Multihull Structure Thoughts

The Casa Cat 20 is the Swiss army knife version of a trailable cruising catamaran. It has a full 6 foot headroom main cabin, it has the possibility of 2 double berths, it is trailable and relatively easy to build. The cat is 20.6 x 12.2 foot capable of being compressed to 6.5 foot for trailing. The special trailer aids in the expansion to the 12.2 foot width. The maximum displacement would be about 2400 lbs with a build weight of about 1200 lbs. The initial boat had a small biplane rig with what look like windsurfer masts. The hull length to beam is about 12 to 1. I suspect this cat is more of a motor sailor than a pure sailor judging by the size of the rig and the windage of the bridge deck cabin.

The Casa Cat is built from plywood (probably 6 mm) with timber framing and stringers, gunnels (probably about 25 x 19 mm) etc. The whole boat has west type epoxy. The exterior is covered with glass cloth and epoxy. Almost all panels in this boat can be built on a flat surface and bent to shape as required. The structure of the main bridge deck cabin box is very important as it acts as the connecting beam structure between the 2 hulls. There is an upper and lower metal corner connecting bracket on each corner of the central box that connect to corresponding metal brackets on the hull units. The main cabin roof structure is not as important but requires so careful thought for ease of rising and lowering.

The accommodation is very good for a 20 foot cat. There is a 3 foot wide single berth in each hull. Now, you may notice a square hole (with a covering panel in the sailing shots) in the outside topside of the hulls, these are foldout panels to allow the hull single berths to turn into a 4.5 foot wide doubles. The main bridge deck cabin has a seating table area, a small galley and a loo area and all with 6 foot headroom. These great accommodation features alone would definitely make this cat a bay or river cat not a deep sea boat. Windage in relation to its sail area and lack of keels would be the enemy of this cat.

The 20 foot version was an innovative design that had a follow up 23 foot version done with an on water expansion method using 200 mm aluminum tubes and electric motors. Neither was a great commercial success but the ideas were good.

The jpegs give the idea. It is design to get a few people out onto the a variety of quite waterways for a bit of fun and then be able to store the boat at home. Interesting concept.

 

This wee house cat does look very neat indeed.
Some info about her on the Duckworks site -
Duckworks - Casa Cat https://www.duckworksmagazine.com/09/designs/casacat/index.htm

Here they mention the home page www.casacat.com but when I click on it I just get a message saying that this domain name is for sale.
I hope that the Casa Cat folk have not gone bust.

 

Bajansailor. Yes, Casacat disappeared several years ago but an interesting site about the Casa Cat 20 is Building a trailer cat https://buildacat.com/cc1.html

 

At the end is a gift but a deviation into small monohulls to show how we got to the gift. Michael Storer is a very good small boat designer who works mainly in plywood. The small boats range from 8 foot to some very good 15 foot sailing dinghies (Goat Island skiff most famous) and up to 30 foot power boats. What piqued my interest is the 8 foot Oz racer which started out as a PDRacer. PDRacers are 8 x 4 foot built of any material which has a tight set of rules controlling the shape of the bottom of the boat but you then have a lot of options of how you “improve” performance. You can use any rig or foils you like. “Performance” of a standard PDRacer is about 5 knots in normal racing.

So, Michael decided to try and improve the performance. Result is the addition of a lateral DSS foil, very deep centreboard and large outboard rudder. Michael claims “Hydrofoil sailing is possible for homebuilt boats. It can be used to reduce heeling so the crew doesn’t have to work so hard.” Interesting approach to gaining performance. No idea if the boat was any faster but I applaud anyone who is willing to experiment on the cheap to understand what is possible.

Michael also designs some boats that can be modified to a trimaran or tacking proa. The Quick Canoe (15.5 x 2.75 foot) can be converted to a trimaran by the addition of floats and simple crossbeams which improved the performance. Also notice the outboard for comfortable cruising. The plans for both the canoe and floats are available from Storer or Duckworks websites. The attached jpeg gives the idea.

Now the gift. Michael Storer has a link to Hullform 9p software. Hullform is a simple software package that you can design hulls on. You can start a new hull shape, fair a hull and get a table of offsets for either a chine hull or a round bilge shape. You can also get full hydrostatics such as displacement, wetted surface, prismatic coefficient, center of buoyancy etc. Plus it also has a drag prediction feature to allow you to optimise the hull shape. This program is excellent but the original software writer stopped updating before Windows 10 was produced. Older versions of the software had difficulty running on Widows 10. This version has been modified to run on Windows 10 as the original writer left the source code available for others to update.

