Multihull Structure Thoughts

I hope you don't mind adding this video of Mocking bird to your thread Old multi, I just thought it was a neat fit. The approach to the rudder is so clever and the boat is breathtaking, and when I use your thread for reference I'll get to see it again..ha.. regards
and thanks captain -crunch and Sailing Anarchy who I lifted it from.

 

TTLF all contributions welcome. There are other Newick tris to discuss at a later stage. Ever heard of Trea? A 26 foot sister of Tremilino.

 

M A Cooper designed a 21’6” x 16’2” foot tri with a 4’9” main hull maximum beam, 11” to 48” draft and 1,550 lbs. displacement (includes crew and gear), 1,000 lbs. empty weight, and 220 square ft. of sail. The main hull (vaka) is multi-chined with a narrow flat bottom, graceful stem profile, and vertical stern to carry the kick-up rudder. The boat is built of marine plywood over a light spruce frame. The topsides are ¼” (6mm) while the bottom and decks are 3/8” (9mm). The main hull features a small cuddy cabin forward, a storage compartment aft, and a self-bailing centre cockpit. The pontoons (amas) are a V-section type also made of ¼” plywood using the “stitch and glue” method. The cross beams (akas) are a hollow box section made of laminated spruce and fir. The boat uses a standard Hobie 16 or 18 rig: mast, mainsail, standard rigging. The jib is custom made and mounts on a roller furling unit. The rig is readily available new or used. The Hobie 16 and 18 rigs are efficient and practical and make ideal “power plants” for the COMET. This would be a simple fun boat of reasonable performance.

 

No I haven't , I just looked up one being built on facebook, foam sandwich and partly built, nice shapes .

 

Cheers was the first Atlantic proa to be noticed by the public after its OSTAR performance. Its 40 x 17 foot with a 3200 lbs displacement with 380 to 635 square foot of sail. Cheers was cold molded with 3 layers of 3 mm mahogany on hulls and deck with 12 mm ply bulkheads in each hull either side of the crossbeams and 12 mm ply bulkheads either side of the rudder boxes. (total 8 bulkheads/hull) the gunnels were 40 x 40 mm. Daggercase rudder boxes have 12 mm sides with 40 x 25 mm logs top and bottom. Cross arms are box beams with 2 laminations of 205 x 19 mm flanges top and bottom. Fore and aft faces a 4 lamination's of 50 x 40 mm. Sponson box is 6 mm ply. Mast is 30 foot with a 3 foot bury. The mast is a square box 170 mm at base tapering to 95 mm square at the tip. The mast walls are 35 mm at the base taper to 16 mm at the tip. The booms are 13 foot long 95 mm square with 16 mm walls except the track wall which is 35 mm thick. This boat require an expert to sail it but it was very fast in most conditions, but it would nose dive if pushed to hard. This was very innovative at the time and started a whole new dimension in boating.

 

Tribune is a 19’ x 12’ foot plywood cruiser designed in 1991 displacing 1500 lbs weighing 1000 lbs. The building jig was made from surplus 4x4” lumber/timber. Marine plywood 6mm hull and frames with 3 mm 2 layer cold molded bottom. Stringers made of white pine all same size 3 x 1” inches of various lengths. Keelson 2 layers 19ft x 10”x 1” inches (laminated together), cross beams two 12ft x 6”x 4” (triple laminated), rudder made from hardened wood, tiller and hiking stick wood of your own choice, centreboard same as the former. All stainless steel bolts, washers, screws, rigging, sails, sheets, lumber/timber, glues, fastenings, cloth, resins, weighed before use within construction. Tribune’s rigging was formulated on computer the same unit designed the New Zealand admission to the America’s Cup, in other words it won’t come down. Mast height as taken from the cabin top 23ft, no spreaders used all s/s rigging and the deck gear supplied from ‘Harken’ through our local supplier. The two hatches over forward and aft stowage lockers to include companion-way hatch are all made from white pine material and marine grade plywood. For the windows we installed smoked/dark Perspex. This boat sailed well as a mini cruiser and daysailer. For a 28 year old design it was reasonable good but the floats would be fuller forward if done today. Also as a small boat it has a 6:1 main hull for space and load carrying.

 

This came from Kurt Hughes blog to answer a question about how strong is glass compared to plywood. Here is a table comparing strengths with out of plane loads (like water pressure).

