Multihull Structure Thoughts

[oldmulti]

Hi
Over the years I have seen a lot of multihull designed and built and have regretted that some really good ideas were not more widely known that could make building a boat faster and lighter. Gary Baigant is really innovative with the structure of his tris. In the past other designers such as Lex Nicol built some very innovative designs.

So I will start this thread off with a description of Devils 3 a 36 x 24.5 ft racer/cruiser tri with 900 sq ft of sail that weighed at launch 3800 lbs. Her main hull was 200 gsm glass cloth outside (doubled over bottom to WL) 12 mm airex foam 200 gsm glass cloth inside. If there was to much flexing in an area a vertical rib was glassed on the inside. Now the interesting part at the gunnels and keel/stem of the main hull was an aluminum space frame of100mm diameter x 3mm wall tube to take the rig loads.
The floats were 200 gsm cloth outside 9 mm airex foam 200 gsm cloth inside with no aluminum structure. Decks for main hull and floats was 200 gsm cloth outside 9 mm airex foam 200 gsm cloth inside. This boat has lasted 35 years and won many races and had good room inside.

The X beams were aluminum 100mm diameter x 3mm wall tube top and bottom of a space frame structure with intermediate verticals connecting top and bottom tubes. All aluminum is 5083 H32 (high strength aircraft quality).

---Added---
Index Updated

 

[bajansailor]

Old Multi, have you got any photos of Devils 3?
Googling found one on a post on this forum in 2011 - Lex Nicol, did he ever create any stock plans? https://www.boatdesign.net/threads/lex-nicol-did-he-ever-create-any-stock-plans.32027/
Scroll down to post #6.

 

[bjn]

Thanks for the info! 200gsm glass on fairly thin foam seems like an extremely light construction. Like 2kg per square meter. So the composites of the three 36 foot hulls would be just a few hundred kg in total.

 

[oldmulti]

No I do not have any photos of Devils 3 as I spent more time yelling at it as it passed us when racing. bjn 200 gsm on thin foam is light but it was very well built. Using light materials requires good building technique. Professional designers up specifications on home build designs because they know that home builders are not that good as builders. Another designer/builder of that time Chris Mashford built a 38 x 25 ft by 7000 lbs cruising tri (Whitebird) from 2 layers of 330 gsm glass cloth on the outside 12 mm airex foam 1 layer 330 gsm glass cloth on the inside for the hulls deck and floats. This was considered a "conservative layup" for cruising.

Next boat for discussion. Attitude. A 28 x 17 ft x 2000 lbs weight with 450 square foot of sail area. Its a 8.5 meter hard raced cat and is a race winner in NZ. The boat is a tube cat and has 4 berths. Displacement is 2800 lbs. It is built from 4 mm gaboon ply (compound or tortured ply) sheathed in 300 gsm glass cloth in epoxy. 2 cedar stringers one midway up the hull and one at the gunnel run the full length of the boat. the Fore and aft bunk tops act as stiffeners. There are vertical foam glass ring frames (approximately 2 x 1 inch) every 40 inches in the hulls. The deck is glass foam. There are minimal BH's. It has 3 aluminum cross beams.

This Way Up is another tortured ply design 30 x 20 foot with 500 square foot sail that weighed 1700 lbs. A tube cat with 4 bunks. Again it was raced hard. The hull was 5 mm gaboon ply covered with 200 gsm glass cloth in epoxy. It has 3 permanent bulkheads to help shape the hull. It has 2 fore aft stringers and bunk tops to act structural components. There are vertical foam glass ribs every 3 foot or so.

The biggest tortured ply (of flat one thickness plywood) I know is Cee Bee 38 x 19 ft a wing deck cruising cat that displaced 8000 lbs. It was built with 9 mm ply tortured like a tornado. The 9 mm ply was scarfed together and then cut to shape and had a 65 mm x 65 mm gunnel strip glued in. The keel line and lower stem were wired together by 3 mm copper wire every 6 inches. The panels were then set apart by 120 degrees from midship to the stern. A epoxy bog was poured along the keel line then had glass taping on the inside. The outside was cleaned up and also taped before compounding. The hull sides were progressively brought together by Spanish windlass's then temporary cross braces were put across the gunnels then 3 bulkheads were inserted and glassed to the hull sides. The hulls had 330 gsm glass cloth on the outside. The rest of the boat was plywood.

