Multihull Structure Thoughts

Next version of a 30 foot tri is an aggregation of several designs. This will be a trimaran 30 x 22 feet x 5000 lbs with about 500 sq ft of sail, composite construction. For a foam glass main hull and float 600 gsm biax 45/45, 12 mm airex (corecell etc) 450 gsm biax 45/45, done in vinylester, The biax is doubled below the waterline an additional 300 mm wide 450 gsm biax along the keel line. If the hull is Durakore or WRC 270 gsm biax 45/45, 13 mm durakpore or 12 mm WRC 270 gsm biax 45/45 doubled below the waterline with keel strip. Bulkheads 12 mm foam with 450 gsm biax glass either side or 9 mm ply with 19 x 45 mm timber edging. Gunnels on all hulls timber 19 x 50 mm. Keel on foam hulls timber 19 x 50 mm. Stem timber. Transom 2 layers of 9 mm ply. Decks 12 mm foam 450 gsm biax glass layup either side, preferably not Airex foam as it softens under high heat. Internal floors 9 mm ply, seats etc 6 mm ply, cabinet fronts 4 mm ply. Cross arms can be aluminum with water stays or timber or foam glass as previously shown in Twiggy mark 3 cross arms. Finally, we will do a high performance 30 foot tri after a small deviation about rudder shafts.

 

Rudders shafts is a difficult subject. They have to be very good as in most multihulls there may be up to 3 rudders. Rudders should be light but robust to handle the loads. BUT it depends on where the boat is going to be moored and the use of the boat. If you have a racing boat moored and sailed in deep water, good build a foam fiber glass or carbon fibre rudder system. Real world sailing is often being moored or sailed in semi shallow water which you may touch bottom or hit objects at some speed (eg sand banks, lumps of wood and other rubbish etc). So what does a designer do? 2 options for spade rudders. The rudder is bulletproof and will damage the hull if its hit hard enough. Second option is to develop a rudder that will bend or partially break if it hits something. I will give you the spade rudder shaft diameters for tri's from 28 foot to 33 foot with weights between 4500 to 6000 lbs. All shafts are 316 stainless steel tubes. 28 foot 33 mm OD with 3 mm wall, 29.5 foot 33 mm OD with 6.5 mm wall, 31 foot 37 mm OD 9 mm wall and 33 foot 45 mm with 3 mm wall. All the blades are WRC glassed over. One designer with thinner wall tubes is working on shafts bending if the boat hits something. These shafts will theoretically bend if the rudders are turned hard over at 25 knots (good luck if you do that in reality rudders will be the least of your concerns). Next designer is taking a half bet with a 6,5 mm wall. The final designer with the 9 mm wall tube has very solid support structure for the rudder shaft support base. If this rudder is hit hard enough it will cause damage to the hull. Again trust your designer and understand what he is trying to achieve. Final problem trying to find thick wall stainless steel shafts is not always easy.

 

Second part on rudder shafts. The reason stainless steel rudder tubes are better than many other materials is there ability to bend before breaking. Carbon fibre elongates 1% then breaks. Stainless tubes can bend 90 degrees and still not break. If a cat dries out on a falling tide it often is bumped up and down and dragged from side to side in the process if there is any wind or wave action. This often puts real pressure on spade rudder shafts. Stainless steel shafts are resilient. But as mentioned earlier there is a difference in the diameter and wall thicknesses of shafts. The thinner wall shafts often have a solid stainless steel insert of about 400 mm long "glued" at the point of the rudder shaft exiting the hull, 200 mm above and 200 mm below the keel line. If you don't choose to do the insert path then thicker rudder wall sections are required. How thick? EG. On a 55 foot cat a 75 mm OD tube with an 18 mm wall. Strong and heavy. The pictures shown a spade rudder that hit a whale and bent. Still fixable. Carbon fibre rudders and shaft would require a new rudder.

