NC so you might struggle for room in the footwell.

I have set the number of waterlines and stations in the in.mlt file to give good hull resolution. You should import all of these and not cut any out. From memory it is 17 stations and 9 waterlines. You can check the in.mlt file.

The version of Delftship I have has auto fairing but it is not all that good. I usually just persist with the points until I get any bumps removed. It does not take too long and it gives you a good feel for the shape.

When I make the plug I loft from waterlines at 1" for the underwater section and this relates to 1" thick foam. It will take for 1" layers. Above the waterline I go to 2" sheets. I then glue the layers together and just fair out the steps in the foam. This means if there are some bumps in the model they can be faired out at the foam stage. The plug for V11G was milled from full thickness foam and I faired these thoroughly. I find making the plug from 1" and 2" foam sheets can be done in a day so having the plug milled is really an expensive luxury - unless you have a tame shop nearby. The sheets we get here are metric and are 2.5m long so work well for 7.28m hull - 8ft sheets could mean you want to reduce the length a tiny amount. If you plan the cut out you can save material.

There are a few tricks to making life easy with Freeship. Moving the image about and zooming in and out with the mouse are essential features for doing fine work. I also work between view windows. The 3D view with shaded rendering is very helpful to identify where things may be going wrong.

Rick W.

[ediestel]

I think that I will stick with a round hull.

I like the idea of flat chines in the forship , but I rather add them in case they are needed for lift.

My feeling is that the reduction of the pitching is more important than the question whether the deck is wet or not.
The speed is going to be higher in the surf and gravity is pulling so there should be enough energy to keep moving.

[Rick Willoughby]

If you want to try the hard chine you should set that as a constraint in Godzilla and have it optimised with the chine.

I did a run and produced hull. I have modified it substantially so it should be rechecked in Michlet and compared with the NC hull to see how it compares.

I have tried to open the fbm file with two versions of Freeship and Delftship. None work. It is easier for me if you use Delftship. It also has a free version and is almost identical to Freeship.

I have done a hard chine version. It requires 6 panels for construction. See lineplan.

If you make this one you do not need foam. I will explain how you make development from this if you want to proceed.

I have the Delftship fbm file attached.

Rick

[Rick Willoughby]

If you are keen on the NC with the rounded chine then it is easy to set waterlines at 1" (25mm) to get cutting lines for 1" foam sheets. This is done using the intersections function in the view menu. You set the start to 0 and the end to the gunwale height and then set the spacing to 0.025m.

You can then printout the waterline coordinates. These are lofted to the sheets for cutting lines.

Rick W.

[ediestel]

Rick,

meanwhile I had a chance to open the file in DelftShip; the boat looks nice.

My file was made with FreeShip plus 2.9 - Delftship opens earlier files from Freeship 2.6 - but not from 2.9.



Developing with hard chines: how many stations would be needed; would you need a strongback ?

The plates would be made out of saturated fiber glass and then cut to fit ?


Performance: would you expect the hard chined boat to behave similar or different form a soft chined/ round hull boat ?


For soft chines: could a strip-plank approach with cured fiber glass strips be successful ?

[Rick Willoughby]

I usually use stations at 1m. I would suggest a strongback or nice long flat surface for reference. I can fit the 7.5m on my garage floor and it is nice and flat.

I did not check the performance of the faired HC hull. You should export a Michlet file from Delftship and run the numbers (post them for me to look through). Then compare with the No Compromise hull. If it gives away a lot then go for the NC. The Godzilla HC was actually better than the NC before I did the fairing.

You can make panels from glass and a good PVC or Corecell foam. I would suggest say 200gsm on either side of 1/4" core. You might want to add say 100gsm of smooth finishing cloth on the outside of the panel to reduce effort in surface finish. These panels will be stiff with adequate strength once layed up.

I am only starting to learn about this sort of construction and making joins at the chine and deck but there are others here who can give better advice.

I have very little reserve buoyancy in the stern of HC. The intention is that you play with outriggers to get the stern to lift as you would like. You could easily add foam to the flat deck to add buoyancy. I have ideas on outriggers but not sure if they are the best. The ones I like the best for rough water are back-to-back cones. These just knife through waves without upsetting trim but they have enough buoyancy to give good long-term stability.

Rick W.

[ediestel]

This is the 'out' file from the Michlet run.

I will run the NC and post that out file for comparison.
The limitation here is that I do not know how to properly fair the hull.
It looks pretty clean to me.


What I can see so far is that S is about 1.8 in my current OC 1,
in the NC version with 7.28 it was 2.17, in the HC hull it is 2.34.

[Rick Willoughby]

This is the 'out' file from the Michlet run.

I will run the NC and post that out file for comparison.

Eckhart
I normally use the ship_output_by_speed.mlt file. The reason being that it is easy to import this whole file into Excel as a text file and do comparative graphs and play with units. Irrespective the drag data you got is a fraction higher at 5m/s for the faired hull than the NC. Something like 84N v 81N at 5m/s.

