Multihull Structure Thoughts

Stealth cats started with a boat built in Australia. It was known as “Cut Snake” but its full name is “Mad as a Cut Snake”. The design started with Robin Chamberlain 37.5 foot “Excess” hull with a modified bow and stern. Alan Carwadine then got advice from Jon Sayer and Brendan Egan to complete the design. Each of these names are good designers and believe in light weight. Result, Cut Snake was 39.5 x 21.6 foot and weighs 6720 lbs with a displacement with 5 crew loaded for racing of 10,700 lbs according to the QMYC OMR rating. The small sectioned fixed aluminium mast is 56 foot high of the deck with wide double spreaders. The total mast, boom, rigging weight 290 lbs. Alan was associated with a mast making company and choose this option for performance. Most sails are carbon with a mainsail of 660 square foot, a 365 square foot foretriangle, a 397 square foot genoa, a 700 square foot screecher and a 1667 square foot spinnaker. The heavy headsail furls but the 105% lighter headsail does not furl. The daggerboards draw 7 foot. Underwing clearance is 2.5 foot. I do not know the length to beam but I am assuming at least 13 to 1.

The performance of this cat is very good. Its OMR rating is .901. According to one test report the cat exceeds wind speed often and fly’s a hull in 15 knots of true wind speed. Cut Snake won the OMR division of the 310 mile light wind 2013 Brisbane to Gladstone in 36 hours, a Seacart 30 did it in 34 hours.

The accommodation has a double berth single berth and a loo in each hull. The main cabin has the galley and seating. This boat is not lavishly fitted out, it is practical and minimal to save weight. Stealth cats are about useful performance not luxury. EG There is a window at the front of the main cabin that opens to allow crew to access the forward deck area.

So how do you build a 40 foot cat that when weighed comes in at 6700 lbs. Very carefully. All PVC foam glass panels were vacuum bagged and full sanded/finished prior to assembly. Peel ply is a way of life here. Any joint lines have rebates in the edges so joining glass are in recesses. The only fairing compound is to smooth the recessed rebate joins. When the entire structure is finished it is painted. No gelcoats as they are too heavy. The V shape forward bulkheads are at 24 degrees so the mast is in the centre and the bulkheads have the Cap shrouds attached at the other end to keep the rigging loads contained. All fabrics are non woven with unidirectional and biaxials in epoxy resin. Carbon fibre is used in cross beam structures, rudder, boards, fore beam, prodder, boom and spreaders. ATL featerlight is used for non structural furniture. The production version of Stealth cats are epoxy resin infused then painted and have carbon fibre mast options.

The latest Stealth 12S specifications are slightly different to Cut Snake but have improved build processes and still maintain the same configuration and 6720 lbs weight. The Cut Snake hull moulds is used for all Stealth cats from 11.8 to 14 meters with some inserts.

The Stealth cats are well designed and engineered to provide a very high performance cruising catamarans and 22 have been produced so far from Asia Catamarans. They build 3 to 4 a year on a semi production basis. Alan Carwadine has just had an all carbon 12 meter version of the Stealth 12S delivered for cruising. Jpegs give the idea.

 

10kW is 13.4 HP. which is not a lot, enough to keep a large catamaran going at 5 or 6 knots say. 10kW per hour with 10kwh means your batteries will last one hour at this rate. You may want more than this if you are been blown onshore in high winds and waves. What are we up to now, 12,000 euros, and that is before inverters, cables, switches, electronics etc. You may want sails for longer passages, travelling at night, sails will shade out the solar cells reducing energy. Alternatively you could put a 25hp outboard in a well, run it at 11 litres per hour at full throttle, but say half throttle for more economy your using 5 litres an hour, say 5 USD an hour, and less where fuel prices are cheaper. In the future solar cells will get cheaper, batteries will get cheaper and lighter, higher energy density. At the moment is difficult. It can work for inshore very light catamarans, that use very little energy

 

Reading in between the lines, is there an underlying assumption that "all electric" isn't feasible for a cruising boat on a relative budget?

