Multihull Structure Thoughts

The Tiki 38 is a popular well known Wharram design. The standard Tiki 38 is 38 x 20.3 foot with a weight of 7,220 lbs and a displacement of 12,200 lbs. The standard rig is a ketch rig with 30 foot masts and 225 square foot wing sails on each mast. The jib and staysail are 140 square foot each and a 536 square foot spinnaker can be carried. The draft is 3 foot loaded over its semi low aspect ratio keels.

The standard Tiki 38 is built from plywood with timber stringers and plywood timber edged bulkheads with timber ply cross beams. All epoxy coated and e-glass covered exterior surfaces. Further details on page 68 of this thread. So far so good. The first 2 jpegs are the standard Tiki 38 rig and layout.

Now the purpose of this update. Several builders like the basic idea of a Tiki 38 but like to improve it to suit their idea of a cat. Jpegs 3 to 5 are of a Tiki 38 “Festina Lente”, an Italian naval architect was requested by a client to remodel the deck, sail plan, interior lay-out and motors of the Tiki 38. The rig for example was changed from a wing sail rig to a fully battened mainsail and foresail rig with a more powerful headsail with the total sail area increased from 588 square foot to 720 square foot. The deck layout was changed for winches and a more forward cockpit, the cabin tops are longer etc.

The next Tiki 38 (jpegs 6 to 8) was built in Asia for an Australian, who modified the structure and redesigned the hulls by having them built in cedar strip planking instead of plywood. The boat was completed with honeycomb decks, honeycomb sandwich interior and furniture. The final result is a much lighter vessel, that is floating about 125 mm above the expected designed waterline which indicates about 2,200 lbs less weight with a slightly fuller waterline.

Finally, jpegs 9 to 11 show the Tiki 38 “Aluna” which was built strictly to plan except the rig, which is a Polynesian crab claw rig, taken of the James Wharram Tama Moana design and scaled up 15%. The rig is built with fibre glassed bamboo spars and high quality agricultural tarp sails. The main sail is 258 square foot, the mizzen is 140 square foot with a second mizzen of 95 square foot for stronger winds. The cat was launched in 2008 with System Three epoxies and Meranti and Okume marine grade plywood. The rig and inventory tells you a bit about the owner.

The ground tackle is a 35lb Bruce anchor with 16 foot of chain and 300 foot rode, 40lb CQR anchor with 16 foot chain and 160 foot rode and a 20lb Danforth anchor with 16 foot chain and 125 foot rode. Also, there is a 18ft military grade parachute sea anchor. But the engine is a 6hp extra long shaft Tohatsu. The 16’ Gary Dierking outrigger canoe has two adult paddles, can be rigged for sailing. But the sanitation is a Porta Potty and bucket toilets. This boat has crossed the Pacific cruising for 6 years. The owner built a good basic structure and then found out what it was going to cost to complete the cat and went “minimal” on everything else.

Each of these variations of a Wharram Tiki 38 may be useful boats but they may not improve the resale value of the cats. Wharram buyers are loyal to the basic concept and marketing and will pay accordingly. But variations can cost more and not always get better resale value. Please understand a Tiki 38 will take as long to build and cost as much as most other 38 foot bridge deck cats if built of similar materials. And often the 38 foot bridge deck cat will have a higher resale values due to there bridge deck cabins. The jpegs are below.

 

The Eco 520 is a new minimalist cruising cat design from Bernard Kohler. It uses the same principle in shape and build as the DUO 480 C cat with a bit more interior room and a mast aft rig. The Eco 520 is 17 x 8.2 foot with a 23 foot wooden mast that carries a genoa and staysail totalling 215 square foot. The weight is 630 lbs and a displacement of 1,300 lbs. As with most of Kohler designs the length to beam of the asymmetric hulls are 14 to 1. He does this for both performance and to provide the widest hull centreline separation possible for stability. The downside of such slim hulls is you cannot overbuild or overload the cat as it will pickup wetted surface very rapidly which will slow the cat in lighter winds.

The mast aft rig needs a strong mast base and slightly heavier rigging wire to minimize forestay sag. Again, not a problem on such a small boat as the wire is likely to be about 3 mm 1x19 mm stainless steel. The diamonds help hold the mast in column from the forestay loading.