I think you will find other issues will be more significant than this so the HC might be a good starting hull for first round - maybe the last one of it works well. It will be quick to build if you have the materials and a good place to work.

You can make nice flat surfaces using ply with plastic stretched over them. Epoxy will readily release from the plastic.

By the way the value of surface finish is usually overstated. For long hulls like these most of the hull operates in a well developed boundary layer. If the boat feels smooth to touch then that is good enough. Achieving a mirror finish gets very little return on effort.

Rick W.

[ediestel]

Attached is the out file for the
7.28 m hull, derived from the NC.

The hull lools reasonably clean in shaded and gaussian views.

I am not sure that I faired it very well.

I will try to make the file DelftShip readable.

[Rick Willoughby]

The HC hull is probably reasonable for developing from.

I do this by exporting the entire hull as a part and then import the hull to layers set up for say bottom, side, aft_deck and front_deck. I then strip out all the rest of the hull in each layer apart from the nominated piece. If all layers are turned on the boat will look the same. You then develop each layer using the function in Delftship.

You could make a model out of cardboard or balsa to check how it works. You might make some changes after you see the model.

As far as surface area goes it is what you get out of Michlet that counts as it looks at all significant causes of drag. Surface area is only one component. I have numerous examples of the accuracy of Michlet.

The lowest surface area is half a sphere but it makes a lousy boat.

Rick W.

[ediestel]

The HC idea is the better approach, you are right.

Do you expect the HC hull to behave more or less alike to the NC hull, especially in the surf ?

Ease of take off, glide, pitch/roll control and control are the main issues.

In other words what will I be able to learn from the HC hull that can be used to determine if a NC hull would be worthwile to be built ?

[ediestel]

Rick,

I do not have the Excel spreadsheet, so for me it is a little hard to read.

[Rick Willoughby]

What I would do is model the existing hull. I know at my power level the Yellow OC1 is about 2kph slower than my optimised black boat. However I can only get about 1kph faster at full power from the black boat. I think if I optimised for 5m/s instead of 3.3m/s the result might be more in favour of Godzilla.

You may in fact find that the HC hull actually works better than the NC because it has a flat bottom. It will eventually plane. It will have some damping in the ends. It will have some intrinsic roll damping in the main hull although this will be tiny compared with the outriggers.

Designing for flat water is easy as the Michlet model perfectly matches real life. Handling waves is another matter.

You will have an easily driven hull that should rise and take off quite early on a wave. In the end it gets down to comparing different hull forms both above and below the surface.

You will have a good base to get the footwell and seat exactly as you want. You can easily make different outriggers to trial.

If you found the HC did not outperform your existing boat then I would not recommend making the NC as you would conclude the design constraints have not been selected well.

The biggest difference I see is the very low KMT. In a pinch you can balance a typical OC1 with the paddle. You may find that more difficult with either the NC or HC. The twin outriggers should make a secure platform that is well ballanced thereby enabling relaxed power paddling. If you never have to really worry about capsize you can concentrate on moving the boat through the water.

So two things - model existing hull in Delftship as best you can, export to Michlet and compare with HC. Make the HC model so you work out how to develop from Delftship and see that it looks right.

Also post as you go so you keep us interested.

Rick W.

[ediestel]

The plates have been isolated, see attachment.

I think I can load them individually and save them from the lifeplan view.

I have a plotter available that should be able to print the plates in their entire length.

In which format should these plates be saved so that the plotter can read them ? Would that be .dxf or should I go through gnuplot ?


Eckhart


PS I have not figured out how to load them into a layer; iow I have not arrived at a model where I can switch layers off and on.

PS: file name should be bottom not buttom :)

[ediestel]

Rick,

my current outriggers have a flat bottom that rises towards the bow; they are not piercing at all. We try to get them on top of the water. They are ~ 6 1/2 inches wide and getting wider.

Do your outriggers have a specific displacement volume and how far do you set the lateral distance to have them ride the bow wave at design speed ? Length/beam ?
Outrigger baseline compared to the baseline of the main hull ?

How do I approach the buoyancy in the stern through the outriggers ?


Eckhart

[Rick Willoughby]

You need to get into the layer properties. You need to nominate each of these parts as "Developable". You can then use the development function of Delftship to give you the shape of the cut out on a flat surface.

Also when you import a part to a file it automatically goes to a new layer. You can rename the layer. You can have different colors for layers and many other features. You need to have a look at the layer control.

You can have a single file containing the original hull on one layer and all the parts of the hull in separate layers.

It is worthwhile taking the time to learn how to operate the layers.

It is also important to understand that you want the flat projection of hull parts and that is what development does. Makes it very simple to cut out.

Rick

[ediestel]

The layers and plate development are working now. I think that the free version of DelftShip does not allow to save the developed plates.