"Sailing Uma" of YouTube fame has had two semi-homegrown electric motor setups... first with lead acid batteries and a forklift motor, now lithium ion batteries tied to a OceanVolt saildrive... (their second setup has something like 5-10x the range of their first system)

In both cases it sounds like they did not ante up for all the pieces of redundant systems that cost tens of thousands for an all electric system, they went with just the essentials... they have a 36 foot Pearson, and have replaced just about everything, dumping the "always a power hog" equipment as they went.

For what hasn't worked... They relative recently TRIED to get regeneration out of the saildrive to power up the batteries as they sail, but they found they don't get enough power for net input with the size of boat and propeller they have, so they're still limited to just solar.

What has worked... if they manage their loads carefully, and plan carefully so they need to motor only minimally... with this approach maybe a couple times over a few years they did cut it pretty close, and had to go into a semi-lockdown mode - to squeak through the need to motor more than they planned, with less sun than they hoped for. But they've never been totally screwed.

They've said several times they're glad they have this setup, because it forces them to be more planful sailors, and more skilled sailors, to manage with almost entirely sail power. 22 countries, 18,358 nautical miles sailed, 0 gallons of diesel used.

 

A short item about utility over looks, the 42 x 16 foot Trawler Catamaran launched by Brix Marine and designed by Juston power cats. Recently launched in Port Townsend and soon on her way to her home port in Alaska. Powered by twin John Deere 180 hp diesels she has a cruising speed of 12 knots with a fuel burn of 4 liters/NM. Top speed is 16 knots. Built in aluminum. Do not know much more about the vessel. Looks very seaworthy and a good cruiser. Web site: CatamaranDesign.ca http://powercatamaran.ca/blog/

A little addition. Scott Juston knows about power cats and aluminum because of his time in Australia where he did work with Austal, the big cat ferry company. When I spoke to him he educated me about pressure distributions on a hull and how in some cases you can lengthen and add weight to a boat and end up with a faster boat without changing the engines at all. It is due to the pressure distributions on a hull being more balanced causing less drag and turbulence. Tank tests help here but some designers have a better understanding than others. Scott also designed monohull and multihull pleasure craft whilst in Australia before he moved back to Canada. His largest sailing cat was a foam glass open bridge deck 55 footer.

 

Cut Snake was the original Stealth cat built in Australia. The Asia Catamaran Stealth 11.8 is the equivalent “production boat” built in Thailand. The 11.8 is 38.7 x 21.15 foot that has an OMR weight certificate of 6,600 lbs ready to sail prior to crew and liquids. The 52 foot aluminium or carbon fibre wing mast carries a fractional sloop rig with a 600 square foot mainsail and a 272 square foot foretriangle. You can add genoa’s, screechers and spinnakers up to 1600 square foot. The daggerboards draw 6.5 foot and has stern hung daggerboard rudders (spade rudders optional). Again, the length to beam on the hulls is about 13 to 1.

The owner of a charter 11.8 “Java” claims “Since the launch of the Stealth’s in 2013 “Java” or her sister yacht have won every regatta they have entered. So far, every regatta she has entered she has ended up on the final podium placings. Even at speeds of over 20 knots Java is a very stable platform to watch the rest of the fleet behind you. The only danger to this boat are her sisters cats”.

The accommodation layout is the same as Cut Snakes and Java is powered by two 20 HP outboards that run on tracks so they can be stored onboard under the rear stern steps. In one of the jpegs you will see the forward steps in the main cabin to allow access to the forward deck areas.

The 11.8 is 2,000 lbs lighter than Cut Snake probably due to better build processes and refinements like carbon fibre masts etc. Moulds were made for all curved panels including the large cabin and cockpit roof structure and an all new hull and deck mould. All flat panels are made on the large glass vacuum table. The techniques used result in light and strong epoxy, foam cored structures with very little fairing needed. Production Stealth cats are epoxy resin infused then painted and have carbon fibre mast options. Carbon fibre is used in cross beam structures, rudder, boards, fore beam, prodder, boom and spreaders. ATL featerlight is used for non-structural furniture. As I mention yesterday stealth number 20, a 12.2 metre “Coconuts” was built for Alan and June Carwardine and will be cruised through Asia competing in as many regattas as possible. “Coconuts” is 80% carbon PVC in its structural build.

