Multihull Structure Thoughts

The following catamaran is a Vellum 22 which was designed by Eric Jean as a 20 meter catamaran but extended by the builder, Boatyard-Chantier Naval scotto, to 22 meters. The cat was built in 2009 and refitted in 2018. The "VELUM 72" is a high standard cruising catamaran. She has been designed with safety, comfort and performance in mind for ocean going cruising and offshore racing. Good start let’s have a look at the Pod catamaran. It is 72.3 x 38 with a weight of 38,000 lbs. The sail area is unknown but I am guessing a 90 foot plus fixed carbon mast and several thousand foot of basic sail area. The length to beam on the hulls is about 14 to 1. Now things start to become inconsistent. The draft varies from 1.8 foot to 4 foot over the daggerboards according to the advertising and the jpegs support the idea of short boards but I doubt it. The rudders alone for a cat this size would draw 4 foot. The engine power is 2 x 75 HP Yanmars. It can motor at 10 knots using 7 litres/hour. Motoring range is over 1200 miles.

The accommodation has in each hull has one double bed cabin, one single bed cabin and one bathroom. Between the 2 hulls is a nacelle with the salon, dining table, chart table and galley. This cat has separate entrances to each hull and nacelle which allows real privacy. Unfortunately, if it is wet and windy you get wet going to your bunk after your meal.

Both hulls are built in e-glass Western Red Cedar strip-planking epoxy and the beams are in carbon composite. The mast, boom and prodder spars are also carbon fibre epoxy.

This cat has crossed the Atlantic on several occasions. My simple calculator says this cat can be very fast (I made an assumption on sail area). I cannot find verification but averages of up to 15 knots, 360 miles per day with peaks of over 25 knots are indicated.

The jpegs give the idea. An interesting boat that could have been yours for 500,000 euros. The last 2 jpegs was the original Eric Jean design

 

This is the first multihull on this thread that I will say now, I would not want to actually sail on. It may be high style design but its function as a good sailing catamaran would be limited. This concept vessel was designed in 2011 to create interest. It did, but it only lacked one attribute, good aero and hydro dynamics required of a good sailing catamaran. The cat is 67 x 43.8 foot with a calculated guess displacement of 140,000 lbs. The mast is 92 foot that carries about 2500 square foot of sail in a to be developed sail plan. Its hulls have a 4.5 to 1 length to beam ratio and a draft of 3.8 foot over the single chine hulls. It has 2, hopefully, very large motors although I cannot see where they are located in the design.

The automotive designer Samuel Errico Piccarini and the design studio C-Yacht joined forces to offer a Pininfarina-branded yacht to influential and demanding customers. In this specific case, the client is the 45-year-old Ryu Cardoso, founder of a textile company in Japan. The goal, as mentioned, was very clear: to design a boat that would satisfy the needs of worldliness and also of solitary yoga of the client tycoon.

As you can see the vessel is see through at most levels giving the feeling of large spaces. The accommodation has 4 double berth cabins in the lower deck in the hulls with an owner’s cabin on the upper deck there are crew cabins somewhere. The main saloon and galley are aft on the lower deck in a hull with another saloon on the upper deck. Steering and sail handling are done from a flybridge. Any further detail on a conceptual design is limited. Just don’t try and climb those wide staircases without hand rails in a seaway.

The construction is unknown but the glass panels and external venetian blinds better be strong to handle any rough weather. Also, I hope the engines are powerful to deal with the 14 foot high freeboard for most of its length and associated windage. Upwind would be engines, down wind sails, reaching speed would be limited by the weight and very fat hulls.

High visual impact, limited sailing ability. The jpegs give the idea.

 

Good grief - that Pininfarina cat is really rather hideous, in every aspect.
I wonder how they furl the sails?

 

The following design I do not know much about but it looked interesting. A gifted home designer did this work and from the limited information provided he has done some serious calculations of the specifications. At one point he had a finite element analysis done for the structure which showed up, amongst other things, the need for extra reinforcement around the main openings into the hulls due to torsion load in a 'lee bow bury' scenario.

The cat is 32.9 foot long, not much else is known. The cat is demountable to allow transport to differing locations but it is a crane job to disassemble or assembly. The hull bottoms are flat dory style but could be round.