The DUO 480 C can carry 600 lbs, the Eco 520 can carry 670 lbs which does appear to be much of a gain but at this size several things happen. The accommodation gain in the main cab is from 5 foot long in the DUO 480 C to a 6.2 foot long cabin in the 520. This allows a good size double berth, table and a small galley with storage space. The second advantage is a stability gain of 28% from the 480’s 3530 ft lbs to the 520’s 4550 ft lbs. These gains make a real difference in small boats. Miss Cindy, the 16.25 foot biplane ocean crossing cat had a 3300 ft lbs stability for the same sail area as the 520.

The performance of the Eco 520 is anticipated to be a maximum speed of 16 knots with averages of about 5 to 7 knots. Miss Cindy peaked at 16 knots and did 90 to 140 miles per day averages singlehanded.

The build is the same style as the DUO 480 C. Basically plywood (6 and 8 mm) and timber (eg 25 x 25 mm stringers). In the Eco 520 a scarphed 17 foot long sheet of 6 mm plywood is laid on a flat surface then bulkheads are placed at right angles to the shaped flat panel. A keel panel, stingers are placed on the bullheads, then the inner tunnel ply sides are draped over the frame with some pressure to shape the bow panels. Result a hull in a very short time. Repeat for the second hull. Then the main cross beam bulkheads and underwing is installed. Cabin top to finish. A fast builder could build one in 250 hours, more realistically it would take 400 hours. The whole boat should be epoxy coated with a light glass (200 gsm max) on the exterior surfaces. The attached material list gives more details.

The jpegs are a mixture of ECO 520 and some build diagrams from a DUO 480 C. A very interesting and simple boat that will give you a good performance camp cruiser for 2 crew.

 

The Bernard Kohler Eco 75 power cat design is interesting for its structure. It is a bridge deck cruising catamaran. The cat is 24.5 x 10.5 foot and weighs 1790 lbs it displaces 3130 lbs. It has a 20 to 40 HP outboard as power which will give it from 18 to 25 knots speed maximum. The hulls L:B is 9:1. The hulls start with a deep V on the bow and a wide V at the transom. So, the boat will be sea worthy. The hulls are designed for medium speed. Speed with a 25 HP engine will be around 25 knots full power and 18 knots at half power.

The cat has 2 single berths and a convertible dinette double berth on the bridge deck. The height on the bridge deck saloon is 4 foot. Just enough for seating on the settee. The hulls have standing headroom in the galley and the toilet.

The structure of the design is plywood and timber. The planking for the chine hulls has a constant radius and is tortured plywood in some spots. The hulls are 6 mm plywood side and two layers of 6 mm ply on the bottoms and have biaxial glass 340 gr/m2 and one layer 200 gr/m2 glass fabric. The roof is 2 layers of 4 mm ply and forward under wing is likely to be 3 layers of 4 mm ply. The under wing is 12 mm ply. The attached sample PDF plan gives more details of the structure. The jpegs show the hull bulkhead and stringer layout arrangement. This is a smart design with each component adding value to the overall strength whilst providing privacy as required. Also, the forward cockpits entrance by a door from the galley hull side is an interesting design feature. Make sure the door can handle a wave impact.

The jpegs give the idea and provide a big hint as to how the Eco 7.5 sailing cat is built. When you look at some of the builder jpeg’s please remember the boat has a reverse bow and the hulls are built upside down. Interesting structure.

 

I quite like that boat, but there is no way it's going to go 25 knots with 25 hp. More like 18 at full power if that. I'd like to be proven wrong, so please do so.

 

Yes, at 1400 kg even the 40 HP is going to be struggling to get near 25.

I crunched the numbers with the data available and the 25 is going to be struggling to get past 15.

 

The power speed numbers for the Eco 75 power cat yesterday were the designer’s numbers not my estimate. I would agree that the numbers are optimistic. But today back to sailing vessels.

The Slyder 49 is a performance orientated bridge deck cruising catamaran designed by the Yacht Design Collective (François Perus-Romain Scolari) and built in China by Mazarin German Yachts. The cat is 49 x 25.2 foot with a weight of 26,700 lbs. The displacement is unknown. The mast is 64 foot long fixed carbon mast with fibre rigging carries a 969 square foot mainsail, a 452 square foot jib, a 1184 square foot code 0 and a 1485 square foot Gennaker. The hull length to beam is about 12 to 1. The hull based curved daggerboards draw from 3.6 to 7.2 foot. The rudders are spades.