Hullforms takes a bit of time to use well so please download the Hullform software and manual listed on the following web site: https://www.storerboatplans.com/boat-design/hullform-9p-now-working-with-windows-10-download-links/

If you want a direct link to the Hullform software try: Download Hullform 9.12 https://www.softpedia.com/get/Science-CAD/Hullform.shtml#download

 

Michael Storer also has a great page on using Paulownia or Kiri, it includes a risk assessment of using lighter woods than specified and formula you could use with any species of timber to asses the suitability of a substitution. A great tool for wooden boat builders.
https://www.storerboatplans.com/faq...e-newest-lightweight-timber-for-boatbuilding/

 

This is a Tim Mann designed Pacific Proa that was conceived mid 2019. The Proa is the Splash class called Taumako and is 38 x 22.6 foot overall but the hull centrelines are only 13.3 foot apart. The weight is 4,240 lbs with maximum displacement of 6240 lbs but this proa also has a 3300lbs capacity refrigerated fish hold which indicates its maximum overload displacement of 7800 lbs. The 33 foot mast carries a 160 square foot mainsail, 120 square foot roller furling jibs fore and aft, 200 square foot drifter that hanks on to a shroud fore and aft. Pacific Proa’s staying is reasonable in normal conditions but if your Pacific Proa is caught aback in strong wind conditions your rigs staying may not hold the rig up. Proa skippers need to be good sailors and seaman to sail pacific proa’s over a long term.

The mainhull length to beam is 11.7 to 1. On the outside of the main hull is the fore and aft “centreboard” rudders. These combined with a low aspect ratio keel on the main hull. The float has water ballast for additional stability. Even though the proa has a relatively small rig, the hull centre lines are close and the additional water ballast will assist the stability and sail carrying power of the Proa.

The 38-foot long Splash class is based on a traditional Polynesian canoe design but uses all modern components such as epoxy, fiberglass, marine plywood, timber and dacron ropes and sails for strength, safety, and longevity.

Tim Mann says “Splash is designed for safely and profitably fishing with a crew of three, within 75 miles or 5 hours return travel of a port of refuge in unpredictable weather, and within 500 miles or 48 hours travel of a port of refuge during good weather with dependable forecasts. Although she can easily and safely travel longer distances through very bad weather when lightly loaded, the fisherman hopes she is never “lightly loaded” when it’s time to go home”.

Splash was designed for Taumako, a small island in the Santa Cruz islands group, and is home to 592 souls. The islanders suffer from the usual political crap that prevents them sailing their 500 hundred year old Proa designs from island to island without a power boat escort. They are having this design built to act as a “transport taxi” and fishing vessel to gain some income and food stock for the islanders.

This is a design that could be converted into an interesting cruiser for the adventurous GOOD sailor. The jpegs give the design. The PDF’s are a repeat of the jpegs. The web page is The Taumako Boat - Ocean People https://oceanpeople.org/taumako/

 

The following jpegs tell you why designing and building big multihulls is not a small task. The first jpeg is a part of the aluminum hull structure of the 140 plus foot Hemisphere catamaran. There are 36 flat bar stringers in each hull with a T section frame at every 3 foot (about 47 per hull), that’s 166 items to be cut, welded together from many smaller parts, finished and all you end up with is an unskinned shape of 2 hulls. The deck framing is even more complex, as are the hull and crossbeam bulkheads. The design work alone to do accurate drawings of all components that integrate the total structure together is in the thousands of hours on a design this big, even using advanced CadCam software. Also, the separate hydraulics, electrical, plumbing, tankage, deck gear, rig, anchoring etc design and drawing work is massive.

In larger multihulls no single person or firm can do everything. For building you have to subcontract out jobs or components to be built by others and then bring the components together. You either have to be a really good project manager or be capable of adjusting the construction process and design along the way to compensate for any design or build flaws that may occur. So, what do I call big? Try about anything above 60 foot. I know “home builders” who have built below 60 footers well, in a reasonable time, but once you get above 60 foot things become just to big and time consuming.

One of the really painful aspects of building large is, if you are a home builder it takes time, years of time and unless you are made of money you buy things along the way. The problem is you build EG a deck structure with a hatch surround structure you choose from a catalogue and then go and buy the hatch 2 years later. Occasionally you hear “sorry we don’t manufacture that size any more”. Time to rebuild the hatch surround.