Plywood/Glass Thickness Conversion comparing bending strength. (Assumed 60,000 psi bending strength for glass and 10,000 for plywood). Assumed that triaxial glass amount is on both sides of a foam core.

Ply thickness Triaxial thickness Core

3mm ply equals (12 oz) 400 gsm w/12 mm core

4mm ply equals (17 oz) 600 gsm w/12 mm core

6mm ply equals (22 oz ) 750 gsm w/12 mm core

9mm ply equals (34 oz) 1150 gsm w/19 mm core

12mm ply equals (2) (22 oz ) 1500 gsm w/19 mm core

As you can see good quality plywood is quite stiff for a given thickness but foam glass weighs less especially when ply thicknesses increase. A nice comparison chart. PS DO NOT substitute 4 mm ply for EG 600 gsm triax either side of 12 mm foam core. This is only measuring bending strength not eg rupture resistance etc.

 

Just had to comment on the thread after having been absent. I guess a first reaction of shock and disbelife is slowly turning into pure awe and admiration for the amazing contributions of oldmulti! Thank you sounds inadequate... but it's all i've got for now. Will probably take me the better part of this month to catch up. Can't wait to read all... and probably several times

 

Btw, that super light carbon swedish built trimaran mentioned early on in this thread, has been on the market for a couple of years now. About 23k€ or 240kSEK. Fascinating high level and almost OCD attention to detail and optimization. Blocket https://mobil.blocket.se/annons/stockholm/sport_trimaran_for_racing_och_noje/83511685

 

In post 159 of this thread I spoke about carbon fibre monohulls. “The weakness of design is occasionally designer group think gets them into trouble. In a recent monohull transatlantic race 8 brand new IMOCA 60 foot mono's from various racing designers competed. Each of the mono's had the latest construction technique. A less than 1 mm thick carbon fiber skin backed up with foam carbon ribs every 300 mm or so for the length of the hull. 6 of the latest design, multi million dollar 60 x 18 foot yachts broke. The hulls flexed breaking some ribs which allowed the skin to flex and fracture. Time to stop racing and hope you save the boat” Here is a link to photo’s of the build of one of these boats: New imoca boats http://forums.sailinganarchy.com/index.php?/topic/154035-new-imoca-boats/&page=12

The following photo may give a big hint as to where designers got the idea for the design and construction of carbon fibre IMOCA yachts came from. BOEING was designing and building the Dreamliner at about the same stage. Theoretically planes designers would have spent a fortune on analysis of the lightest strongest structure possible. The only problem is planes and boats have different forces on them. Planes have big longitudinal forces, pressurization, flexing forces etc. Boats have similar longitudinal, flexing forces but are not pressurized (pressurization can strengthen a structure). The main difference between a boat and planes though is wave action and impact forces. In short although the design concept is similar there are big differences in the forces impacting the structures on a daily basis. The sample 60 photo shows how extreme the designs are and the forces on the keel etc would be extreme. The sailors say the boats are very uncomfortable knocking you around all the time they are above 10 knots. The price mono's pay for speed.

 

The Tridarka Raider is a design collaboration between Steve Isaac of WaterTribe, Inc., and Matt Layden (Paradox fame) to be a fast cruiser. The main hull is 6mm (1/4") marine plywood for the vaka, 4mm marine plywood for the amas, standard stitch-and-glue technique with 6 oz. fiberglass tape with 6 oz and 4 oz. fiberglass cloth over plywood. The cockpit is uncluttered by using a leeboard instead of a centerboard.

Length Overall: 20'-7" x 12’-6” beam with Vaka deck beam 2’-4”, DWL beam 1’-8”

Design Displacement: 650 lbs Empty weight: 350 lbs Max payload: 900 lbs

Design Draft Board Down: 3'-8" Sail area (Working): 115 sq ft Mast above DWL: 19’-0”

Notice that the akas are asymmetric. This allows one ama to be moved inboard when the overall beam needs to be reduced (CP1 for example). The akas are hollow box beams to save weight. Also, the akas are lashed to the vaka and the amas.

Mast is tapered carbon fiber engineered by manufacturer to the following spec:

• Length overall 231", Bury 28", CE above the step 139", Round cross section
• Max righting moment 11,000 lb-ft

Vaka At Max Load 1250 lb. Total Displacement
Vaka Upright, 0 Righting Moment
(At anchor in a protected lagoon with the family.)