The deck gear is minimal to save weight. Two halyard winches and two sheet winches for the boat. Another example is on “Java” there is only 2 lightweight anchors with a minimum of chain and rode.

The jpegs give the idea of an excellent “production” cat for those who want a very high performance “cruiser”.

 

OldMulti,

Thanks for the write-up on Asia Cats! As my experience is decidedly more European with designs, would you mind terribly answering the following questions?

1. The mast bulkhead creates a chevron (spreading aft) with two smaller bulkheads (spreading forward) ahead of this, thus creating a modified 'X' pattern. Seems very stable. Is there anyone else doing this in their designs? And what might you say about this geometry good/bad?
2. In an interview, Alan talks about how they lift up the entire outboard and place a "plug" in the hull. Seems perhaps they are using a small moon-pool to prevent water from flooding into the hulls as they lift and rotate the outboards into the storage position. Is this now commonplace in Oceania Regional designs?
I dare say that if this was more well known that many North American/Caribbean Cruisers might opt for this over fixed diesels.... If you look closely at the youtubber videos of previous Stealth cats for resale. The hulls have an uninterrupted flow across the bottom. Rudders up & removed, outboards up & stored with no visible penetration point (in the video shown) and the dagger boards up (with almost no) instream penetration: no wonder they are winning the regattas!

Cheers - Ogre

 

Coastal Ogre. The x pattern has been used on several multihulls from 20 to 50 foot. Some have a pure X pattern on open bridgedeck cats (a 50 footer by John Hitch, a 38 and 30 footer by robin Chamberlin) and other semi bridgedeck and bridgedeck cats have the variations of the same approach. The main point is the design of the load distribution. Some designers put more emphasis on the straight cross beams carrying the torsional loads with minimal extra strength in the "chevron" beams others use the chevron as the main strength beams which requires some fancy design work. If the deck structure is designed correctly you do not need chevron beams. Again it just needs good design. Large X beam tris beams are done that way to resist torsional loads as well as resist the cap shroud loads imposed by the rig. The large X tris have very substantial travellers out the back for a reason and its not just to carry mainsail loads they also help resist torsional loads between the hulls.

As for the outboards and foils. The outboard arrangement on the stealth are very good and come in 3 versions. Cut Snake had a straight lift off the rear beam, Java have outboards that sit on the stern then have tracks that allow the outboard to liftup and roll forward into lockers seyt up under the aft steps. the final option is an outboard well that has a plate on the bottom of the outboard that seals the outboard slot when it is lifted. This is common in Australia. Look at plans for the Buccaneer 33 earlier in this thread you will see a sheet for the construction details of that option. The lifting daggerboard rudders on Java are very common in Australia. The daggerboard keels are common and the preferred option for anyone who wants some performance in their multihull.

Stealth cats are good but a similar type of cat design is available from Schoinning, Grainger et al and they can direct you to very competent builders. French cats do not represent the world view.

 

OldMulti,

Thank you for your response.

- Ogre

 

This is the second item in multihull miss steps. There is a discussion going on about the scantlings for a steel trimaran of 50 foot with many views on its viability. I know of an open wing catamaran and trimaran that had hulls built in ferrocement. I can point you to a 36 foot steel bridgedeck design that was built and is sailing. What I am indicating if you are prepared to accept some design limitations you can build a multihull out of many materials BUT there are limitations.

If you are going to build a boat in a heavy material such as steel, your shell weight in a 40 foot hull is going to be 3 mm with framing and weigh about 5.5 lbs/square foot. A ferro cement hull (if done very well) could be 12 mm weighing 7.5 lbs/square foot. A 9 mm solid glass hull would weigh 3 lbs/square foot. A foam glass shell could weigh 1.2 lbs/square foot. A 40 foot cat hull shell would be about 600 square foot so the result is one hull in ferro would weigh 4,500 lbs, in steel 3,300 lbs, in solid glass 1,800 lbs and in foam glass 720 lbs.

This is a simple generalisation for just 1 hull no decks. Yes, you can design a cat hull that will support any of these materials but the ferro hull will have a length to beam of EG 8 to 1, the foam glass hull will have a length to beam of 13 to 1, due to the hull materials alone, irrespective of the deck and crossbeam structure materials.