 

For those who have a real desire to understand marine composite construction the following 10 meg 380 page PDF will assist in understanding of glass, kevlar, carbon fibre structures materials and production techniques. http://www.ericgreeneassociates.com/images/MARINE_COMPOSITES.pdf

 

The Seacart 30 is the very definition of a high performance trimaran with minimal accommodation. The 30 x 21.6 foot, 2570 lbs tri carries 600 square foot of sail. The Seacart is very fast basically day sailor but is capable of things like round britian races etc. This boat has a very sophisticated build. It uses an autoclave with vacuum bagging. An autoclave is a pressure chamber that is heat controlled. This pressure chamber needs to be the size of a hull as the entire hull, cross beam or mast needs to "cooked" as one unit. The initial makers of the Seacart were carbon fibre mast makers who had a big autoclave. So we have an autoclave that can cook a hull up to 4 atmospheres of pressure. This is not back yard building. So the hull layup is 3 layers of 200 gsm unidirectional carbon on the outside of 12 mm Nomex with 285 gsm biax carbon on the inside. By using an autoclave you can get under 40% resin and over 60% carbon in your layup and the entire epoxy resin carbon mix is completely cured. This is where the big weight savings are achieved. The hulls have no gelcoat or is any prepreg material to save weight. If the autoclave is not used and normal vacuum bagging was done with gelcoats and prepregs the boat would have been 250 lbs heavier. The built weight of the boat is 1800 lbs. The boat is painted after assembly. Cross arms are carbon fibre tubes with 9 mm rod water stays under. There is not much point in detailing much more about this boat as it is not in realm of the average builder.

 

Tony Grainger does an interesting guide to composite construction at GUIDE TO COMPOSITE YACHT CONSTRUCTION https://www.graingerdesigns.net/guide-to-composite-yacht-construction/ It has many interesting points including not roughing up your layup to much if you are doing secondary bonding interesting stuff. Also have a look at his Raku 32 cat. Nice design.

 

That is a really useful guide , the water droplet height to determine surface energy is something we can all use, thanks. A simple scotch brite scourer can change the surface ,.. I think., if it smells it's probably active.

 

Two things that interested me about Grainger Raku 32 design was the shape of the hulls and where the mast beam is. The shape of the hulls makes building easier as the majority could be done with flat panels. After a search of Graingers web site I found a line plan of his 42 foot tri which indicates the same basic shape applies to tris as well. The proportions and rocker lines will be different but the concept is the same. Tony is very experienced at creating fast boats so I will assume the boats will be very good. The R42 tri is meant to be a very fast cruiser racer (more like a racer cruiser). The lines are attached. The mast beam on the Raku 32 is under the front of the double berths and is only about 400 mm high but very capable of taking the mast compression forces to the hull sides and associated bulkhead. This is very similar to Woods Flicka 35 foot cat design where the mast load are taken by a crossbeam under the front of saloon seats. The Woods beam is in timber. The Raku 32 beam is very likely to be a foam glass (or carbon) structure. These structures allow more flexibility in mast placement and interior layout. As has been said by Kurt Hughes "with enough money and carbon you can build anything."

 

Constant camber boat building was invented by Jim Brown john Marples and refined by Chris White. Constant Camber is the same as concept as Cylinder molding. That is if you need bent plywood then make bent plywood. The refinement of Constant Camber is you build a mold that has a bend in 2 directions then build up layers of thinner wood/ply to the hull thickness required. As one shape is used for all the hull sheets a short mold can be made and join the short panels can be joined together to form a hull length. The hull panels are then joined at the keel line and tortured into shape. The main advantage of this technique is you can use multi layers of lower quality timber to form a hull panel. The layers of timber all have the same shape and have epoxy spread over them and are then vacuum bagged to form a solid panel. the hull panels when together basically form a monoque hull structure requiring minimal stringers and framing. The disadvantage of the technique is the hull shape is not always optimal but it is soft riding. It can be used on cat and tri hulls. Thickness of hull panels are 10 foot 4.5 mm, 15 to 32 foot 9 mm if displacements are below 4000 lbs. 32 to 40 foot 12 mm. 40 to 45 foot 15 mm. 45 to 55 foot 18 mm if displacements are below 15000 lbs. 45 to 55 feet 18 mm. 55 to 65 foot 25 mm. And an 80 foot design that had a shell thickness of 40 mm. Below is a small diagram of a typical hull shape. A mold. A plywood layup on mold. A vacuum bagged panel and various build shots. The final shots are of a 3 meter tri , a 32 foot tri and a 64 foot tri built in a jungle named THAT (build story on the web) that are constant camber builds.

 

A bit more about Constant Camber boat building. This page will show you how to build a mold Shutterfly https://phantomboatworks.shutterfly.com/pictures/13 and attached is the original patent for constant camber. This is an easy way to build hulls but the finishing of the rest of the boat is the same. A few photos of a CC tri being built and an example of a cross beam required. The second patent US 602 etc was developed by Jim Brown et al before this patent was registered. It shows a refinement that allows asymmetric shapes.