Is there a way to export the offsets/coordinates of the plates ?
I could then feed them into gnuplot and arrive at the target this way.


"Save as" does not seem to work.

Can you save the HC hull in a down-compatible version ?

[Rick Willoughby]

Rick,

my current outriggers have a flat bottom that rises towards the bow; they are not piercing at all. We try to get them on top of the water. They are ~ 6 1/2 inches wide and getting wider.

Do your outriggers have a specific displacement volume and how far do you set the lateral distance to have them ride the bow wave at design speed ? Length/beam ?
Outrigger baseline compared to the baseline of the main hull ?

How do I approach the buoyancy in the stern through the outriggers ?


Eckhart

I like the outrigger I have on my OC1 for a single sided outrigger as it is always loaded so it needs to plane. If you have two you can balance them so they operate above normal water level. My requirement is to have them generate uplift quickly so they have a flat rocker with a long pointy nose and stern.

This video shows the cone type in operation on the yellow boat. You will see they are mostly unloaded in calm conditions. In waves they drive though with very little added resistance.
http://www.youtube.com/watch?v=ckWqIgmVM4Y

The outriggers on the black boat are slightly different in design. They work on a torque arm and roll over waves. The bow lifts rather than plough. They submerge in the middle and the stern lifts as the wave moves under. You would not be able to do this with the limitation of two iakos.

For twin outriggers I would first try the conical type. Say 6ft long bow and 4ft long stern - so 10ft overall. The mid section diameter around 4 to 5 inches. The cones are tilted downward so they have a flat rocker. For ocean use where there will always be some wave they could have in-line axes.

The front of the outriggers would be aligned with the front of the cockpit and the stern just ahead of the stern of the main hull. You need calm conditions to get advantage of the bow wave as it is very small. I normally set the outriggers about 10mm above the water level when I am on the boat. I doubt that you would detect any benefit from doing this.

I find outriggers displacing around 15l placed about 1m out work well. You need to elevate the iakos with faired struts to prevent them from clipping waves. I suggest at least 1ft above water level. You rolling moment will not be very high because you are sitting low. If you have a preferred side to paddle on you might want to create some initial roll so that the boat sits balanced when you stroke. It might pay to get the set up efficient in calm water and see how this translates to rougher water. (By the way I hate paddling. I can pedal at twice the speed I can paddle. Hence I am not the best person to discuss paddling set up. Although I agree that sitting low has merit on a number of fronts.)

Long slender outrigger hulls like those described will just rocket through waves and the roll is controlled mostly by buoyancy rather than induced lift. Set slight bow up so the stern hits first and they do tend to lift than dive. Remember induced lift will add drag that more than offsets any benefit from the lift.

You only need say 3mm thick corecell to form the cones and then glass over the outside with say 400gsm total. Weight of each should be less than 1kg.

You can use the cylinder function in Delftship to create the outriggers. Also make sure you place them on a separate layer. These will be easily developable. On the other hand diving can be nasty as it can tip you at speed.

Rick W.

[Rick Willoughby]

The layers and plate development are working now. I think that the free version of DelftShip does not allow to save the developed plates.

Is there a way to export the offsets/coordinates of the plates ?
I could then feed them into gnuplot and arrive at the target this way.


"Save as" does not seem to work.

Can you save the HC hull in a down-compatible version ?

There should be an icon on the menu bar of the Plate Development window that allows you to save the coordinates for each plate in a txt file.

Rick W.

[ediestel]

FreeShip can't read DelftShip files; DelftShip free does not allow to save the plates.

I will try to reengineer the HC hull in FreeShip so that I can save the plates.


I'll rest my case for now until I have figured all this out.

Thank you very much, - thanks to you my plans are more concrete and realistic now.

The key issue will be how the boat responds in the wave.

[Rick Willoughby]

My version of Delftship allows you to Save As in FreeShip 2.6. I have attached HC including outriggers.

Rick W.

[ediestel]

Rick,

There is actually a way of using a delftship file in freeship; it is probably not intended use, so I will hold on posting it here.

Thank you for the other file.

Next I will create some stations and see that I can print them. I will report back once I have worked out the plotting of the plates.

I am happy - it is just the beginning, nevertheless, I think I learned a lot and we were able to clarify the concept.

The video is nice; the yellow outrigger looks like a Pahoa. The double-conus outriggers scare me a little bit - they look very different from what we have in use here.


Eckhart

[Rick Willoughby]

You can turn each piece off in the Development window so you only get the one piece at a time. This means you can create a coordinate file for each plate (composite piece).

I usually mirror the pieces that cut along the centreline so I get a single piece in the development. You can choose each layer to be symmetrical or not. Does not make much difference if you loft manually but if you want to plot a cutout it makes it a bit easier.

I think I mentioned earlier that the yellow boat is a Pahoa. I have tried the original outrigger and the two pointy ones. The two pointy ones are superior on many counts. They really do just slice through the waves. A planing type with a flatter ski-like nose causes a lot more drag.