 

[oldmulti]

Moolabah Fire Truck is a 50 x 42 foot wide aluminum racing trimaran that displaced 8000 lbs with a 1000 sq ft sail area. The boat was built from design to in the water in 1200 hours. The boat is still sailing after 35 years and was raced hard for 15 years. The build technique is the great part of this boat. Buy a pile of 5 mm 5083 H321 marine grade aluminum sheet. Build an oval tube 1070 mm wide and 1870 mm high. This is done by rolling 5 mm sheets into a half semicircle with a 535 mm radius. This forms the top and bottom of the tube. Separate the top and bottom semicircles by 5 mm thick 800 mm wide plates either side. Now you effectively have a 50 ft long oval tube. Weld a 6 mm x 50 mm aluminum stringer on the mid point of the flat tube on the inside on either side. Now cut a dart about 4 meters long in the bow in the top and bottom of semi circles. The dart will be triangle with curved sides that will allow the bow to be formed by squeezing the sides and what is left of remaining semicircles to form a bow shape. The stern is made by cutting a dart in the rear side panels of the tube and pulling them together. The floats were smaller but built the same way. Aluminium BH's are placed every 1.8 meters, at the cross beams and at the rear cabin BH. The daggercase sides were 5 mm aluminum. The cross beams 5 mm aluminum starting as an 600 x 600 mm tube in the centre tapering to a 450 x 240 mm tube at the ends. There are 2 transverse 8 mm x 50 mm internal stringers top and bottom of the cross arms. The float to crossbeam connection was reinforced later by a 45 degree strut of about 600 mm length. The boat had side pods added for limited bunk space added later.

The same concept could be used to build EG a plywood tube of say 2 layers of 4 mm ply to form a ply tube cat of say 10 meters with minimal internal structure.

I have more interesting designs to tell you about. Next will be light weight foam kevlar design developed of a light foam glass design.

 

[oldmulti]

Shockwave 37 and its development Super Shockwave 37. Crowther tube racing cats. The Shockwave 37 was designed in the late 70's. A 36 x 20 foot cat that weighed 3000 lbs and displaced 4500 lbs carrying up to 1000 square foot of sail area. Several were built and raced hard but more a coastal racer. The initial hulls were 330 gsm glass cloth 12 mm airex foam 330 gsm glass cloth doubled below the waterline. Foam glass BH's. Decks were 330 gsm glass cloth 12 mm airex foam 330 gsm glass cloth. The 3 aluminum cross beams were E8150 (Canberra lamp posts) 203 x 140 x 4.8 mm sections with extruded ribs inside. As products evolved so did the design and a later version was created.

Super Shockwave 37 a 37 x 25 foot tube cat again displacing 4500 lbs sailing with 1000 square foot of sail. The hulls had a bulb bow and fuller stern. Crowthers own boat had 165 gsm Kevlar style 285 on the outside and inside of 20 mm airex foam for hull and decks. The resin was epoxy. Theory was the thicker foam would allow a lighter skin layup but still provide the same "strength" (read stiffness). The main mast carrying crossbeam was a composite construction with aluminum forward and rear beams. The Super Shockwaves were faster across most wind ranges and pitched less and were offshore capable. Several tricks were used in the building of one Super Shock. When the foam was on the male mould and fully faired they ran a screed of microballoons and slow setting epoxy to fill the foam holes prior to applying the kevlar skin under a vacuum bag, then within 12 hours of the kevlar going on they ran a final screed of microballoons and epoxy. The idea was to minimise the weight gain of pure epoxy in the foam and have the entire skin filler cure as one unit using less epoxy and protected the kevlar in the final fairing. Again it require good building with several people to coordinate the tasks.

 

[oldmulti]

The Grainger R42 trimaran design was built on the gold coast near brisbane Australia. The racer/cruiser tri is 42 x 31.5 foot of 10200 lbs displacement carrying 1150 square feet of sail. The boat is made from pre manufactured flat panels of carbon fibre foam sandwich for the basically multichine hulls. The idea was to speed up construction and have reduced weight by having the pre manufactured panels done under controlled conditions with high fibre to low resin ratio's. As this is a recent design I will not give to many specifications on layup etc. But the professional builder said that he regretted building the hulls from pre manufactured panels. The amount of work to join the chines together, bog the joints, layup joining carbon fibre then bog and fair the carbon fibre joining strips added weight and took considerable time. The builder said he thought it would have been faster to do a male mold, fair the foam, lay up the Carbon fibre and fair the hulls. He also said it would have been lighter and slightly stronger. Excluding the BH's (some of which had to have significant additional reinforcement) the remainder of the boat had to be custom built on site with a series of molds. The "time savings" of the flat panel construction proved to be elusive. The builder is excellent and very experienced. R42 BUILD PROJECT GALLERY https://www.graingerdesigns.net/video-and-photo-galleries/r42-build-gallery/

Carbon fibre is an excellent material when designed and built correctly BUT as mast makers learnt the strength of carbon fibre can be deceptive. Some early masts were engineered to meet all strength requirements in tension and compression but the masts failed. The masts failed because if they went slightly out of column the walls buckled leading to failure. Aluminum masts have thicker wall sections and can resist the buckling better because thicker walls are stiffer. Stiffness is only partially related to strength.

