Multihull Structure Thoughts

I have only seen 1 jpeg of this cat and a very short description from the old Quorning Boats (DEN) “1981 To test new markets, the one and only catamaran built at the yard, the “Combi Cat” was built for a German client, based on the floats from the Dragonfly 25.” The design was done by Børge Quorning.

The catamaran was called the “Combi Cat”. If it is built on Dragon 25 floats then the cat would be about 21 x guess 8 foot. The sailing, with 2 crew, displacement would be about 1500 lbs. The mast is about 29 foot. Sail area unknown but I am guessing about 230 square foot. The length to beam of the hulls is 13 to 1.

The accommodation looks like a small cubby cabin for some storage and a berth with maybe 3 foot headroom. The cockpit could fit 4 people.

Quorning at this stage were producing foam or balsa fiberglass sandwich structures so I assume that would be the build of this cat.

Sorry, I do know any more but an interesting cat that appears to be well done but did not find a commercial market.

An interesting design and nice looks. Sorry only 1 jpeg. The second jpeg is of the original Dragonfly 25 design of 1980 to show the floats.

 

oldmulti, do you mean this catamaran?

 

And a third one:

 

Manfred. Thankyou, yes that is the cat. Its interior is smaller than I expected as the "cabin" is a cubby to protect your head from the rain. The cockpit/cubby could be a comfortable double berth area with a tent over. But a very nice cat. Any idea of performance?

 

Oldmulti thank you for your kind acceptance. As far as I know the catamaran was no succes and Borge and Jens did not build more than this one.
I liked the form of the floats and made some tests with them from rubbish. I cut them open from the stern and inserted a wedge. The flow of water around them and the hydrodynamic lift were fine and the floats were very light. Thus I`ve built one outrigger and three trimarans, sailed them and gave them to other sailors.

 

You want a cruising cat that can sail well, have enough headroom to handle your 6 foot plus height throughout the boat, have some really comfortable berths, an owners cabin and can fit through the 16.3 foot wide French Canals. OK. This is going to be an interesting one. The latest (well 2018 and still in production) Broadblue 346 may fill the requirements.

The Broadblue 346 is 33.3 x 15.6 foot with a “displacement” of 10,750 lbs. The 43 foot fixed aluminium mast carries a 301 square foot mainsail, a 204 square foot self tacking jib, a 485 square foot gennaker and a 1050 square foot spinnaker. The mast is set well aft as this gives the headsail more power and cuts out on pitching. The length to beam on the hulls is about 9 to 1. The draft is 3.4 foot over fixed low aspect ratio keels. The engines can be a 25hp Outboard or 2 x 16hp or 2 x 20hp diesels with saildrives and propellers.

The 345 was launched in 2008 and was designed by Plymouth based multihull specialist Dazcats and was initially known as the Voyager 10 before she was acquired by Broadblue and rebranded the Broadblue 345. The 346 was a 2018 update where the designers: Mark Jarvis, Darren Newton and Derick Reynolds (basically the Dazcat team) kept the basic design but redesigned the coachroof and did other upgrades.

The standard accommodation for this yacht is two large double aft cabins and a single cabin forward in the port hull as standard. The master cabin to starboard has an ensuite forward and there is the option to increase the number of berths by incorporating a single cabin amidships on the port side. The main saloon has a galley seating and navigation area.

The hull, deck, engines and interior joinery are completed at Broadblue's factory in Poland, and the boats are then motored to the UK (already a good shakedown, not least for the engine installation), where the mast, rigging and sails are fitted, and finishing work completed.

Both Hull and Deck are vacuum bagged, foam core, with woven matt construction with solid GRP below the waterline. The hull-deck join are fully bonded and protected. All of the structural bulkheads are of vacuum bagged foam core construction.

The performance is good with test reports showing 7 knots in 10 knot winds and 9 to 10 knots in 25 knot gusts. The 346 can sail upwind relatively well but don’t expect monohull pointing ability. This cat is a good cruiser not a racer. It is classified as ocean capable and as long as you choose your weather and don’t push things you will arrive safely.