The next problem is if you plan EG your deck gear after you have completed the shell, often you have to put reinforcing addons to the shell structure to support the deck gear. One boat I sailed on had ongoing electrical problems at night, which no one could solve, until they found a deck gear bolt hole had been drilled grazing a wire to a navigation light, shorting occurred at night. Planning a complete build solution takes time and the bigger the boat the more time it takes.

I have focused on aluminum boats at the start of these comments, but building a plywood timber multi takes just as many components and as much additional electrics, plumbing, rig etc as an aluminum multi. Only foam glass boats become a little easier to build in larger sizes due to things like flat panels, resin infusion, vacuum bagging etc. The real trick to building a multihull is to have as few components to build as possible. If a single mould can be used to create the hull bottoms on a cat or one mould creates all the cabinets in a boat the faster the build will be. But foam glass boats require more preplanning as you need to understand where you are going to put EG timber inserts into the foam to act as a hard point for deck gear etc. again you have the same electrical, plumbing, rig etc design and build issues.

Build small and simple with as few “luxuries” as possible and you be on the water faster with less design and planning work. The jpegs give the idea of the complexities of aluminium multihulls. PS I like aluminum as a material for multi’s above 60 foot. Have fun.

 

The following is one mans dream that he spent 3 years conceiving, designing, detailing and then passed the concept design to Jonas Panacek, a German naval architect, to verify the calculations, structure and CADCAM the design. The vessel is a Wharram type truncated V hull shape cat design of 50.8 x 26.3 foot that weight about 20,000lbs and displaces 28,000 lbs. The sail area is unknown but the jpeg indicates a cross between a Gunter and lateen mainsail and a conventional jib on a relatively short mast. The hull length to beam ratio is 11 to 1 at the waterline on a 41.5 foot waterline with a waterline beam of 3.75 foot although the hull flairs out to 8 foot at the gunnel level.

The design is intended to be a cruiser that can make money fishing if required. The cat has a 3000 lbs fish holding tank in each hull. There is a deck house on one hull leaving the main bridgedeck area clear for fishing and sail handling duties

The main interest in the design is its structure which is aluminum plate in marine grade (5086 or 5083). The keel plate is 20 mm thick, hull sides to the waterline are 6mm, from the waterline to the gunnel the hull sides are 5 mm plate. The decks are also 5 mm. The stringers and deck beams are 45 x 6 mm flat bar supported by 6 mm ring frames at 700 mm centre lines. The main cross beams are 380 mm high by 280 mm wide box beams built from 6 mm plate. The beams have a slight curve in them which made bending a more expensive mast section for a beam impractical. This is a relatively strong structure. EG Mumby who builds aluminum cats in Australia uses 4 mm aluminum plating on his 48 foot cat (last 4 jpegs).

This is the usual trade off. Lighter skins require a bit more framing or closer stringer lines. The opposite approach is to use the Strongall build technique of EG 10 or 12 mm thick aluminum skins with minimal framing. Really thick skins make the cat nearly bulletproof but the weight gain is substantial. EG a 40 foot Strongall cat displaces 27,000 lbs.

The final point about aluminum multihulls is electrolysis. You really need to understand how to handle electrical flows between different metals. Understand how to isolate differing metal components and what sacrificial components are required. There are thousands of long lasting aluminum boats around but care needs to be taken.

This boat was to be built in New Zealand several years ago, but I do not know if it was done. The CADCAM was done to allow the aluminum components of the cat to be precut to allow rapid assembly. The jpegs give the idea of what was intended.

 

There was an aluminium boat in the 40ft fleet when I sailed at Fremantle, we used to call it the Aspirin.

 

The Broadblue 346 is a family cruising catamaran that will fit into the French canals and “standard” size marina berths. The 346 is 33.5 x 15.9 foot and weighs 10750 lbs. The 41 foot fixed Selden aluminum mast is mounted on the main cabin top, carries a fractional rig with a 300 square foot mainsail, a 205 square foot self tacking jib on a Harken furler, a 485 square foot gennaker and a 1050 square foot spinnaker. The fully battened main has roller bearing cars and 3 reefs (2 x single line). The rig has polished stainless steel chain plates and 1 x 19 SS standing rigging. The cat has low aspect ratio keels. The length to beam on the hulls are approximately 10 to 1.