In short, the performance of a EG cat built in heavier “cheaper” materials will be slower. To improve the performance of heavier cats you need to put on taller masts, bigger sails, bigger deck gear and larger engines. Also finishing ferrocement and steel shell builds requires very good preparations and paint finishes. As one professional fisherman said it was cheaper to install an aluminium deck house on a steel hull due to the reduced finishing and maintenance cost compared to having the deckhouse built in steel.

So, what has been built and has travelled some distance. There is a 45 foot wharram cat with steel hulls and ply timber everything else. The mainly steel cat shells include a 40 foot bridgedeck charter cat in Queensland named Tongarra page 76 of this thread, Catrina a 36 foot steel bridgedeck design by Boden boat plans on page 2 of this thread. Llianase 80 foot steel cat page 80 and 81 of this thread. Each of these boats are best described as cruisers that can sail but don’t expect above about 12 knots. It is possible to build a lighter steel cat using thinner steel on the shell but it will require more framing and excellent welders to avoid surface distortions during welding. Also, you maintenance regime better be very good or the thinner materials will rust in a few years. PS a good designer would also be required.

Len Hedges designed a 40 foot open bridgedeck ferro cement catamaran that was built in Melbourne and another man built a 48 foot trimaran with a ferro cement central hull and small floats that made the 2000 miles trip to Queensland from Melbourne. These 2 boats could sail adequately but were slow in light to moderate conditions and had upwind difficulty according to the tri owner. Both boats done cheaply in the 70’s. There hulls were 18 mm thick ferro with ply decks etc.

Summary. I strongly advise against building a multi less than 50 foot in steel or ferro. It may appear to be a cheap build but I can assure you will be paying for bigger rigs, engines etc and the cost of insulation materials, finishing materials and very reduced resale value will cost you a lot of money.

It is cheaper to build in a light material like foam glass, strip plank, aluminum or solid glass up front. But the cheapest approach is to buy a good second-hand multihull that will only require a minimal maintenance upgrade.

The jpegs give an idea of what is possible. Don’t be to inspired.

PS the final jpeg is of the designer and builder of the steel 40 foot Tongarra catamaran followed up with a trimaran build that was recently launched in Australia. Built of balsa wood and fibreglass, the tri is 14.5metres long, with a 5.8metre beam and 0.6metre draft. Its futuristic-looking shape is all about "function over form", according to Joe, who is a mechanical design engineer. The tri can do 15 knots under power.

 

I love watching them, but they have a truly unique lifestyle. And have had to rely on shore power a tremendous amount in northern climates where they have been recently. I love that they are pulling it off, but if you do a job that isn't you tube and you value a bit more independence in climates with less sun its not really tenable. Most, and when I say most I mean 99% of my stomping grounds does not have the infrastructure they relied on all winter, nor the environmental capacity to regen in my climate without fossil fuels. Kudos for them for doing it, but its far from widespread scalable.

 

Based on that design, looks like a mashup of two well known and quite old designs. But structure, and even prototype wise, could be very interesting.

 

The Explocat 52 is built by Gracia yachts. It is a blue water bridge deck aluminium sailing cruising catamaran that claims it can travel anywhere. More on that later. The Explocat 52 is 53.5 x 26.9 foot that weighs 42,000 lbs. The 74 foot mast carries 1800 square foot upwind and a 1990 square foot Gennaker. The low aspect ratio keels draw 4.9 foot. The length to beam of the hulls are about 10 to 1. The bridge deck clearance is 2.8 foot. It has two 60 HP diesel engines and 1200 litres of fuel. Water tanks contain 700 litres.

The 3 or 4 cabin accommodation layouts are shown below and have all the features that you would require.

So why the interest? The following quote from Yachting World Magazine “By contrast, the thinnest plating of the Explocat 52 is 5mm, which increases through 8, 10 and 12mm thicknesses, before reaching an enormously reassuring 14mm at the bottom of the hulls. The boat has framing of up to 14mm and is structurally engineered to eliminate flexing between the hulls. A substantial keel with a long chord length is welded to the bottom of the hulls. They are marginally deeper than the rudders, which offers some protection, as well as providing a firm base on which to dry out on a beach. At the same time the key elements that have made Garcia’s Exploration monohulls so successful are incorporated.”