The accommodation is a 4 cabin layout with 3 double berths and a 2 single berths in a fourth cabin. There is a head per hull. The double berths are narrower than French cats due to the slimmer higher performance hulls. The bridge deck cabin has a dinette, galley and navigation area. There are several accommodation options such as an “owners” cabin in one hull etc. The Slyder 49 is a semi production cat that allows some accommodation customisation.

The cockpit is covered, but the steering wheels are in aft gunnel based exposed mini cockpits. These give an good feel for wind in the hair but the down side is you are exposed and cannot see past your own bridge deck cabin in close quarters unless you run from side to side. Cabin/Bimimi top steering positions are much better for steering and sailing control.

The structure of this cat is well done. The Chinese factory built all of the 70-foot Clipper Round the World Race yachts. They know how to build a boat. The Slyder 49 is built using a 100% Vinylester e-glass composite Corecell PVC sandwich under vacuum. All load lines have been reinforced by unidirectional carbon fabric and the bow area has been reinforced for impact protection with Kevlar. This is a light, stiff boat. There is an option to have an all carbon fibre build if you want a 2000 lbs lighter boat for more performance. But as per usual you pay for the option. The base boat cost $US700,000 the carbon option costs $US902,000. (2017 numbers).

I have not read any tests for the cat but the numbers indicate a high performance cruiser. Peaks of over 20 knots and 10 to 12 knot averages should be possible. This is a potential 300 mile per day cat.

The jpegs give the idea. Slyder also produce a 59 foot version.

 

I like this design too. So if you don't mind crunching some more numbers; what hp outboard would this boat need to maintain a 10 knot cruise at around half throttle at full disp?

 

“Liquid Sky” was a beach charter catamaran between 2001 and 2005, that was home designed and built mainly by Mark Preuss. The cat is 32.5 x 18 foot with a weight of 5000 lbs and a 36 foot aluminium mast on a pedestal with a cutter rig. Liquid Sky was rigged with 6 mm galvanized cable and 12 mm turnbuckles with stainless steel forestays for the hanked headsails. The original mast was a wooden 50 foot wing mast that proved to be to effective on a mooring with the cat “sailing” around when moored. Result the rig went to a conventional mast and cutter rig. The sail area is 520 square foot. With mast head 44ft off the water, this rig could be controlled by a lone sailor. By performance catamaran standards the boat was underpowered. The hull length to beam is 10 to 1.

The “accommodation” is limited with some spaces internally for 4 berths and a toilet and a small galley. The headroom is 5 foot over a limited area. The cockpit is large to satisfy its function as a charter cat.

The interesting part of this cat is its build technique. The ideas are good but the actual structural dimensions and cross beam connection approach would need to be reviewed. Also, the rudder locations and tiller arrangements could do with a rethink. The designer builder did some worthwhile research, did some “plans” and started building frames of the correct size. When the frames were ready the cat hulls were built in components. The set of bow frames had stringers inserted. Then a skin of multiple layers of 3 mm plywood were added. The large skin panels are glued with polyurethane and hundreds of small screws are used to build up the 3mm plywood. The screws are removed before the next layer of plywood is added.

To quote the designer “Some places only had one 3 mm layer, some had six. Four of the frames were robust with large tabs passing through the hull. These tabs would attach the hulls together.” The tabs are solid timber that attach to internal frames at 4 locations. The timber cross beams slot over the top of these tabs to form the complete cat. The second set of hull components were built next. The hull components were then taken to an assembly location where the hulls were completed along with the cross beams etc.

The original timber wing mast was a series of timber bulkheads and stringers with a 3 mm plywood skin. The internal space in the mast was filled with cheap Styrofoam for ???. The mast structurally worked; it was removed for other reasons.

The rudders are built on a 25 mm stainless steel shaft with a timber blade and glassed over. The rudders are mounted inboard of the hull stern to allow them to be withdrawn vertically for beaching. The tiller steering is aluminium tubes. The cat rudders and steering system evolved over time.

The entire boat was finished with coal-tar Epoxy paint covered the non visible internal hull surfaces. Styrofoam was built into the bows to provide positive flotation.

This cat worked as a day sailing charter cat in probably relatively calm weather so a lot of its design features were probably not fully tested in a serious deep sea storm. With that reservation some of the build ideas are worth some consideration. The jpeg here tell more of the story.

 

Further Liquid Sky catamaran build jpegs.