The build is plywood with plywood timber frames and stringers. The cross arms are timber plywood that fit together and are locked in by wedges as bolts can create single pressure points. Friction over mechanical point loads. The lower part of hulls could be strip planked, the rest plywood or could ideally be all foam sandwich with mainly flat panels. The plywood hulls could be 6 mm or 9 mm etc fiberglass covered externally.

The accommodation is hull based with 4 berths, galley seating, toilet etc in a practical layout. The folding shelter in the middle would slide forward on rails for standing at the wheel or back for sitting at helm with better weather protection, or fold away altogether. This is a realistic design.

The jpegs are limited. An interesting concept.

 

A daring design!

 

Hi folks. A lightening strike within a kilometer took out our digital internet for a few days. We have just been reconnected. I will post tomorrow morning. Have fun out there if the weather isn't causing problems.

 

The following item is about when reality overrides dreams. There are many people who dream of building large boats at home without realising the work involved. Richard Woods has designs up to 50 foot but recommends if you are home building don’t build over 40 foot. Wharram has designs up to 65 foot but says the simpler sheet plywood designs are OK to 63 foot but if you want a load carrying designs with a more sophisticated hull shape it should be built by one of their authorised builders.

So, let’s look at the Wharram designed Islander 55. The Islander 55 is described as “eminently suitable for charter use or as a large ocean cruiser”. The 55 is 55 x 27.5 foot with a weight of 15,700 lbs and a displacement of 26,900 lbs. The 42 foot masts forming a schooner wing sail rig has a 430 square foot mainsail and fore sail, a 275 square foot jib, a 200 square foot stay sail and a 405 square foot Genoa. The length to beam ranges between 12 to 1 to 11 to 1 depending on the load. The draft of the cat fully laden is 3.4 foot.

The accommodation layouts vary according to the intended use of the Islander 55. The original design request came from islanders who wanted to carry loads of fish etc. The design also has a flexispace approach which can have up to 5 double berth cabins in a charter mode. It also can have a small or large central pod cabin with a steering space and cockpit to a full cabin with galley, seating, navigation and steering.

The reason Wharram requests this design be built by professional builders is the size and mode of construction. Again, it is plywood, timber and epoxy but the hulls are double moulded plywood. The way Wharram does the structure is time consuming, but it is strong. The real issue is can you find the lengths of quality of timber that this design would require. Have no illusions when you build this large, think a 1000 hours just doing a lot of epoxy work, fairing and painting. A build like this is at least 8,000 to 12,000 plus hours. You need a lot of friends or employees to help out.

So why no home builders when a Pahi 63 can be home built. The Pahi 63 is 63 x 28 foot with a weight of 13,300 lbs and a displacement of 24,500 lbs. It carries virtually the same rig as the Islander 55. Its length to beam on its hulls are 17 to 1. Not much difference. Now we come to the build. The crossbeams and rig would virtually be the same in both cats as would the fitout. The only significant difference is the Pahi 63 has sheet plywood hulls over a frame and stringers versus the Islander 55 having a cold moulded hull over stringers, yes you will spend more time building the hulls of an Islander 55 but it is not significant. What is really being talked about here is a perception of complexity and “quality” of the build. Home builders can produce the same quality but would take years longer and may take short cuts. Wharram wants quality products out there, not sometimes questionable results.

Nice cats if this is your style, but if you’re going to build this big and spend this amount of money, save yourself a lot of building pain and buy a good second hand French glass cat. Sailing is way more fun than building. The jpegs give the idea.

 

The next Wharram that can only be obtained as a professional build is the Islander 65. The Islander 65 'Vaka Motu' was originally designed in 2000 as an island-hopping cargo/passenger/trading vessel. The Islander 65 is 65 x 31 foot with a weight of 22,400 lbs and a maximum displacement of 45,000 lbs. The two 45 foot timber masts carry a Wharram Wingsail Schooner Rig (Total area): 1238 square foot. There is an alternative Crab Claw rig available for those who want a cheaper more traditional option. The length to beam unloaded is 12 to 1, when loaded the length to beam is 11 to 1.. The draft is 3.9 foot when heavily loaded. Engine power is delivered from the deck of the 'Islander 65' via the 'Long Tail' system. When the boat is sailing, the propeller and shaft tilts out of the sea to avoid drag.