Another aspect of carbon fibre is that it requires very good building technique if the weight and strength advantages are to be gained. EG do not mix uni directional glass and uni directional carbon together in say a 50/50 ratio. Because glass elongates more than carbon, all the initial loads goes on the carbon which will break first before any significant load goes to the glass. In short design a carbon component well or stick to high strength S glass/epoxy and you will get to the same point with a slight weight penalty.

 

[oldmulti]

Carbon fibre cross beams for a wharram Tiki 26 catamaran. Tiki 26 are tube cats 26 x 15.5 foot displacing 3000 lbs with 285 square foot of sail area. A british guy got feed up with wood cross beams so he ordered a set of carbon fibre cross beams from C-Tech New Zealand. The mast beam was 158 x 122 mm inside diameter with a 3.9 mm wall thickness. In the centre of the beam at the mast point there was an additional 9 layers of 200 gsm carbon cloth up to 1 meter long on the top (and bottom) of the beam for additional reinforcing. Inside the beam under the mast is a 40 mm carbon fibre tube to separate the faces. The ends of the beams at the hull attachment points were reinforced with extra carbon fibre wraps. The mast beam can take 40,000 lbs of down thrust with 8 mm of deflection.

The rear beam was 158 x 122 mm with 3.3 mm walls. The owner wanted this size of beams but C-Tech said the beams could have been 138 x 94 mm of the same build and be strong enough.

I got the above from Robert Hughes post on 11 april 2016 in Wharram builders and friends "Building techniques and solutions".

 

[oldmulti]

Ben Bolt a cliffhanger cruiser racer tri by Lex Nicol. The tri is 42 x 28 foot displacing 7000 lbs carrying 900 square foot of sail. The main hull has an aluminum tube space frame around the gunnels and along the keel and stem. When the hull was built and the internal frame attached glass was wrapped around the tubes which then "shrank" fit the hull tubes to the tubes. All rigging loads were basically taken by the perimeter tubes allowing the ulls just to deal with water forces. The tubes are 100 x 3 mm 5083 H32 (high strength aircraft grade). The 3 cross beams were also 100 mm tubes for the top and bottom trusses with 50 x 4 mm tubes acting as truss frames. The inner ends of the cross beams where they leave the main hull are 1 meter high, the outer end of the cross beams where they enter the float are 400 mm high. This boat has done thousands of miles of racing and cruising and looks as good as new but the cross arms need constant inspection for stress cracks as they have had to do some minor repairs on them over time.

The main and float hulls were constructed from 200 gsm kevlar 12 mm airex foam 200 gsm kevlar (doubled outside on main hull bottom to waterline). The decks were 330 gsm cloth, 300 gsm cloth 12 mm airex foam 200 gsm kevlar on the inside. The BH's were foam glass. The dagger board was aluminum with a 25 mm leading edge tube, 3 mm aluminum walls second vertical tube 75% aft of cord and some shaping BH's at stress points and to handle uphaul and down haul.

Comparing the Gringer R42 and Ben bolt. They are 35 years apart in design, i is in carbon fibre the other in kevlar and glass. I think in the real world there displacements would be similar. The R42 as slightly more stability and sail area and probably better hull shapes which will reduce pitching etc. But some old designs were light for there time and structurally sound. later designs have better hull shapes more efficient rigs and better weight distribution that allows less pitching etc therefore they are faster.

 

[trip the light fandango]

 

[trip the light fandango]

This is a really interesting thread, reading about all these classic old designs and building techniques , I'll have to do some reading on Lex Nicol.
Escape 10 caught my eye because it has similar camber or angles on its amas /floats to what I think I'll be setting mine on. From my novice backyard building eye they make sense to me. It may be because I worked at the boatyard mariner in Southport helping to fair an aluminium Ragamuffin for a bloke named Rod,(ex side car motorbike racer) who was the welder, who lived on a trimaran that had a similar look. each morning before work he'd say "remember, always work safe", nice bloke, I think it was a bright colour...a few months.. early 80's..anyway. I don't see modern trimarans with that much angle and wonder why. Thanks for the thread It's a beaut.

 

[oldmulti]

Hi. Trip the light fandango. You may be interested in the PDF attached. The boat was 35 x 31 foot and displaced 9000 lbs. It was raced hard by several owners and had no problems. It also was a good cruiser.