If you get some thin corecell or PVC foam you should be able to make a couple of pointy outriggers in a day. Form up two cones for each outrigger with the foam and glass over. Worth a trial.

Rick W.

[ediestel]

Rick,

we have discussed the design in its current stage with regards to Hawaiian open ocean conditions.

Two points of interest came up:

1. A hard chined boat would be too 'stiff' allowing no lateral glide/movement of the hull.
This becomes an issue while paddling in sections where ground swell, wind swell an backwash cast waves at the hull from at least three directions, such as close to island shores/cliffs.

The same is true for boats that are too narrow, beam to draft.
It leads to an uncomfortable, not efficient ride and back pains.

My current boat has this problem, there are sections in local races where it has a terrible time.

Our thought, the chines have to be at least soft, or the hull should be rounded with a higher beam to draft ratio. The ideal number is generally 2.8.
We don't want the boat deep in the water.

2. There was a concern that a two outrigger layout would break in the ocean.
Examples for Trimaran failures were given. I do not have an opinion on this concern.

One main ama and a rescue ama would be an approach to consider.


Eckhart

[Rick Willoughby]

Rick,

we have discussed the design in its current stage with regards to Hawaiian open ocean conditions.

Two points of interest came up:

1. A hard chined boat would be too 'stiff' allowing no lateral glide/movement of the hull.
This becomes an issue while paddling in sections where ground swell, wind swell an backwash cast waves at the hull from at least three directions, such as close to island shores/cliffs.

The same is true for boats that are too narrow, beam to draft.
It leads to an uncomfortable, not efficient ride and back pains.

My current boat has this problem, there are sections in local races where it has a terrible time.

Our thought, the chines have to be at least soft, or the hull should be rounded with a higher beam to draft ratio. The ideal number is generally 2.8.
We don't want the boat deep in the water.

2. There was a concern that a two outrigger layout would break in the ocean.
Examples for Trimaran failures were given. I do not have an opinion on this concern.

One main ama and a rescue ama would be an approach to consider.


Eckhart

The first point makes sense. To get sideways glide you need a lifting bottom when pushed sideways. The current hull will dig in.

The idea of the single hard chine was to make simple construction. Adding another panel to soften it as well as Veeing along the keel adds complexity and the foam mould becomes more attractive. I think you will find it is not the beam to width ratio so much as having a shape that lifts when pushed sideways.

I have capsized a hard chine dinghy driven by an outboard. I have cornered much faster in Veed bottom boat without tripping up. I believe lift is achievable without going to the 2.8 ratio and getting into the pack. The way you will go faster is to keep it narrow.

On point 2, the long slender outriggers work well in waves. They do not generate uplift other than buoyancy so the force can be easily determined. You do not need strong iakos to take the loads. I am recommending around 15l of displacement volume total. Maximum force is therefore 150N - peanuts.

Rick W.

[ediestel]

What principle do you apply to lift the bottom in this scenario ?


The outriggers have more strain to endure when you are on a wave face, broaching and the outrigger goes 'ama under'; this usually does not happen, but I think the construction would have to account for that.

I do not see how two outriggers would increase this risk either.


A temporary mold will then be the better option. We have not entirely decided on the legth of waterline yet; we may stay a little shorter than the theoretical optimum to gain a little control/manoever.

[Rick Willoughby]

Eckhart
Given the apparent requirement to glide sideways I would have the mid section as follows:
WL beam around 12"
Sides down vertical to about 0.5" above the waterline
A chine panel around 3" across sloping 40 degrees to flat
A keel panel from the chine panel to the centreline that slopes 15 degrees to the flat

You would need to fiddle with this to blend along the hull. The chine panel disappears toward bow and stern.

After you have the new shape you check how it performs by exporting to Michlet. This then gives an idea of the cost of softening the chine.

Using a foam plug you would soften the corners and make slight curves instead of flat panels but basic shape would be as I describe.

There is no reason why you could not experiment with different shape amas on you existing boat. I would be interested to learn how the dart type perform in the ocean. They are simple to make.

Rick W.

[Rick Willoughby]

What principle do you apply to lift the bottom in this scenario ?


...

It is just a lifting surface.

I have found that an edge at 45 degrees to the flat will dig a deeper hole. Around 40 degrees it starts to generate lift.

This is from experimentation rather than any soundly based theory.

Think of the side of the hull as a ski. As it slips sideways it needs to lift rather than dig a hole (low pressure) in the water and trip up.

I expect this makes it easier to turn as well.

Rick W.

[ediestel]

The outrigger will likely be built first.
I am intrigued by its shape. I just have to find a good way to connect it to my existing boat.

I have two options at this point:

I entered the data of my current OC 1 into michlet, then opened it in Delftship. Here I added length, scaled it to the desired beam and draft.