The 'Tiki' Soft Wing Sail schooner rig can be handled by two people in gale conditions, reefed with a following wind and gives good sail drive (if cut by a trained sail maker) 37-40° off the apparent wind. Wharram claims it will give easy 24 hours averages of 150 to 200 miles (more if driven in ideal sailing conditions). A realistic number and similar to the Pahi 63 averages. This is only 8 knots with peaks around 17 to 18 knots. Size does not always mean speed especially if you can carry a 22,000 lbs payload.

The Islander 65 again is a plywood/timber/epoxy build. The hulls are a bulkhead, frame, stringer structure with cold moulded skin. The crossbeams are mainly top and bottom timber flanges with a plywood web. The central pod can be from simple to a full cabin structure. The rudders are attached to the stern posts by Wharrams rope (Dyneema) attachment. Remember, this is a 65 foot cat, a lot simpler and cheaper than stainless steel tubes etc. This is a very big build, don’t try it at home.

The accommodation option varies from a cargo carrier with a limited accommodation to full charter cat option with 8 double berth cabins and a large central pod with galley, seating and navigation. The more useful choice for many is flexispace option which allows space or berths depending on the route and number of passengers.

This is a very large functional cat capable of doing global sailing, it is not intended as a racer but could be a floating “island” being used as a home. The jpegs give the idea.

 

Second entry of the Islander 65 catamaran. Cross beams, rudder, near finish and pod.

 

Goodness me! - thats the catamaran drawings I did 10+ years ago with a view to a retirement project! However, I did not build the boat - I decided that I would rather spend my time sailing and possibly building one or two much smaller boats. Also, even though the design was demountable to require less workshop space, I was not able to find sufficient building space for this catamaran. Instead we aquired a 10m trimaran second hand but actually have continued to do much of our boating with our 4.5m sailing dinghy and 4.5m row boat. Much to be said for little boats. Although a 10m catamaran is small compared to many.

Re. the FEA study, I did that using ANSIS. It was a very simplified model of the structure but was sufficient to show areas where stresses were likely to highest. The 'bow burying' situation was simulated by supporting the model near the bow of one hull, applying upward load at the opposite shroud chainplate and downward at the mast foot. I also considered support at one hull bow and opposite hull stern, so like if the boatyard makes a really bad job of proping up the boat ashore, a situation that any multihull should surely be strong enough to easily cope with. As you say, this study did show that some re-inforcement was needed around the large companionway hatch openings.

The boat was designed to be demountable into two hulls, cross beams, the bridgedeck and the forward central structure. This was to reduce space needed for building and to allow the possiblity of very occasional land transport. As you say, I devised a system of wedges to lock the hulls to the main cross beams but if assembly is only required the one time at first launch it would be little change to the design to make permanently bonded connections.

The lower hulls, up to the 'knuckle' above the waterline, were to be round bilge strip planked with a wide plywood plank on the centreline (so not flat bottomed dory type), everything else plywood. The strip planking would be glass/epoxy sheathed inside and out, all else externally glass epoxy sheathed and epoxy coated internally. Structure other than the strip planks would be mostly 9mm plywood, some 12mm plywood and with a few parts in doubled up plywood. 6mm plywood and thinner only in lightly loaded items such as internal furniture. The strip planked lower hulls would form a 'building jig' on which to assemble the upper structure. Externally protuding stringers attach to the upper edge of these lower hulls forming a knuckle above the waterline to which the plywood topsides attach. The outer topsides are well reinforced with internal stringers - I was thinking of fender loads which can be high. The inner topsides are currently drawn with lighter part length stringers, those should really be full length I think.

I chose plywood (other than the strip planking for the lower hull sections) as the quickest way to get the boat finished. I have done some boat building with plywood and some with foam and glass and although foam and glass produces a more durable and lighter result plywood and epoxy is far, far quicker, at least it is for me - some may disagree! The basic design could certainly be modified to be all foam and glass, or carbon, which could make a light and pretty fast catamaran. Doing this would require working through the drawings again though! A well built plywood boat will last longer than I will, and maybe longer than most people who read this forum!