Vaka Ama Total
Displacement: 1100 lbs. 2 * 75 lbs. 1250 lbs
Wetted Surface*: 44.7 sqft 2 * 9.8 sqft 64.3 sqft
cP: .634
LCB: .534 L

Vaka At Max Load 1250 lb. Total Displacement
Max Righting Moment, Ama Submerged (20 degree heel)

Vaka Ama Total
Displacement: 237 lbs. 1013 lbs. 1250 lbs
Wetted Surface*: 19.8 sqft 56.7 sqft ** 76.5 sqft
cP: .661
LCB: .452 L

Notice the longitudinal centre of buoyancy and prismatic coefficient changes on the mainhull and float between the boat sailing flat and heeled at 20 degrees. This is the reason designing trimarans is a little harder than designing catamarans.

 

Great thread, thanks. Be nice to have a chat with you about the good old days if you are ever on the Gold Coast and want a cuppa.

I don't doubt that Kurt's numbers are correct, but they are out of context and useless for boat design, which is more about bending stiffness than strength. In real terms, 3mm ply is more likely to equal 200 gsm glass either side of 5mm core. 4mm probably the same, perhaps 300gsm. 6mm ply is what is used in up to 30'/9m cats, with frames and stringers; a foam cat would be 10mm with 400 each side, and no stringers. 9mm ply is for 30-40'cats, also with frames and stringers, which would be 15-20mm with 400-600 inside and out. 12mm ply is unusual on cats except where they might see big impacts, which is to do with toughness. I doubt any cat less than 50' has 1500 gsm of glass as it's base laminate.

Rupture resistance is different again, depending on how the loads are applied. ie slamming, sharp point, round point, etc

Stiffness is a function of thickness squared, so 3mm ply is 1/8 the stiffness of 6mm ply. Materials wise, it is dependant on the strength of the outer skin which is why ply is stiffer lengthwise than sideways, For a given thickness, glass is stronger than wood resulting in 1mm of glass each side on a 4mm sheet of ply will be much stiffer (and heavier) than 6mm ply.

 

The following trimaran is 31 x 28 foot weighing 2700 lbs without rig, displaces 4300 lbs. The boat folds like a Farrier although its not a Farrier. This boat has raced extensively and is a proven winner over 20 years. It has accommodation for 5 people and I am sorry I cannot name it due to the detail I am about to give. The main hull is 200 gsm Kevlar 12 mm airex foam 200 gsm Kevlar. The float skin is 193 gsm carbon fibre cloth 9 mm airex foam 193 gsm carbon fibre. The decks are 400 gsm carbon fibre 20 mm foam 193 gsm carbon cloth inside (the photo shows there is very little supporting roof structure which is the reason for the thick foam.). Structural bulkheads are 193 gsm carbon fibre cloth 12 mm airex foam 193 gsm carbon fibre with 6 mm solid carbon fibre around openings. The mast step is 7 layers of 15 mm foam separated by vertical 1.5 mm solid carbon fibre separators and wrapped with CF layers all around. The cross beams are box beams 150 mm wide minimum size (does not include forward fairing) and 180 mm deep near the main hull tapering to the float. The top and bottom flanges are 15 mm thick solid carbon fibre unidirectionals at the inner end. The flanges taper to 6 mm thick at the point of float attachment. The shear webs to form the cross-beam box are 3 mm thick CF. There are beam bulkheads every 500 mm with 12 mm foam and CF. The folding connecting arms are aluminium connected to the main hulls and cross arms with 18 x 3 mm stainless steel tubes. The tubes are attached to the main hull and cross beams by 3 (25 mm wide x 6 mm thick Carbon Fibre straps) of unidirectional and some double bias to bind it together. The straps are splayed into the main hull onto an 8 layer 540 gsm carbon fibre reinforcement from gunnel to gunnel around the hull and deck structures internally.

 

Rob, thanks for the offer of a cuppa. I have relatives on the coast who I visit occasionally. Thanks for the correction on foam glass to plywood chart. As you said theory is one thing practical application is another. I do know of 2 x 40 foot cats that have been built with 4 mm ply skins with very close stringers and framing. The one in Canada broke up. The other only raced a short time in bay events before being "retired" permanently.