The builder is Broadblue and the designer is Darren Newton. The cat was originally designed as the Voyager 10 in 2008 then rebranded the Broadblue 345. In 2018 she had an all new deck mould and tweaks to the interior layout. The cat is ocean capable with a European Category A classification. The accommodation includes two large double aft cabins with king size berths and a single cabin forward as standard. There is a loo forward in the other hull. The main cabin has the galley, chart area and dinette all with 6.5 foot headroom. There is a hot and cold pressurised water system throughout. The cockpit is roomy and there is a “sail station” concept that brings all of the working sheets and halyards to a single forward facing location alongside the helm station. The 346 can be handled by one person, including reefing of both the main and headsails.

The hull and deck are vacuum bagged, foam core, with woven matt construction with solid GRP below the waterline. The deck has an integrally moulded non-slip finish. The hull-deck join is fully bonded and protected. All of the structural bulkheads are of vacuum bagged foam core construction. The Broadblue 346 is virtually unsinkable due to its water tight bulkheads and buoyancy compartments fore and aft. Rudders and steering are formed around stainless steel stocks with self lubricating bearings. The steering system is hydraulic, operated from a wheel at the helm position. The rudders are linked via a tie bar. There is an access cover designed to accept an emergency tiller.

The 346 has been tested in 15 to 25 knots of wind. With a single reef in the mainsail. They were soon moving at 8.5 to 9.5 knots. The 346 has hydraulic steering that has more feel than on most cats that the tester has handled. The 346 does not point or tack as well as a deep keel cruising monohull but performs both tasks competently according to the tester.

This is a size of cat that is of interest to many and gives a guide to a possible style of cat that would be a good cruiser with some performance capability. The jpegs give the idea.

 

This is about the wing sail on the boat not the boat. The Skye Wing was conceived as a result of frustration with the limitations of the Bermudan rig. The Skye Wing rig has a higher lift/drag ratio when going to windward with higher pointing than a standard Bermudan Rig. The rig provides reduced sheeting loads and more effective control of sail shape/power on all points. The Skye Wing rig has easier reefing and would be easy for single handing. The rig has an ability to instantly de-power, even when running with gentler gybing. Other advantages are minimal sail chafe when broad reaching/running and less running rigging and deck equipment to purchase/maintain. Also, there are fewer components so less chance of rig failure. The bottom line is the Skye Wing rig is simple a reefable wing sail rig.

The Skye Wing rig is a fully-battened, double-sided sail that wraps around the mast forming a smooth aerofoil with a closed luff and doubled at the foot, head and leech of the sail. A halyard is reeved through a masthead fitting that is free to rotate along with the top yard and sail. The sail cloth has 7 “conventional” battens per side. There are also 3 reefing points in the sail. There are camber lines, which give independent control of bend on the port and starboard sail battens to help maintain the desired shape and sail twist.

The Skye Wing rig comprises of a 225 mm diameter by 40 foot long free-standing unstayed carbon fibre mast. The bifurcated (aerofoil shaped) boom has Douglas fir wood side panels with oak bracing which wraps around the mast extending forward of the mast. The boom is free to rotate around the mast on a bushed sleeve. A high-peaked yard of similar construction to the boom, also free to rotate around the mast. The Boom and Yard are solid structures that freely rotate around the mast and can be raised or lowered as required.

Spark (the monohull the rig was tried on) was launched in April 2004 and the performance of the rig was described as encouraging. The owner said “The sail set beautifully and the camber line/sheeting arrangement successfully shifted the camber from side to side when tacking. When the sheets were set up on a beat or reach the rig was self tacking - just put the helm down and over it went. In light airs a couple of tugs on the sheets helped to shift the camber across. Running before the wind was a piece of cake, thanks to the fully-battened sail and absence of standing rigging. Steering a course dead downwind was very easy and stress free. Gybing was a joy. Haul in on the leeward sheet while turning and then let it fly as the wind caught the back of the sail was the trick. The boom (which cleared the forward end of the cockpit, swept over the coachroof in a flash and then came gently to rest on the other side.”

The wing set well in light airs providing good lift but tended to twist too much when the wind strengthened, losing power in the upper part of the wing. A method of controlling the twist using a line from masthead was proposed. Hoisting the sail and yard requires some effort but if the yard was made of eg Carbon Fibre it would be easier.