This is a seriously strong catamaran. There are 80 foot cats with thinner bottoms, but more on that tomorrow. What we have is a well built cat that could even go into Artic regions for a cruise. But if you do go to the Artic you will appreciate the 76 mm high density foam insulation throughout the living areas to provide excellent thermal and acoustic insulation. Think about that the majority of the boat has had insulation added to it after the build. That alone is a very time consuming task, as after the insulation it will require non-structural lining materials, for looks, on top of the insulation.

The performance of the Explocat 52 is reasonable with a test report saying “Broad reaching at 120° TWA with full main and Code 0 in 16 knots of true wind we cruised comfortably at 10 knots, reaching an unfussed maximum of 11.8 knots, with the boat still feeling rock steady.” And “In just seven knots of true wind we made 5.3 close-hauled, rising to 6.2 in 9 knots of breeze. Maximum upwind speed was 9 knots in 15 knots of true wind. However, these numbers can’t be achieved if pinching – the boat likes to be sailed fast and free, with tacking angles of at least 105°. This is hardly a surprise for a boat of this style that’s sufficiently fast to have a big impact on apparent wind angles.”

The jpegs give the idea. A true world cruiser capable of going most places. Bullet proof but not rock proof.

 

Yesterday we looked at the Explocat 52 and admired its strong aluminum construction. But Lets look at other aluminum catamarans to understand what is possible. We will compare the Meta Prometa series of cats.

Cat Flotteur 42 is a bridgedeck catamaran is 42 x 21 foot weighting 22,000 lbs and displacing 27,000 lbs. Its keel (hull bottom) is 12 mm thick and hull sides are 10 mm thick with minimal framing and stringers.

Cat Flotteur 45 is a bridgedeck catamaran is 45 x 22 foot weighting 26,000 lbs and displacing 33,600 lbs. Its keel (hull bottom) is 12 mm thick and hull sides are 10 mm thick with minimal framing and stringers.

The Santorini 65 power catamaran is 65 x 24.25 foot that weighs 44,800 lbs. Its keel (hull bottom) is 12 mm thick and hull sides are 10 mm thick with some framing and stringers.

Trawlvius 80 power catamaran is 78.5 x 29.5 foot that weighs 55,000 lbs. Its keel (hull bottom) is 12 mm thick and hull sides are 10 mm thick with some framing and stringers.

All the above cats are designed by Joubert/Nivelt. The decks and deck houses of both are in 4 to 6 mm aluminum with some framing support.

An alternative structural approach to an aluminum sailing catamaran is done by Crowther. Crowther Design 85 which was a 45 x 24 foot cruising cat of about 20,000 lbs weight. Many versions were built including a 50 x 24 foot 24,000 lbs weight aluminum version. This version has a 4 mm plate hull with 37 x 37 x 3 mm T stringers on 250 mm center lines. Frames are 115 x 50 x 5 mm T sections in the hulls at 1200 mm center lines. The Deck and cabin plate is 3 mm with 75 x 4 mm with a 25 x 3 mm flange to form a T section deck and cabin frames with 37 x 37 x 3 mm T stringers. The gunnel deck connection has 100 mm x 5 mm flat bar as reinforcement.

A 60 foot Crowther aluminum power cat would have a 5 mm plate hull with 115 x 50 x 5 mm T section frames and 40 x 40 x 3mm T section stringers at about 300 mm centre lines. The deck is similar construction.

What is being indicated here is the usual designer trade off. Thicker hull plate less framing. Thick plate less framing ends up with a slightly heavier boat weight but less work in building. I would prefer to cruise with thicker skins but I would have to expect less performance. EG Banana Split a Catflotteur 42 can do about 50% wind speed. A Crowther Design 85 designed around the same time can do 200 miles per day averages or about 65% of wind speed.

Also, as you have noticed a Catflotteur 42 shell plating dimensions can and has been used up to 80 foot cats. The only variation is the amount of framing and stringers used (which would still be less than a Crowther cats framing and stringers).