 

I found this on Kurt Hughes site. It is on his blog and you will have to talk to Kurt Hughes if you want more detail of its applicability to any cat you may want to use it on. It is a folding system for a 30 foot tube cat. It is as simple as its gets, but remember it depends on the hinges being well designed and built. I do not know if any designs were built using this system.

 

Hydrofoil sailboats are thought to be a modern invention but they are just a modern refinement. Below are 3 photo's of interesting boats. The first one is Monitor, paid for by the US Navy. It was developed by Sam Bradfield and first sailed in 1957. Sam went on and developed several other boats including Scat. The second photo is of Willwaw a 32 foot plywood cruising tri built in the 60's and sailed in the late 60's and early 70's from California to New Zealand foiling when winds were above 15 knots. There is a book that has been recently re-released "Hydrofoil Voyager" written by Dave Keiper the designer, builder and sailor. The boat had a much smoother motion when foiling but its daily averages were no better than a well designed conventional 32 foot tri. The final photo is of a foiling proa at the 1972 speed sailing week. It could foil in both directions. Foiling sailboats are developing fast but for the average sailer let the professionals develop them first as those carbon foils are not cheap to develop or build. Each set of foils on an america's cup boat cost about $US400,000.

 

We have spoken about various glass materials for boat construction but the following from steam radio (an old web thread) is interesting. “We put Kevlar inside. It is partly because it is significantly better in tension than compression, but also when water penetrates the resin fibre matrix it acts like a wick and the water will migrate into the laminate – not something you want in a material that is very close to the water all the time. Small areas of damage that are not attended to can make this a problem. The third reason is that in an impact the outer skin will rupture – the foam will distort It is is either Airex R63.80 or Corecell A500 and the inside skin will stay intact even if the resin starts to fracture. The idea is that the foam and outer skin absorb a lot of the energy before it gets to the inner skin – which is ultra strong in tension. this combination has proved very successful in a number of cases of grounding or high impact on the hull.

Kevlar does need to be laid in a combination with glass, because the resin does not stick to the fibres very well. For the bigger boats we use a unidirectional fabric with alternate tows of kevlar and glass 50:50 by volume. This means that each layer is 660 gms/m2 Aramid/Glass – 237 gms/m2 Aramid, 422 Glass Unidirectional.”

Regards

John Shuttleworth
for Shuttleworth Design Ltd.

The part that interested me was "resin does not stick to the fibre very well". If Kevlar does not stick well to resin then you have a problem in its use. Needs further investigation.

 

One interesting production technique I liked was done on the Seawind 1200 which was produced in the early 2000's. Its main cross beam structure is an I beam with a top and bottom flange, The smart part of this is the top flange is part of the full bridge deck molding. The deck is a foam glass layup with a 15 mm foam core. The core is removed at the point of the crossbeam and solid glass flange is substituted in the deck molding. The flange is about 15 x 225 mm e-glass unidirectional across the full width of the deck mold. The inner skin of the deck is then laid up. The same applies to the underwing molding where the foam is removed and a solid glass flange is substituted at the point of the main bulkhead. The deck and underwing at this point is separated by a 20 mm plywood bulkhead that is taped to the deck and underwing mold. This is a very effective way of reducing the number of components to build separately and could be applied to a home build if you were willing to build big components in a shed then glass it all together. The photos show a Seawind 1200 and the mast sits on the forward end of the saloon head of the forward double bunks. PS most seawinds models have aluminum cross beams, the Seawind 1200 base boat came from another manufacture that went out of business. Seawind have replaced that model.

 

Builders are often surprised at how modern cats like the Lagoon 50, 40 etc. handle their mast compression loads on a bridgedeck. There is a Russian web site about the build of Mike Waller TC 670 that shows many detailed build photo's (2 photo's attached). The mast compression appears to go thru the middle of the cabin at the front of the central daggerboard case. But the front of the Double bunk (on left side photo) attached to the daggerboard case has a small cross beam attached and under the underwing has several very deep fore and aft underwing stringers that attach to the fore and aft cross beam structures. As indicated above in post 188 Richard Woods Flicka 35 cat has a cross beam under the front of a saloon seat. The beam goes between the inner gunnels. The Raku 32 has a beam under the front of the double bunks. Big french cat designers use some internal "furniture" and very big underwing stringers to discretely disperse the loads along and across the wing deck structure. It doesn't need to be very deep fore and aft stringers just very well designed. Need experts to design but is able to build by home builders. The attached Lagoon 50 show an interior shot with a metal post in the middle which supports the mast.