The fully-battened sail ensured there was no flogging on hoisting or lowering, and the lazyjacks allowed the sail to drop nicely onto the top of the boom. The reefing system worked well, allowing a fair set of the wing although inhibiting the control of the camber, which was not a serious impediment in the stronger winds when reefed.

The limited jpegs give the idea. I do not know if the sail has been deployed else where but it is a good concept that could be used on multihulls.

 

The Skye Wing appears to be similar to the Gallant rig which was originally developed in the 1980's by Jack Manners Spenvcer in England - they are now being made by a company called Autosails Ltd.
Home http://autosails.co.uk/index.html

http://www.voiles-alternatives.com/documents/ailes_souples/gallant/gallant_sailing_word_1991.pdf

 

Bajansailor. Agreed it looks similar to the Gallant rig, but the Gallant rig has full fixed "wing structure" batten arrangement at each batten level. Result Gallant rigs has a more controlled Aerofoil but is also heavier. The Skye Wing sail only has the yard and boom as a fixed structure. The battens in between are "normal" sail battens. Think of the Skye rig as 2 battened lug sails joined together at the yard and boom. The overall structure is easier to make, is lighter and allows some twist in the rig. But its real advantage over the Gallant rig is the aerofoil can be asymmetric (more wing shape) due to the standard battens allowing the rig to generate more power. The Gallant rig is symmetrical due to the solid battens. Gallant rigs have tried improvements to improve lift but eventually the "Swing wing" rig was the ultimate development of the Gallant rig path.

 

I do not know much about this design project beyond what is on the designer’s web site but it indicates the thinking that is being done by clients. The trimaran is 164 x 100 foot of my estimated 160 tons (360,000 lbs) displacement. The sail area is unknown but is a ketch/schooner with mostly self tacking sails. The kick up rudders and centreboards in each float will be spectacular engineered structures, considering the forces that will be involved in sailing this vessel in heavy weather. A large wave will still be able to throw this tri sideways and with 160 tons plus of momentum being resisted by a deep centre board in a float there is a lot of twisting moments applied to the centre board case, float and crossarms.

Its fortunate the tri’s concept was conceived and developed at the Kiel University of Applied Sciences can assist in the design and who have already run wind tunnel tests on the tri designs rig and exterior.

The large tri will be an all carbon design with the construction. The details are being worked out at the SMG shipyard in Rostock. Carbon filament winding technology should be used as far as possible. Filament winding allows eg a float shaped mandrel can be created and a continuous carbon fibre filament is wound on from one end of the float to the other. Multiple layers are built up to the thickness required. Then EG a foam layer is added, then additional filament layers are wound on. The advantage of this technology is you get continuous homogeneous skins with no joining layers required and if set up correctly it can be performed mainly by machines which can lay up prewetted carbon fibre as a continuous run. The overall result is a lighter, stronger and faster built structure. The floats, main hull, cross beam and masts booms can all be built this way. And if you are very good, the foils can be done the same way.

Jonas Panacek, the German naval architect is responsible for the general plan, i.e. the layout and construction, and help with the exterior design. In addition, we are responsible for the conformity and the safety requirements. The ship is to be given full Germanischer Lloyd class certification.

An interesting concept that I hope will be built so we can all learn something about large multihulls. The jpegs give the idea of the tri and the final 3 jpegs are of filament winding.

 

On page 73 number 1086 of this thread is the first part of a story about the capsize of Flicka. Jan Gougeon’s self-designed and built 31′ trimaran FLICKA was capsized by heavy seas in the North Atlantic whilst sailing in a qualifying race for the OSTAR (Original Single-Handed Transatlantic Race) in 1979. The story can be found at Surviving Flicka’s Capsize https://www.epoxyworks.com/index.php/surviving-flickas-capsize/

The second instalment of the capsize story of Flicka’s capsize can be found here Surviving FLICKA’s Capsize, Part 2 https://www.epoxyworks.com/index.php/surviving-flickas-capsize-part-2/#more-10163

This story is very educational about what caused the capsize what worked and what did not. The reality is it appeared to be a wave induced capsize and Jan survived. After that experience Jan had a conviction that all multihulls should be self-rescuing and designed several multihulls that could be self-rescuing from a 25 foot tri Splinter to the 40 foot Strings catamaran. Please read the Epoxyworks magazine article as education is very important to all who sail away from the shoreline.

The jpegs are of Flicka then of Splinter and Ollie 2 self rescuing tris. Strings the self rescuing catamaran is the last jpeg.