I am hesitant to go too long for practical reasons and turning ability in the chop.

I now want to print out some stations and see what it takes to create a fiberglass one-off of the existing hull as male plugand adapt it to the stations/ new design.


Second option: as you described.


I have some problems interpreting the data: basically I am looking for the values under 'power' in the speed-to-hull file, comparing the values. Is that correct ?
The differences are small.

I have not solved the CAD question fully either.

[ediestel]

moving ahead with the plotter software . . .

[Rick Willoughby]

.......

I have some problems interpreting the data: basically I am looking for the values under 'power' in the speed-to-hull file, comparing the values. Is that correct ?
The differences are small.

I have not solved the CAD question fully either.

Yes. You just compare the power values. I normally graph them in Excel by importing that file. The key points of interest will be say 3.5m/s as this is the 1 hour speed in stable conditions and say 5m/s as this is the speed likely to be reached when on a wave. However the latter may not have the hull submerged anywhere near the normal waterline unless it is a big wave.

The CAD sections in the second post do not make much sense to me.

With the outriggers you need to fair the iakos as they get near the amas as they will be driving through waves. They need to be mounted at least 1ft high across the span or alternatively faired for full length.

Rick

[ediestel]

This hull has numbers very close to a Godzilla hull at 6.75 m, 0.27, 0.11, displacement 0.107.
Godzilla narrows the beam slightly and lengthens the waterline but the numbers do not get better.

4 = 2.15
5 = 4.001
6 = 6.51

Next I will strengthen the harden chine and see what happens.

These dimensions seem reasonable to me.
The upper deck is a vague shape.

The wave piercing bow is much debated everywhere - it is diffiult to filter out what that really is. The key is to reduce pitching.

[Rick Willoughby]

The numbers you posted are hundreds of watts.

i.e. 5m/s requires 400W

This is good. It is typical of a round section that has the volume in the right place.

I believe a long boat with flat rocker resists pitching. Some like flat sections in the bow and stern to increase damping.

I have attached the double chine version as I tried to describe earlier. This is the sort of section that will skid sideways. Its other feature is that it has better roll stability. With a good paddler you could possibly stay upright without an outrigger but not for long. It would be stable if you were laying flat.

Sadly it takes extra power to push it. Not a lot more but enough to be ordinary I expect.

Rick W.

[ediestel]

I understand 'rocker' strictly in terms of pressure relationships: if the flow accelerates over the longer surface than the lower pressure sucks the hull down.

This can actually not be the reason for better steering control. Thus it must be the reduction of the lateral plane that allows for rotation around a vertical axis.

The relationship between fore and aft rocker would then be important, a symmetrical distribution would pivot the boat around a central vertical axis, an asymmetrical rocker would let the bow resp. stern skid relative to the opposing end and describe a different arc.

The lateral sliding of the boat is mostly desired in very irregular chop that often comes from two opposing sides, one being the backwash. The back of the boat may be tossed towards an opposite direction comppared to the front.
The boat should therefore not sit too deep either.

Regarding the pitching - this is at least as confusing as the rocker issue. From flared ends to various volume distributions everything is being mentioned.
How flat surfaces reduce pitching is not easy to understand.

My best thought is that the boat has to be somewhat wet, like your ama, to avoid pitching most effectively.

The question would then be: if your bow is pearling but your boat keeps a good trim while doing it, will that be preferable in terms of foreward motion with the wave ?

[Rick Willoughby]

A few comments:
With stern hung rudders, boats steer from the stern. To turn they must move the stern sideways. It does not matter if they pivot around the bow or further aft but the stern must be able to go sideways. I have tried a bow rudder on a couple of boats and it is not good because it induces roll-out in the turn.

You see surf skis with deep entries and flatter runs aft so they tend to steer more around the bow than mid section.

My V11A rides through waves as if they are not there. By comparison V7 (the yellow one) wants to ride over them. V11A is much faster in waves (chop). However in a following wave that has a wavelength just a bit longer than the hull I am able to drive down the wave and into the back of the next one. The hull would be angled at about 10 degrees bow down. I back off when the bow is completely submerged for more than 2m. I think I could push it further under if I keep powering. Not sure if the buoyancy would eventually take over. On bigger waves it would be hard to achieve the speed to get into the back of the next wave.

I do not have any experience with these long hulls in swells or confused chop. I can see potential to bury the bow so having some extra buoyancy and/or lifting surfaces possibly has merit. The bow in my original HC file is about what I think has reasonable balance between wave piercing and lifting but I am not an expert.

Hulls with little rocker and little buoyancy in the ends just glide. There is little tendency to pitch. On the other hand they need a following sea to be under them before they start to respond and pick up speed. I rarely get waves big enough to actually ride. Most of the time I am driving through them. I am able to hold about 14kph by running diagonally across a smaller wave that is moving at say 10kph.

Rick W.

[ediestel]

We have roughly three types of waves:

- The smaller chop that your V11 does so well in.