If this project were to continue the next stage would be add just a few things that seem to be missing in the current model (e.g. mooring cleats!) then generate dxf files from the computer model, one file for each piece of plywood, also formers for strip planking. So several hundred .dxf files I think or even 1000+. There were about 150 dxf files for our 4.5m rowboat. But its just a matter of mouse clicking each surface on the model and assigning a file name. Then the .dxf files need to be 'nested' using nesting software, but first dividing up the parts that wont fit on a single plywood sheet and adding finger joints although scarfing would be an alternative - I think finger joints are easier. Then the files go away for waterjet cutting, (or cnc routing). When I built our 4.5m rowboat I had all the plywood cut out by waterjet and that was a huge timesaver. The building goes so much quicker if all the parts are accurately pre-cut and fit together with no significant need for adjustment. Also a big help if all the fastner holes for all the fittings are pre-cut in the plywood so you just bolt on the fittings without needing to do any marking out first. If all goes well it is like assembling flat pack furniture, but more of it. The basic assembly of our rowboat was a pleasant days work, the internal glass taping and external sheathing then painting did take a bit longer but it was still quick overal.

I designed the boat with pivoting centreboards and rudder blades, also deep rubbing strakes on the hull bottoms for grounding. I drew flaps hinged to the aft edge of each board to close off the slots when the boards are lowered. The lines to raise and lower the centreboards are lead up to the cockpit. There would be a pair of outboards on hinged lifting supports, probably 10hp each. Our trimaran has a daggerboard, I do find that a bit of a worry in shallow water!

I have not drawn bow nets, the thought being that the wide walkway to the forestay would be sufficient, if not, bow nets could be fitted but the anchoring arrangement would then need slight alteration.

The CAD software automatically calculates the weight of all parts of the boat and location of the CoG, provided you assign the materials and densities correctly. I used the weights from the software as the basis for calculating the total weight and including four crew members, rig, stores, fuel, water, etc, etc. the total was a shy under 300okg. The waterline as drawn matches that weight and CoG.

This design was taken to an a considerable level of detail - more than I have seen from professional designers. I guess I find CAD drawing rather adictive, it hss been a bit of a hobby of mine from the '80s, starting with making drawings on a pen plotter from textual coordinate input. I think I did go a bit too far with a few features, for example the design includes a built in system for lowering the mast with a 'gin pole' concealed in the forward structure - I was thinking of cruising in the Netherlands but its not really necessary. Without the folding mast feature (or perhaps a forward folding mast with the boom as a 'gin pole') the shelter over the wheel could be a rigid wheelhouse structure rather than a large folding sprayhood. Having said that, the folding spray hood would be lighter and nice to be able to fold it away in good weather or if the boat were to be raced. This catamaran is light for its size not having a bridgedeck cabin and the furniture that ususally goes with that. With good spars and sails it should be quite quick.

There is relativly little detail for the rig since this would be a fairly standard fractional sloop, all parts available 'off the shelf'.


I drew a plywood tender, although in reality I would probably choose an inflateable. But I did go on to refine the design of that plywood tender and the result won first prize in a boat design competition!

You say the .jpg s are limited - so here are a few more.

Happy to discuss with anyone thinking of building similar.

Thanks to OldMulti for this amazing thread - truely a multihull encyclopaedia.

 

A few more pictures ........

 

The final in the Islander professional build series from Wharram design and this one confuses me. The Islander 39 was specially designed as an eco charter catamaran for warm climates. When you see the accommodation and build you will understand my confusion. The Islander 39 is 39 x 20.4 foot with a yawl rig. The weight is 6600 lbs and the maximum displacement is 11,000 lbs. The main mast is 38 foot high and carries a 333 square foot wing mainsail and a 235 square foot genoa. The mizzen mast is 22 foot and carries a 106 square foot wingsail mizzen. Both masts are timber. The length to beam on the hulls is 11 to 1 with a laden draft of 3 foot. The single centrally placed 9.9hp 4-stroke outboard is to be used only in calms or to get into places where sailing is impossible.

Now to my confusion, the layout shows 4 double berth “cabins” which are actually partially open cockpits with a mattress on the floor and a canvas roof over part of the space for some protection. I hope this cat does not sail where it rains. The claim is it is a simple ecologically sensitive vessel where the crew adapts to the conditions, the boat does not provide an EG airconditioned existence for the crew. OK, I accept some of that statement but you either have a very big day sailor or a limited range camp cruiser. A Tiki 26 will provide more protected cruising cover than this design. The open sided deckpod has seating for the crew of 8 with a spacious galley in which several people can prepare food together. The cooker is screened from wind; roll-up side screens protect the crew from sun, wind or rain. The roof top is used for solar panels that will provide all the electrical needs, which are again encouraged to be kept simple.