An interesting design that for a lot of people would be all that they require for an ocean cruising cat that would be far cheaper to maintain than a 45 foot cat.

The jpegs give the idea.

 

A story about a man who has been around multihulls for the majority of his life and finally designed his dream boat. Harold Aune had a couple of friends, Bill Kristofferson and Nick Shownawolf were both starting to build offshore cruising trimarans for themselves so he helped them build their boats. Harold learnt quickly and a few months later started building his own 37’ trimaran. Freya a 37 foot cold molded wood trimaran was the result, He then in about 1974 built a Kismet 37 Kyst a cold molded tri. After that, he was involved in the boating industry. In 2014 he designed his “perfect” cruising trimaran.

The Aune 64 is 67 x 40 foot with a weight of 53,000 lbs, a displacement of 63,000 lbs and a total capacity 73,000 lbs. The 82 foot fixed aluminum mast that carries 1600 square foot of sail area. The draft over the low aspect ratio keel is 5 foot. The 2 engines are 200 HP diesels with 4000 litres of fuel. It is designed to cruise, with one engine at 9-10 knots and up to 16-18 knots maxed out with both engines.

This should give a hint as to this tri is a motor sailor. With 4000 litres of fuel the range will be good under power. The prismatic coefficient is a low .522 which interests me, normally multihulls have a PC of .57 to .65. A higher PC means fuller ends in the hulls.

The accommodation is vast with 5 double berth cabins with ensuites. The main cabin has seating, large galley, internal helming and navigation etc. This could be a permanent live aboard.

The construction of the hulls are composite/cold mold wood. The keel and rudder are composite. I would suggest that there is a lot of engineering required in this design as any multi that can achieve 18 knots under power needs to understand slamming loads when going into waves.

The sailing performance according to the designer “Under sail there is even greater speed potential readily achieving double-digit speeds. Ocean passages with daily averages of 300 miles are easily attainable. Smooth stable motion whether on or off the wind in all but very rough conditions are to be expected under power or sail.”

I hope this tri was built to see if the capabilities were realized. A floating home that can travel far. The jpegs are limited.

 

Another approach with a float from Dragonfly 800 for more displacement and comfort.

Beams strengthened with carbon top and bottom. Sitting boards are foldable to floats to
enable paddling. Hull inner bottom and sitting boards are sandwich with foam and aircraft ply to keep it as light as possible. Wanted a tracking tri with bicycle.

 

This is about Multihull Hull Shapes. Tony Grainger, multihull designer, has developed hull shapes to suit his range of sailing and power multihulls. He has written several articles about the hull shapes which I will try and summarise here.

To start with a designer needs a concept of what he is designing, then a series of iterative steps is taken to mainly establish 2 things, what the likely weight of the vessel is and where the centre of gravity will be. Then you can design a hull that can support the weight and have the buoyancy correctly distributed to have the boat trim correctly when sailing or powering. Notice a sailing boat may have a slight bow up trim at rest but when sailing the thrust of the rig will push the bow down slightly when sailing.

Now the characteristics of the hull will vary according to need. “A fat hull will have high form drag. A hull too skinny will have higher drag from skin friction. We want asymmetric water planes and a reasonably flat rocker to minimise pitching. A fine entry will minimise slamming in short steep waves and throw less spray. A flat run aft in cross section will promote planing or semi planing at speed. A straight run aft with a shallow release angle in the rocker line will also help to improve performance downwind in fresh air. A rocker line that is too flat will be slow to tack and will suffer in the light from increased wetted area.”

The rocker line in many of Tony’s boats is gently curved coming aft to about 70% to 75% aft of the bow, then it has a subtle but distinctive kick or bustle where it carries the maximum buoyancy before running aft to the transom quite straight and shallow in profile, allowing the flow to exit cleanly

Also, the rig effect. “Moving the rig well aft might be a good idea for high speed reaching in waves, especially if the boat is light. Move it too far aft in a cruising boat and you have problems providing enough buoyancy in the aft sections without creating a lot of drag.”