- The confused chop that hits the boat mostly form the side with a trend to lets say 20 degree aft.

- The larger swells. The larger swells are usually quartered to gain speed.

In a downwind run the angle is determined by the deepest trough in front of you - that is where you are aiming the bow, you actually want to follow aiming at the bottom of the trough as much as possible. The angle here is probably 30 - 40 degree right and left from straigth down the wave face.

True 'quartering' is used to get over the wave and gain the speed on the back face, or when you do not see a deep trough you try to find the smaller waves runing off the larger swell at about 45 degrees and gain speed there.

Your V 11 design may then not do not so well in the confused stuff ?
To do well here the boat must sit high; can length related buoyancy do this ?

When going straight down a wave face ?

Why do you think you would not get enough speed to get on a larger wave ?
Usually we try to keep the hull speed up quartering the wave and then we drop in once we have sufficient speed and sufficient steepness in the wave.


The angle of Hawaiian waves would be steeper, 15 degrees, maybe up to 20.

I would not mind to have the bow in the back of a wave as long as the boat can follow that wave.

My thought is that I would try to allow piercing of the wave at a trim of up to 15 degree unrestrictedly and gradually hinder piercing at steeper angles.

How about keeping the firs 2 - 3 feet of the bow as piercing as possible and then start up building volume to lift the hull starting at 3 feet tot ake effect when the trim gets greateer 15 degrees or the bow gets too deep into the back of the wave ?

[Rick Willoughby]

Last point first.
The idea of allowing the bow to bury to a point and then generating lift behing the bow might be good. My V6 boat had an exaggerated version of this. It carved through small waves well up to the point of the iakos hitting wavetops (see video). It was a true wave piercing design. The bow sat about 50mm below the surface when at rest. It was faster than V7 in most conditions and just a bit slower than V11A.

One other point.
My comment on the larger waves was related to driving into the back of the wave in front. The wavelength is related to the speed. In shallow water things change a bit but that is beside the point. My meaning was that if you are on a wave with say 50ft wavelength it will be moving to fast to drive down the face and into the back of the preceding wave. Rather you would be working just to ride the wave.

Unless you are riding waves they will slow you down. What I have found is that a boat that is fastest in calm water will still be fastest in rough water. However I cannot speak from experience in large waves.

Rick W.

[ediestel]

Rick,

you are right on the performance of the chined hull, the speed drops form 4.7 to 4.0 knots at 20 N.

I will look at your V 7, I do not remember seeing that video.


The wave troughs are not all that long, they get relatively longer when you quarter; also before you drop in only part of your boat is in the trough, the rest extends over the crest.
The key issue with my current boat is that it takes off too late.
I moved my position by several inches forward, that helped.
Up to 1/3 of the hull sticks out of the water in steep waves just before take off. This happens mostly when you have to adjust the direction of travel.

Ideally the boat would easily assume the right trim and do this early.

Eckhart


PS.: I used 'autogroup' for layers and plate development - it seperates all the layers correctly.

[ediestel]

Very nice.


This bow behavior is exactly what I imagine for my OC 1.


The important part would be that this bow could display this behavior with a trim from - 10 to 15 degree or so.


Is the bow just a conus, like your outriggers ? Or is it a z bow under water ? Would a little more vertical volume change the behavior too much ?


One of the important features of your v& bow is that even when you have water on your deck it does not matter much, because the surface area is small enough to tolerate it.
I feel that this is the only way to substantially address pitching issues, isn't it.

[Rick Willoughby]

V7 is the yellow Pahoa OC1. It has a high stern that sits out of the water at rest. It is not wide or flared but quite high - about 8". This part of the hull has a lot of leverage so submerging it gets the stern up quite quick.

If you want the boat to lift onto a wave then I expect that you need buoyancy right aft. You could try this on you existing hull by just adding some faired foam above and behind the existing stern. Just tape it in place with duct tape to see what it does for a brief trial.

Rick W.

[Rick Willoughby]

V6 - note V6, bow is just a cone made from aluminium - shaped like I have for the orange outriggers fitted to V7, the yellow Pahoa. Thin vertical volume does not make perceptible difference. Having it submerged eliminates the bow wave but it is a small saving and only applicable to very calm conditions. Even a ripple and it breaks the surface.

You should look it my web site it helps understand my boat designations and the development. Godzilla was the greatest find but I was going in the right direction before I started applying it.

Rick W.

[ediestel]

Yes I have been on your website before, I had not seen the V 6.


You may be interested/have read this thread; some of the desired aspects seem to be represented there.
http://www.boatdesign.net/forums/showthread.php?t=18037

[ediestel]

Rick, I am currently trying to find sufficient information reagrding open ocean behavior of possible hulls. It is quite difficult to make a good judgement.

I found that the added drag of the chines cost almost 4/10th of a knot in speed at 20 N.