Next confusion is the “professional build” status of this design. Why? This cat could be constructed in a back yard and would be actually easier to build than a Tiki 38. It may have slightly rounder hulls in strip-planking or double diagonal plywood and epoxy, but a good home builder would be able to construct this with ease. The rest of the build is pretty much the same as a Tiki 38. Finally on construction, a constant camber hull could easily be done for this cat concept which would make the hull build a lot faster, cheaper and simpler and may I suggest probably stronger. EG a Constant Camber CC 40 trimaran has a 12 mm thick hull with virtually no stringers, minimal bulkheads and has an 11,000 lbs displacement.

There is a potentially good cruiser design here if the hull accommodation was reorganised to a more practical solution. This design will probably sail at about the same speed as a Tiki 38 which is 6 to 8 knot averages with peaks of 15 knots. I do not know if any have been built. The limited jpegs give the idea, the last 2 jpegs are of a CC 40 tri which gives an idea of Constant Camber hull shape.

 

Fountaine-Pajot (now owned by Dufour) has released a new $US1.1 million cruising cat that had more than 150 sold off the plan before a cat was built. Hmm. The Aura 51 is 50.6 x 26.5 foot with a weight of 39,900 lbs. The aluminium (or optional carbon fibre) 64 foot mast carries a 1023 square foot mainsail and a 635 square foot furling genoa. There is optional EG code Zero and spinnakers. The hulls length to beam is about 9 to 1. The draft is 4.4 foot over the fixed low aspect ratio keels. The underwing clearance is 3 foot near the centre. The standard engines are 2 x 60 HP with a fuel economy of 15 litres/hour at 8 knots. The entire coachroof and flybridge are bordered by an area of solar panels – up to 2,000W.

This is a sports cruising cat. The accommodation options range from the Double Maestro Version; 2 Owners Suites + 2 Heads / 2 Double Cabins + 2 Heads. The Full Maestro Version;
1 Big Owner Suite + 1 Head / 3 Double Cabins + 3 Bathrooms. The Maestro 5 Cabins Version; 1 Owner Suite + 1 Head / 4 Double Cabins + 4 Heads. Or the 6 Cabins Version; 6 Double Cabins + 6 Heads. The main saloon has a large galley, large seating area, entertainment area but no formal navigation area. The electronic world has taken over and the entertainment area has a computer that provides most navigation functions. The attached cockpit has very large open doors to allow cockpit seating to be integrated with the main saloon. The majority of sailing controls and helming is done from the fly bridge. The winches are electric if ordered and will be required for a short handed crew.

This cat is luxury plus, but still can sail well. Test reports say it can do nearly 6 knots beating into 12 knots of wind. Even pinching to around 32 degrees apparent, it lost less than a knot of pace. Another report said. “On clearing the harbour, we were picked up by an ideal 11-16 knot northwest wind. The Aura 51 immediately demonstrated her excellent capabilities under full mainsail and genoa, settling at a speed of 8-9 knots with the wind on the beam. The Incidences D4 sail wardrobe provided a welcome effect. During a more pronounced gust, the GPS even indicated 10 knots. The sail plan adapts easily to sailing close to the wind, especially the overlapping genoa with its sheeting point set well inboard. At speeds below 5-6 knots, the relatively short skeg keels won’t eliminate all the leeway, but above a Force 3, you’re able to make a course a little sharper than 50° without stalling. Under gennaker, the Aura 51 was able to maintain an average speed of over 10 knots.” Translation, the Aura can sail at about 50% to 60% of wind speed with peaks of about 16 to 18 knots of boat speed.

The structure is has long-term design input of Berret-Racoupeau. The hulls are built in high-quality vinylester and fiberglass with foam and balsa cores for strength and rigidity. Smaller parts are injection molded (including the coachroof), and vacuum infusion is used everywhere else. In short, this is a pure production shell and bulkheads with some internal furniture variations in the hulls. This cat is 6000 lbs heavier than the previous Saba 50 model but has larger accommodation and with a similar rig, the Aura 51 is a faster cat according o those who have sailed on both boats.

The jpegs give the idea of an good large cruiser.

 

...at 67 my perception of aesthetics seems to have stayed in the late 70ies, when I look at cats as this...

 

I hate those things so much that I'll pull the anchor and move if one comes into the bay.
As Jimmy Buffet said about mobile homes: "They looked a lot better as beer cans". These things looked better as condominiums.