Translation. Everything in hull design is about intended purpose EG Racer or Cruiser and what compromises you want to take. Are you sailing in a light wind area with minimal sea conditions and you want to weekend cruise then a 10 to 1 length to beam with rounded hull and full ends will work. But if you want an ocean racer that can sail upwind in strong winds then a EG 12 to 1 hull that has a flat stern and fine bow will probably work better. It all depends on your statement of requirements.

A few jpegs of hull shapes. The Livewire 28 hull is a sample of a racer cruiser, the Chincogan 52 is a fast cruiser. The TR52 tri is a recent design, look at the rocker shape of the main hull. Each has a hull shape to suit the need.

 

A client/friend of mine that I do vintage car/motorcycle restoration and maintenance work for gave me a copy of his Hagerty Drivers Club magazine because he thought I would find the article about this Trimaran concept interesting.

It was a pitched publicity/marketing stunt for which John Marples and Jim Brown were tasked to design the ‘54 Tri modeled below to carry a Chrysler LaBaron which could power the boat to 5-6 knots at 1500 rpm by using the front wheels to drive dyno rollers on swivels that also allowed steering wheel movements to turn the rudder.

I can’t help but wonder what a modern day version using an electric car and solar panels might be like…

Anyway, a pretty wild concept/project to say the least.

You can read more about here if you like:

The Audacious PR Stunt to Prove That the Chrysler LeBaron Was a World Beater - Hagerty Media https://www.hagerty.com/media/automotive-history/the-audacious-pr-stunt-to-prove-that-the-chrysler-lebaron-was-a-world-beater/

 

This is about a power cat build with only 2 jpeg. The real interest is the professional builders use of resin infusion. Dave Walworth is the designer of the 35 foot charter cat and he knew the boat’s build-team members, Joe Kitchell. The owner wanted an eco friendly charter cat (read hybrid electric) that intended to do 3 hour dolphin cruises twice a day over a short range in Key West. The benefits of electric propulsion, was Belafonte was expected to burn as little as 3 gallons of fuel on each 3½-hour tour. The idea is that Belafonte will cruise to the inshore dolphins using the diesel generator and batteries, then slow to a near idle once the dolphins are found. “Because it’s an electric motor, the boat isn’t going to stall,” Walworth says. “He can go as slow as he wants with the dolphins.”

The design and construction of Belafonte, a 35-foot, 20-passenger power cat built by Kitchell Composites in California (now Left Coast Composites). Belafonte was built with epoxy resin infusion — Kitchell’s skill.

“It took us a little while, but we figured out epoxy infusion and started building boats that way,” Kitchell says. “I built him two 57-footers. Kitchell was sold on epoxy infusion and made it his specialty. At the time he started with the process, other builders were using polyester and vinylester infusion, he says, but those processes use thinner resins. When people tried epoxy infusion, they often stumbled into expensive mistakes. “Epoxy is thicker,” he says. “You have to have a true vacuum — and that’s a lot trickier than it sounds. A lot of things can go wrong.”

With epoxy infusion, a traditional mold creates the shape of the boat. Workers lay up foam and cloth inside the mold and then put a bag over it. The bag is sealed, and there’s a hose for the epoxy that will be pumped into the sealed area. There’s also a hose for the exhaust going out. “That’s it,” Kitchell says. “It takes generally about an hour and a half to pull the epoxy into a hull, and then you let it sit.”

The process also saves on labor, he says. “With epoxy infusion, me and two guys can build a 72-foot hull in a week,” Kitchell says. “We lay up everything dry, we put it in a bag, and we displace the vacuum. You’re basically sucking the epoxy into the part.” Some cats Joe built had the pre-preg carbon cooked at almost 200 degrees (Fahrenheit) for hours

The basic hint here understand your materials, do test runs and don’t assume that the resin infusion done by others using differing fabrics and resins will work on your project. The works “You have to have a true vacuum” is not to hard but requires effort and attention to detail.