The shown hull in Gaussian coloring is my best so far with 3.91 N at 5 m/s.
I started with Godzilla, length 6.9 m and cut .2 m off the stern.

[Rick Willoughby]

My view is that you go for the lowest drag underwater section for calm water. Create a deck design that you believe will best cope with the conditions.

Build a boat and see how it performs in your conditions.

You can go on for a long time trying to postulate what will be best. I can tell you that no theory copes well with waves and chop so the easiest way is to experience it. There will be cues from other designs so if you can actually sample their merits then they might give pointers.

Rick

[ediestel]

I was out in the open ocean today in about 4 - 6 ft swell; while paddling I thought of boat design ...


There is no way to fully predict a hulls behavior in these constantly changing conditions.

Many theoretical considerations seem to have merit. But which one will perform most of the time in most of the conditions at average or better ?

Example bow design: do you want it very buoyant to keep it out of the water, or do you want it minimally buoyant to keep the boat wet and reduce the amplitudes ? Both thoughts are valid in my opinion.

I agree that there are many variable that can well be assessed by trial and error.

I will take your advise and engage a more pragmatic approach by building a hull and changing it fit.

I am glad that your design has made it to North America with the purpose of breaking the 24 hour distance record; I stumbled on the website of gregsadventures.com.

Could it be that the density of the water accounts to a degree for the lower average speeds that Greg has achieved so far ? Is it favoring him that he is sitting higher ?

Thank you very much for your gracious help. I think that I will be building with female molds and stripped foam.
The LR 2 class A catamaran of John Lindale is an inspiring project, as is yours.

Aloha

Eckhart

[Rick Willoughby]

....

I am glad that your design has made it to North America with the purpose of breaking the 24 hour distance record; I stumbled on the website of gregsadventures.com.

Could it be that the density of the water accounts to a degree for the lower average speeds that Greg has achieved so far ? Is it favoring him that he is sitting higher ?

Thank you very much for your gracious help. I think that I will be building with female molds and stripped foam.
The LR 2 class A catamaran of John Lindale is an inspiring project, as is yours.

Aloha

Eckhart

Eckhart
Greg has just about got there. He has now used all my developments but one and he is working on that now - he will remove the seals from the gearbox. I have a box that is well run-in and uses light oil. His is new and costs about 6W in losses. Mine is under 3W.

To get the design speed of 12kph at 150W he needs dead calm conditions. The boat is very close to that now.

I started a lighter version of my hull this weekend using the old boat as a plug. The old boat still has the original foam plug inside so is about 6kg too heavy.

If you make sound observations and take an analytical approach you should get a good result.

Greg has a power meter on his crank that gives highly repeatable input power this helps in quickly analysing performance.

I have a Garmin GPS that includes heart rate monitor so I can guage my working level from week-to-week. I plot my performance and make notes on weather. You would be able to use the same method to assess performance. If you are racing with others then you should also have a good basis for comparing benefit of changes.

Will look forward to production of your hull. Sometimes it is quicker to actually get your hands dirty than researching a topic.

Rick W.

[ediestel]

If it comes to speed I would consider a foil under the outrigger.


I have built a variety of foils - attached to the rear iaku, about 1 1/2 ft deep.
They work, the boat starts to glide.
Pitch and roll are reduced, the glide per stroke is longer, stability is increased and a huli - that i s capsize -more or less excluded.

My versions did not do well for open ocean: you cannot fly the ama anymore, and there are situations where the foil works as a break.

For flat water and a record attempt - it would work. It would allow you to set the regular outriggers higher, above the water at all times. The balance gets real easy while moving, almost like a bike.
One side with a foil should be enough.


Eckhart

There is one thing that works for the open ocean actually: it is a foiled rudder.
Unfortunately I lost my last version after I forgot to tighten the screws. While attached it controlled pitch and roll, - less than with the outrigger-foil -, and prolonged the glide in the swell.
My interpretation was that the boat being closer to ideal trim and having less pitching amplitude made for the facilitated surfing. That day we had three ft wind chop.
It was a simple aluminum bar, less than 5 inch span and 0.5 inch chord. Not finished, just for initial tests.

[ediestel]

Rick,

within my given parameters D, L, B = 0.28 this is my best design with 4.01 at 5 m/s.

Can this be improved with form parameters ? I tried many, many different options so far, but there may be a culprit.

How can I judge the stability ?


Eckhart

[Leo Lazauskas]

Rick,

within my given parameters D, L, B = 0.28 this is my best design with 4.01 at 5 m/s.

Can this be improved with form parameters ? I tried many, many different options so far, but there may be a culprit.

How can I judge the stability ?


Eckhart

The hull is just a Wigley hull before you run the optimisation. Do we have to run the optimisation ourselves? Or did you have a set of results after your optimisations you want us to look at?

Also, the number of waterlines and stations in your input file is far too low. Use more, e.g. 33 x 33.

Have fun,
Leo.