When I said that the Explocat 52 is tough, I meant it. It not only has a thick hull plating but it also has a significant amount of framing and stringers of significant dimension. The result is the Explocat weighs 42,000 lbs compared to the Crowther Design 85 weight of 24,000 lbs and the Cat Flotteur 45 weight of 26,000 lbs.

But if you want an interesting variation, there is a 50 foot racing tri (Mooloolabar Firetruck) in Australia that had its hulls and cross beams built from 5 mm aluminium with 1 stringer per hull side and 5 frames. The tris OMR weight was 9000 lbs. It was built 40 years ago and is still sailing. This was a pure racing machine with zero internal accommodation.

You take your choice and the designer designs accordingly. There is no right or wrong answers only compromises to suit your statement of requirements. Jpegs are of some of the boats discussed.

 

The final bit on aluminium cats for a while. An unusual Russian design built in 1965 gives an idea of a small cat structure. The design is not “conventional” but still works as a cat. The cat is 24.8 x 11.8 foot that displaces around 2000 lbs. The 45 foot mast is a 50 mm inside diameter with a top section of 50 mm outside diameter aluminium pipes. The mainsail is 320 square foot with a 190 square foot jib. The length to beam is 14 to 1. Centre boards down draft is 3 foot.

The main accommodation is in a cabin unit of 18 x 10 foot that bolts to the hulls. The cabin contains 5 berths and stowage lockers. It only has 3.5 foot headroom.

The cat hulls are built using aluminium-magnesium alloy AMg-3, 2 mm thick. The V bottom hulls consist of the keel, stringers and frames made of squares tubes of the same alloy. All joints are argon-arc welded. The hulls are filled with waste foam for buoyancy. The bridge deck is made of 2.5 mm thick sheets, reinforced with longitudinal and transverse ribs of 40 X 40 X 3 mm square tubes. Since these squares are made of duralumin, which is difficult to weld, they are riveted to the flooring with aluminium rivets. The cabin roof has smaller section square tubes for support but the same thickness of 2.5 mm sheets. The bridge deck unit and hulls are joined by 8 mm stainless steel bolts. Centerboards and rudders are made of 7 mm thick duralumin sheets. Again, a warning, building in aluminium below 3 mm thick requires good welding skills and an understanding of the materials characteristics.

The performance of the cat is described as moderate. Due to the cat weighing more than designed the wing deck is low to the water and pounds upwind. The builder keeps the cats sailing to rivers and lakes. The weight does have one advantage with the cat able to carry full sail up to 24 knots of wind speed. No indication of speeds.

The jpegs show the unusual shape. This is more about the build than the design concept.

 

I am going to have internet upgrades tomorrow so an early update. This is about a low priced bargain. An unfinished $300,000 bargain. Why? The basis of this cat started out as Highest Honor 3 an 85 foot high performance cruising cat. The next owner brought the boat in 2000 and decided he wanted to extend it to 100 foot long. The result is a 100 x 39.4 foot displacing 123,000 lbs. The sail area upwind is 4,300 square foot with a 2670 square foot mainsail 1040 square foot staysail and a 1570 square foot yankee. The length to beam 13 to 1. The new low aspect ratio keel draft is 5.5 foot. The two engines are 250 HP.

The 85 foot Higest Honor series of cats (3 of them) were designed by Van Petegem & Lauriot-Prevot and were seriously fast cruising cats because its hulls and its superstructure in composite and carbon materials. They were very light and had big sail areas for their time. They would have cost, even in 1991, $1 million a piece.

So, when the 2000 owner wanted the cat extended to 100 foot he hired Van Petegem & Lauriot-Prevot to do the redesign and competent yards to do the extension rebuild. The new hull was also certified MCA and Bureau Veritas. The cat then went on sale for $300,000 in about 2008. Yes, it probably had another $500,000 spent on it to finish it for EG charter work or more if it was to be a private yacht. But a new 100 foot cat of equal capability would set you back over $4 miilion.

The jpegs give the idea. The accommodation will be whatever you want. Back to reality in a day or so.