 

Hobart 1100 was a production catamaran from about 1988 and was designed by Claude Constant Saindon for family cruising and travel. The cat is 36 x 17.7 foot with a weight of 6000 lbs. The Francespar aluminum 38 foot mast carries a 370 gram mainsail of 240 square foot on a Leisystem boom furler, the genoa is 333 square foot on a Leifurt furler, a 360 gram solent Dacron jib of 194 square foot and a 55 gram nylon tri-radial spinnaker of 750 square foot. The double backstay, cap shrouds, lower front and rear stays are 8 mm, the forestay is 10 mm and the inner releasable forestay is 10 mm. All wire is 1 x19 stainless steel wire. The hull length to beam of about 10 to 1. The hulls have low aspect ratio keels with stainless steel strips on the bottom that draw 3 foot. The rudders are spade.

Ample accommodation is provided with 2 spacious double berth forward cabins and an aft single berth cabin. The bridge deck cabin has a dinette that converts into a double berth. There is a flush deck version with 5 foot of headroom or the Hobart 1110 version with 6 foot headroom. The hulls have full headroom with in the starboard hull mid ships there is a fitted galley and a port aft bathroom with hot and cold water, sea toilet and shower.

The hull and underwing are solid polyester e-glass. The deck and cabin roof are a polyester sandwich with polyurethane foam and balsa at fitting locations. The bulkheads and interior are in marine plywood with teak and balsa line finish, solid elm wood. Exterior wood iroko Quality fittings and all stainless steel screws. The gelcoat of the hulls is painted white with a two-component polyester paint.

The cat can sail at 50 degrees upwind. Downwind the cat sails at 12 knots with good breeze. The cat can do better if the wind and sea state are favorable. This cat is a 15 knot peak sort of boat, but due to its finer ends and low aspect ratio keels upwind can be impacted. Also, having fine ends and having the main sleeping berths forward of midship would be uncomfortable to many if going upwind if you were trying to sleep. . The main anchor is a Delta 16 anchor with 130 foot of 10 mm chain and 200 foot of 18 mm rope. There is a second mooring in the cockpit with a 20 kg FOB flat anchor with 60 foot of 8 mm chain and 160 foot of 18 mm rope.

The jpegs give the idea of a practical production cruiser with some reasonable performance and space.

 

I am the new owner of the last Seawing 24 built, purchased out of Channel Islands CA and now on the Hawaiian island of Kauai.

I am presently remodeling and upgrading the interior, fabricating a hard bimini top, and getting her prepped and ready for our summer mooring season in a Hanalei Bay.

Skip was a smart and methodical designer. You can tell a lot of thought was put into the design and functionality of this boat, and it’s construction was very well executed by Ventura Multihull. I can’t wait to see how she performs this summer.

Skips skills as an Aeronautical Engineer were quite apparent when I towed the boat from Southern CA to the Bay Area. The design is definitely light and a real wind cheater as I could hardly tell I was towing a big Tri behind a little Toyota Tacoma.

This particular example was setup for racing and features a wider beam, more aerodynamic aka arms, bigger mast and sail plan, among other things like the use of carbon to further reduce weight. The rudder is a spade style with its shaft and bearings built into a drop in housing that cleverly fills a slot in the transom and completes the swim step. This system can be removed completely, but also has two pin-able depth settings for launching and shallow navigation. The cockpit as you can see is very deep and roomy. Interior-wise, the cabin features double single 6’ berths. One of them is a slide out, and I’m modifying the other to do the same.

Other interior mods will include a larger countertop, a large slide out storage drawer that will mount under the cockpit floor and slide into the cabin, a hard bimini with solar that will be adjustable to provide two positions -a morning position which will provide standing height and cover the entire cockpit, and a forward sailing position that will allow for boom and main traveler clearance.

Here are a few more photos. You really get a good sense of how big the Seawing is overall (for a 24’) when you see it sitting on trailer next to my Farrier Tramp.

 

That looks like a great boat! A piece of history too.

 

Yes, it’s definitely a special boat. Amazing what he was able to incorporate into a fast, light, strong and functional design. You really get a sense that he took all the lessons he learned and incorporated them into the design of the Seawing 24, his first production molded boat. And Ventura Multihull did a great job executing the construction. It’s too bad more weren’t built.

 

Today we will discuss a very high performance cruising cat from the design of Christophe Barreau. The TS5 (also known as ORC 50) is 50 x 28.1 foot and weighs 20700 lbs and displaces 28660 lbs. The 67 foot optional carbon mast carries a 947 square foot mainsail and 646 square foot genoa. The hull length to beam is about 13 to 1. The hull curved based daggerboards draw 8.9 foot down and the spade rudders draw 5.6 foot.