[Rick Willoughby]

Rick,

within my given parameters D, L, B = 0.28 this is my best design with 4.01 at 5 m/s.

Can this be improved with form parameters ? I tried many, many different options so far, but there may be a culprit.

How can I judge the stability ?


Eckhart

Echkart
This is better than the Godzilla result. I doubt that I can improve on it.

Leo makes the point that adding more stations and waterlines might improve the result so this would be worth a try but I doubt that you will get better.

Leo - The basis of the hull was one produced by Godzilla with 17 stations and 9 waterlines. There has been a little manual fiddling to fair it.

When I get some time I will try Godzilla with greater resolution to see if does anything better.

Rick W.

[ediestel]

Yes, this is the input file only.

I use Godzilla this way: I run a specific 'in' file, then look at the output/results.

If the results are good, I save the input file directly with a certain label that allows me to see what specific constraint I have in the input file.

During this communication it will allow everybody just to download the file and have it ready to go for a run in Godzilla with equal or new constraints.

It does not show the entire results; it makes it easier to reproduce/modify the run.

Thank you for pointing out that more stations should be used.

The only way I got better results was by allowing a narrower beam, which is not suitable for practical purposes.




The question regarding the parameters is based on one of Leo's articles, where the optimal a1= 1.0 for the waterline shape for similar hulls, that is parabolic if I remember correctly.

I wondered how the input parameter f0 relates to a1 for the waterline shape in the article, and if it should get as close to 1 as possible for the forebody of the ship. I do not fully understand the practical interpretation of a given waterline shape.

I have run quite a few constraint combinations; most results have a similar power requirement of about P = 0.40x for similar hulls.



Regarding stability: is there a pragmatic way in Godzilla to look at stability by setting parameters ?



Thank you for your interest.

[ediestel]

It turns out that the Godzilla run with 33 stations produces a better result for the power requirements at 5 m/s:

0.403 vs 0.396 with Rt = 7.92504e-002 about 1.76 % better



General information on metacentric height can be found here:

http://en.wikipedia.org/wiki/Metacentric_height


What are longitudinal/transversal target value ranges for the hull with following constraints:
D= 0.11 t, L 6.7m, B 0.28 m, T =0.115 m, S = 2 sqm, Cb about 0.5, Cp about 0.64


Center of gravity of the paddler likely ~ 12.5 "; 6'5" height.


PS: it has been said many times before, but it is still worth to mention: thank you to Leo Lazauskas for providing Godzilla/Michlet to all of us; also thank you to Rick Willoughby for promoting and helping with the practical applications.

[Rick Willoughby]

Eckhart
You can control stability by limitting the lower value of GMT for the hull. This value is the height of the metacentric height above the CofG (the height of CofG defaults to waterline unless you change the hull loading parameter). To have a stable hull without outrigers you would need a GMT of around 0.35m.

Delftship give the stability figure as KMT. This is the metacentric height above the keel. Michlet and Delftship will give the same height allowing for the difference in reference.

By the way I limit the number of stations and waterlines to values used to make it easier to work with. In practical terms it will be difficult to assess 1% difference in power but it is lower.

Rick W.

[ediestel]

Rick,

thank you.

I understand the reduction of sections and waterlines.

I also understand the concept of metacentric height.

There are two situations of seakeeping that I think may be important:


1. On the crest of a wave in following sea, heave
2. surfing, mostly done 'quatering' a wave to starboard


1. While in the open ocean, one of the least stable moments occurs when the paddler/OC1 is to slow to catch a wave, the boat is lifted up unto the crest of the wave and then sinks back. I believe that this motion with reduced gravitational force results in reduced stability.

2. surfing is mostly done to the starboard side, many resaons for that including race courses, position of the outrigger, better control as the outrigger is not in the way etc.
Stability in this scenario - on a wave face, 'surfing' with trim minus 10 -15 degree, 4 - 8 ft waves from 3/4 aft, at the beginning of the acceleration - is very important, as regarding roll and pitch.


The answer to soften the impact to 1) may be to have the LCB aft of the LCG
regarding 2) I would guess that a 'round' hullshape may be good as the wave impacts the hull from the back right.


How would or should the metacentric height be adapted for these two crucial scenarios ?

If it is too high then the roll stability will be irritating while quatering the wave,
if it is too low the stability during the heave may be a problem.



Is there a 'rule of thumb' that can be applied here ?
'Minimum' CG to CMT distance ?


The values for rowing shells are 0.1 m, your value for a canoe is about 0.34 m. My current design seems about 0.13 m, thus well below CofG.

Where in this range would you see an open ocean canoe with outrigger with described use characteristics ? I have not found any publications that address the outrigger in this constellation yet.

Eckhart

[ediestel]

http://www.waterbiking.org/

good articles in the design section; great examples

[Rick Willoughby]

Here is an input file for Godzilla to produce an OC1.

Rick W