The jpeg shows the deck layout of the boat and the oven used to cook an 88 foot cat.

 

Peter Brady is an Australian designer who has had 30 plus years of experience. He has designed sailing and power multihulls with his most famous the 33 foot Scimitar 1010 power catamaran. The power cats have displaning hulls that are designed to be just as efficient at lower speeds as they are higher speeds, cutting through the water rather than trying to plane over it. This seamless transition between displacement and planning allows the skipper to choose the safest speed for the prevailing conditions. The hull shape is versatile in the styles of power catamarans EG sports fishing boats, family coastal cruisers, long-range offshore cruisers and commercial fishing boats

This hull design is a further development of the Scimitar 1010 and the Leopard Power Cats and has not been equaled or bettered in terms of fuel efficiency, range and seakeeping in its size.

The so-called 'displaning' hull form has narrower sponsons, a higher tunnel clearance, and a wider tunnel width than most planing cats. Planing power cats have not always been entirely successful in applications that required a reasonable load carrying capacity, because when loaded they cannot not always ‘get on the plane’, whereas the displaning hull form has no such problems. Peter’s designs are forgiving of high loads, only affecting their top speed by a knot or two and making EG 10 lph reduction in fuel consumption at cruising speeds. When overloaded or over weight, planing power cats are not able to ‘get on the plane’ so the wingdeck clearance and tunnel volume is reduced, resulting in their wingdeck structure taking more of a pounding, which in turn creates both structural problems and an uncomfortable ride. The planing catamarans generally narrower, lower volume tunnel created by wider planing hulls then become even more of a problem when not ‘on the plane’, as wave interaction and increased wetted surface decrease performance further.

Translation. There are 3 basic types of power catamaran hulls. Planing where the idea is get the cat to the plane and then gain the speed advantage. If these cats are overloaded they may not get onto the plane and be inefficient. Second model is semi displacement cats, Marples, Grainger and Crowther etc fall into this category as does Brady’s cats but with a slight variation. Final category is Tennant’s full displacement canoe stern power cats. Each solution can provide a good result but the full planing and Tennant models are weight sensitive and operate best is specific speed ranges.

Brady’s “displaning” operate with less weight sensitivity and over a wider speed range but do not have the outright speed of a high powered planing power cat. Displaning cats have very good fuel economy when correctly powered.

There are a few jpegs of the hull type below.

 

The following is the final for now on power catamaran hull shapes. I will not do an item on Tennant’s canoe stern (CS) shape as I have spoken about before. The final one is about Tony Grainers Sensori cats which initially had Tennant’s CS shape then evolved into a semi displacement hull shape. Using a lot of Tony’s word’s, we will study why.

The initial hulls of the Sensori range were built on the CS concept developed and made popular by Malcolm Tennant in the 1980's. The CS has its advantages, in particular the fine hulls have excellent sea handling characteristics and wave interaction in the tunnel is minimised thanks to the narrow hull and fine entry at the bow. But are they the right choice for fuel efficient operation?

In order to be able to make an informed decision on the preferred hull form for the new Sensori cats I built a CS hull and a Semi Displacement hull of equal displacement for each of the cats in the range and examined the hydrostatics for each in turn. For the semi displacement hull I used a relatively deep V entry with the maximum draft well forward. The V form extends well aft of amidships and gradually rolls into a flattened bottom with a modest transom immersion at half load. For the CS hull I stayed very close to the form we had initially employed on the Sensori cats. The two hull forms can quickly and easily switched in our design software and examined on each of the models.

On studying the hydrostatics of the two hull types 4 characteristics are apparent.
1. The semi displacement hull provides greater internal volume for engines and sleeping accommodation.
2. The reduced sinkage rate for the semi displacement hull (cm immersion per kg weight) allows for greater load carrying capacity and there is less change in the hull characteristics such as wing clearance as fuel and water load changes.
3. The CS Hull will inherently require greater transom immersion and hence turbulence in the wake.
4. There is a significant increase in wetted area for the CS hull. Around 19% to 20% higher on average compared to the semi displacement hull.