 

[oldmulti]

Lex float angle was more than most other designers. I think it came about because of the hard driving racers that crewed his other designs. The racers really pushed there boats hard and often either buried a float or in his lighter boats lifted the main hull (this was before rudders were on floats was standard) which caused a few control problems. Many designers have some angle on floats but it only needs to be 5 to 10 degrees if you are in cruising mode because that is the maximum you will will probably heal. Also if you are building with aluminum tube beams angling the floats requires extra work. Curved beams make it easier to angle a float.

 

[oldmulti]

Strip plank cedar is one of the slowest fast build methods I know of. If you can find long, uniform WRC strips it is a lot faster. Cutting up, planing down and joining WRC is the slow part. But the technique allows any hull shape and if done well last a long time. But like all boat construction techniques the design of the structures vary greatly. Turning Point a very successful Grainger tube racing cat 32 x 19.5 foot IMOR weight of 2800 lbs with 600 square feet of sail had a 200 gsm uni glass inside 8 mm WRC 200 gsm uni glass outside (doubled over bottom to WL) epoxy resin for hull and deck. The cross beams were aluminum. Compare this with Brady sailing cat (semi bridgedeck) 31 x 17 foot of 4500lbs displacement that had 600 gsm biax glass outside 12 mm WRC 400 gsm biax inside. The decks were 9 mm ply with 200 gsm cloth covering. The Grainger 30 x 19 foot solid semi wing cruiser racer cat Rum Tugger displaced 6000 lbs and had 565 square foot of sail had 330 gsm cloth inside 12 mm wrc 440 gsm double bias outside doubled over bottom to WL. This boat was fast, comfortable and reliable.

Shawn Arbor (now retired) one of the best boat builders in Australia said that WRC construction requires equal or thicker glass on the inside of the hull because all the stip plankers he repaired had fractured inside. He had designed (but not built to my knowledge) a 30 foot racing tri that was going to have hulls of 153 gsm uni glass 6 mm WRC 153 gsm uni glass. Adrenalin the F40 racing tri 40 x 40 foot displacing 4000 lbs with 900 square foot of sail had 200 gsm uni carbon fibre 9 mm WRC 200 gsm uni carbon fibre on the main hull. The floats were 200 gsm uni carbon fibre 2 layers of 2.5 mm oregan 200 gsm uni carbon fibre. This boat was still sailing in NZ in 2010 after 35 years.

Denny's Harrgami proa 35 x 18 foot displacing 3000 lbs used 440 gsm double bias 8 mm wrc 440 gsm double bias inside. Harry proa's structually are different to other designs because rob has localized the forces to the centre portions of his boats.

Graingers Flat Chat a very fast and successful tube racing cat of 40 x 25 feet displacing 5000 lbs with 1000 square feet of sail was originally built with 300 gsm uni glass outside 10 mm WRC 300 gsm uni glass inside worked well for a few years before it's hull split. It was thought the lack of some longitudinal glass to cover any weakness in the WRC (like a fracture or small knot) combined with only vertical uni glass caused the problem. A suggested layup was 400 gsm double bias either side of 10 mm WRC.

There is a discussion about the need to scarf the WRC strips or just depend on the edge gluing of the strips to provide the fore and aft strength. If its a heavy cruising design with thick WRC strips and a relatively thick skin then no scarfing may be OK but if its thin skins and thin WRC then scarf as the WRC is providing most of the fore aft strength of the hull.

 

[trip the light fandango]

Lex float angle was more than most other designers. I think it came about because of the hard driving racers that crewed his other designs. The racers really pushed there boats hard and often either buried a float or in his lighter boats lifted the main hull (this was before rudders were on floats was standard) which caused a few control problems. Many designers have some angle on floats but it only needs to be 5 to 10 degrees if you are in cruising mode because that is the maximum you will will probably heal. Also if you are building with aluminum tube beams angling the floats requires extra work. Curved beams make it easier to angle a float.

Those beams on Escape are really pleasing to the eye, lovely curves, if those floats were bigger ,the loads would increase but boy those designs would fly, and they look ageless to my untrained eye..I'm building larger floats for a Tremolino which has curved aluminium beams, for sort of micro cruising.. There's weather beam wind/ water deflection , maybe softer at rest, and perhaps a safety factor in swinging them out.. .But mainly it's the original design beams/ castings/ posts that make my new floats too deep [both displacing], so the further I swing them out the less wetted surface, anyway it's good fun. and great info, and a great file, thanks.
`In a couple of seasons I'll look at building wider beams cheers