Why do I say high performance? The company claims the boat can do 350 mile plus days. OK, let’s get some supporting information. In the 2019 ARC transatlantic race “Hallucine” TS5 took 11 days and 16 hours to do the 2900 to 3800 mile course (depends on how far south you go before heading to USA). Assume the TS5 sailed a 3400 mile course. That is an average of 291 miles per day or a 12.3 knot average. OK. Next the builder claims the cat can reach 25 knots in a 28 knot wind. On the ARC Hullucine sailed almost all the way with a full main and heavy A2 spinnaker. They were able to gybe through 145-150° and were making 17-20 knots – “usually 17-18 knots steady”– as soon as they reached the tradewinds. The wind never topped 17 knots apparent except for a few squalls. If given the right conditions and a bit more wind I could believe 350 mile plus days and 15 knot 24 hour averages.

To achieve this performance Hallucine was a product cat with a carbon fibre wing, fibre rigging, carbon fibre fore and aft beams options and very good sails. The crew opted for minimal fuel and water (read 30 litres of water) and used a 27 litre/day water maker to supply additional water on the trip. Being French food was good quality.

The second boat in 2019 ARC (10 hours later) was a Volvo 65 racer that had 15 crew who lived on freeze dried food and no bunks. Hallucine also beat an IMOCA 60 and a 105 foot super yacht, as well as a variety of monohulls and multihulls.

OK the TS5 is fast, but in 2019 but the fifth place cat was a TS42, Elektra, a 42ft smaller sister from the same French builders (page 44 of this thread), which crossed in just under 13 days. The 2020 ARC was won by Banzai, a TS42 in 10 days 22 hours. That is a 312 mile/day average or 13 knots. This builder produces some very fast production “cruising” catamarans.

Some other cats in the ARC 2019 showed what can be done even with a French cruising cat “A 55ft Bali cat was holding up with an X-61, which is a fast monohull, and if you pick out comparable elapsed times and distances sailed you can see that a Lagoon 42 is going the same speed as a 46ft Bavaria fast mono and significantly faster than a Discovery 55, if you’re sailing them well and getting the best out of them.”

The TS5 is built by Marsaudon Composites in France. The ORC50 (TS5) is built using resin infusion process in female molds. The PVC foam-glass-polyester and vinylester sandwich is used throughout the majority of the hull, decks and bulkheads. As mentioned, use of carbon in the bulkheads, forward beam, compression beam and the rotating carbon mast can be added to suit the owner´s requirements. By using resin infusion, the builders can get very good weight control throughout the structure.

The other weigh saving approach is the interior of the TS5 is pretty spartan, it´s all about weight saving inside. The underwater hull of the TS5 (ORC50) is shaped for speed with slim hulls designed to minimize resistance and reduce the wetted surface, so that means less living space in the hulls. The TS5 still has 4 cabins with 6.5 x 5 foot bunks and 2 toilets in the hulls. The main saloon has a dinette, galley and navigation location. Unfortunately, the steering locations are very exposed on the rear aft quarters.

So, you have a very fast cabin cat in the TS5 but speed can have one downside. Hallucine, with 4 crewmembers in November 2020, off La Coruna, Spain struck a partially submerged container at 20 knots and pitch-poled, with one crewmember lost. The 1600 plus lost containers per year if hit at 7 knots would do damage but may not cause a capsize. Hit the container at 20 knots and you’re dealing with a lot of momentum which in this case resulted in the pitch pole.

The TS range of catamarans (TS5, TS42 and TS3) are all very high-performance cats that have a proven track record of speed. Good designs and especially a very good builder. The jpegs tell the story

 

"So, you have a very fast cabin cat in the TS5 but speed can have one downside. Hallucine, with 4 crewmembers in November 2020, off La Coruna, Spain struck a partially submerged container at 20 knots and pitch-poled, with one crewmember lost. The 1600 plus lost containers per year if hit at 7 knots would do damage but may not cause a capsize. Hit the container at 20 knots and you’re dealing with a lot of momentum which in this case resulted in the pitch pole."

Please, what is the origin of your capsize diagnostic ? Looking at the photo, there is no sign of collision on the under water hull or any appendage ? To my knowledge, sometimes ago there was no reliable diagnostic ?