Wetted area is the primary drag factor in the low to mid speed range and this is also the speed range where most owners will choose to operate if range and economy are important. Another factor in favour of the Semi Displacement hull form is that they are likely to take advantage of some lift from the hull form in the higher end of the speed range, something the CS hull is very unlikely to be able to achieve.

Transom Immersion causes turbulence. We don't want that. We want the water separating from the hull as cleanly as possible. This presents a delicate exercise for the designer. The CS hull relies on a certain amount of the box section at the stern to be immersed to maintain trim and resist pitching. The same applies to some extent to the semi displacement hull but its hull form has minimal transom immersion and more forgiving to trim adjustment.

Translation. The semi displacement hull is more forgiving in loading, trim and has less wetted surface area. This helps reduce fuel consumption over lower to mid speed ranges. The hulls also have more internal room for engines and accommodation.

There are limited jpegs.

 

Having come across the van der Werff and Bergsma methods in a Professional Boatbuilder online article ( Van der Werff's Curved Wood - Professional BoatBuilder Magazine https://www.proboat.com/2024/04/van-der-werff/), it's interesting but has a ways to go to be a radical approach to combining sustainable materials with open-ended form design. Adaptive CNC molds have come into use in the last 4-5 years, and Adapa is the leader in the market if not the only marketer due to patents: Adapa - adaptive moulds - Adapa - adaptive moulds https://adapamoulds.com/. It's cool technology, but really expensive to cover a relatively small area. The grid structure is over-specified on one axis vs another if you are molding a material that is anisotropic (like strip planking) vs isotropic (like foam core). Scoring core material, or cutting it into patterns to make auxetic surfaces, let's it adapt easily to the mold surface. But do that to long-grain wood when it already has strength and flex characteristics oriented to the global loads of a hull or deck structure? The smoothness of a surface off an adaptive mold has to based on the transition from the actuators, through the x-y rods, and into the semi-conformable membrane.

The comparison to Constant Camber seems marginal. Both the adaptive mold in the photos and CC generate panels that need to be butt-blocked to make longer hullforms. Attention needs to be paid to stress and shape discontinuities at seams. Laying up foam or wood strips into CNC'd female molds goes really fast, and flipping the molds is simple to make port vs starboard. CNC routers are cheap, and I haven't made a router template for years. An Adapa Mold probably not in the budget for a long time...

 

A short one about an idea of a transportable 38 foot power catamaran. Perter Brady (Australian designer) was asked by a client to design a transportable power catamaran for trailing around Australia as required to get to new cruising grounds. The client who owns an engineering firm wanted something his family could enjoy. So, Peter designed a 38 foot catamaran that could be transported by a Semi Trailer.

The Transformer 38 (also known as 12 meter) is 38 x 16 foot of unknown displacement. It can be collapsed to 8 foot wide for transport. The catamaran has a central module that slides inside the hulls on the trailer but extends out to make the overall beam 16ft on the water. The Transformer Cat has a displaning hull form to cruise economically at 15 knots and a top speed of around 18 knots, powered by 2x 70hp high torque outboards.

The accommodation has not been detailed but the cat probably can sleep 6 people with a large main saloon when fully extended for motoring.

The build of the cat hulls and topside panels will be lifted from our variable dimension power cat moulds and a section with a flatter run aft grafted on to support the weight of the outboards being located further aft than my usual inboards would be. The Transformer Cat will be constructed from infused composites featuring solid glass bottoms from the keel to the turn of the bilge, with cored construction above this. The central pod section will slide into the hulls so a great deal of detail has had to be worked through in regards to how the layout will work and in waterproofing the joins. The fit-out will be lightweight using cored panels, and the interior finish will be painted for minimum weight, with comfortable simplicity the key.

An interesting design of which there is 2 magazine articles but no indication of it being finally built. The engineering company owner had the capability of building any folding or sliding structure required but there is a lot more to a design like this especially the line about waterproofing the joints.

Sorry about the limited jpegs.