Multihull Structure Thoughts

A timely reminder, the baffles I'm making to stop water rushing up and down the extra skin/added hull bilge,..they could act as an apex especially when beaching , I'll add extra tape to both skins at those stations and give the baffles more thought...another quality post ,thanks.

 

The following plan gives details of how foam glass ribs were constructed in the 80's. These effectively strengthen hulls with large unsupported panels therefore minimizing the number of bulkheads required for support. The materials have changed (eg very little CSM and better foams) but the concept is the same. The advantage of a rib is that a hull can have a lighter support structure and have more internal space for storage etc. If you find that your boat hull is to flexible in area eg bow then a rib or ribs can stiffen up a structure in that area. Ribs can be applied to ply, glass or foam glass hulls or under decks. The plan also shows how a home builder can build a fairly large mast from a basic tube to a full mast with some stainless steel and correctly sized bolts. The mast head unit can be made by any competent welder. Again detail is very important. Smooth and round all edges and holes thru stainless steel to avoid any stress points. Do not mismatch materials EG use 316 stainless for all parts. Try and avoid welding across a tang if you are going to use a doubler plate, only weld around the edges. Any bolts thru a mast should have compression tubes on the inside to stop the mast being crushed by over tightening the bolts. Please use aluminum spreaders not wood or inspect your wood spreaders frequently.

 

OM, you're doing a really good job on this thread-thanks!

 

Klis 3 is an interesting trimaran that has been developed over 3 iterations. The original Klis 22 x 14 foot weighing 1100 lbs displacing 2200 lbs carrying 260 square foot of sail. crossed the Atlantic holding the single handed record for a decade in the 60's. Klis eventually sailed across the pacific to New Zealand. Klis was constructed of 6 mm ply with 9 mm ply decks with stringers and frames. The crossbeams were 100 x 150 mm Columbia pine boxes bolted to gunwales that were doubled to accept the loads. The mast was a 150 x 75 mm solid pine mast. The rudder blade and centre board were 9 mm aluminum. Bernard Rhodes Klis 3 designer upgraded the design to 24 x 17 foot weighting 1200 lbs displacing 2400 lbs carrying 380 square foot of sail. The hull and floats are still 6 mm ply (with some torturing) and decks 9 mm ply with frames and stringers. The cross arms now are a top and bottom timbers of 25 x 225 mm oregan glassed over. The timbers are separated by 300 mm at the inboard end and meet at the outboard end at the float. The cross arms have stainless steel fish plates with 6 mm bolts on the inboard end to allow the floats to be taken off for trailing. The mast is now a 28 foot mast with a set of diamonds. The really interesting change has been in the centreboard. Instead of a single board the boards have been moved to the floats and angled to provide lift as well resistance. According an owner and Rhodes who have sailed the original Klis and Klis 3, Klis 3 is closer winded, faster and more seakindly. The boards and rudders are built in timber. There have been at least 6 Klis 3's built in 3 countries and all are well used. This is a minimalist ocean capable cruiser. It may not be pretty but it does its job well.

 

Chainplates for boats can be stainless steel bolted onto the structure of the boat or can be a composite with a stainless steel reinforcing ring for attaching the shroud to. Some example diagrams are below. An EG a 20,000 lbs breaking strain shroud would be attached to a stainless steel chainplate that would have a breaking strain of 30000 lbs plus. The stainless steel EG 30 x 6 mm chainplate would be attached to a bulkhead with five 9 mm bolts etc. A composite chainplate would use either e glass, s glass or carbon uni 100 mm wide wrapped around a stainless steel thimble then thru a slot in the deck onto a bulkhead and/or hull side over 600 mm down. Uni directional Eglass will be thicker to obtain the same strength and more resistant to flexing due to its thickness. The resins must be a good quality epoxy. Epoxy resin and uni directional eglass has about the same elongation factor of 4.5%. Design the composite chainplate with a safety factor of 5 times the breaking strain of the shroud attached to it.

 

The attached document gives a overview of the issues in yacht design and structures by many professionals in the field. It helps put things in context and gives some ideas about composite chainplates etc. Its long but useful.

 

Saw such chain plates in a Bernd Kohler K-designs KD860 (page 2) slide show video at ± 2:52 to ± 3:05.

 

I found an interesting "production" carbon fibre chainplate from Flink design in the UK. Also another variant on a replacement chain plate for a 41 foot monohull.

 

Daggerboard cases come in a few formats. Many plywood and early home build designs have plywood cases that have ply sides a timber front and back with some reinforcing timbers on the side and at the top and bottom. These cases need to have all pieces of the cases epoxy sealed really well and glassed prior to assembly. Once the dagger case is in place, interior maintenance is very hard. Remember the forces on these cases is substantial which will reveal any poor building. Also the water forces that build up in these cases at speed is substantial. Finally work out if you are going to have a timber insert in the bottom of square shape case. The insert should be the shape of the foil to minimize any noise and minimize water ingress into the case. Alternatively the top half of the board s left square to fit in case with only the bottom half of the board shaped like a foil. More noise, more water sloshing around in the case. The attached gives a timber case in a foam glass hull.

 

The Tiki 30 masts vary a bit depending on the builder. The standard design says a 31 foot tall timber box mast of 170 x 170 mm built from 25 x 150 mm walls and 25 x 170 mm side walls. In each internal corner a 40 x 40 mm triangle. The mast is then rounded to 160 mm. Alternatively a timber birdsmouth mast is built from 25 mm timber wall material. You can choose a 33 foot aluminium round tube mast of 150 x 3.5 mm, another builder used an aluminum tube mast of 125 x 4 mm claiming he had consulted the designers. Finally a builder paid for a Ctech carbon fibre mast 30 foot high of 125 mm internal diameter tube with a 3 mm walls for his Tiki 30. All boats are running the standard gaff wing sail. Tiki 30's are good sailing boats which work with any mast type but less weight aloft helps reduce pitching. But the most extreme Tiki 30 rig/mast is PHA a transatlantic crossing cat. PHA has a freestanding mast on each hull that measures 33 foot high is 200 mm wide at the deck 100 mm wide at the tip and is built from 12 slats of 25 mm white pine, rounded and wrapped in 2 layers of biaxale cloth with a layer of carbon fibre from the mast base to 1 meter above deck level. This boat sails well and was the basis of a Tiki 46 foot freestanding mast wharram.

 

Oldmulti; I would like to say a robust 'thank you' for all the information on this thread. I actually just now joined this group (my first post actually) to say 'thanks'. I'm a long time (production boat) multihull owner, but I'm a freshman hack when it comes to working with composites in large formats.

I have a (perhaps newbie) question about the use of the UD for the hull reinforcement shown in the 06/25 12:00 AM post above. How would you keep the fibers parallel to the side curves shown in the image? Would it be acceptable to foldback the UD near the deck creating multiple layers at the top and thus essentially keeping the fiber orientation truly perpendicular to the thimble? Or is it sufficient to use the UD with a vertical orientation and just cut the shape as needed?

Regards - Coastal Ogre

 

Coastal Ogre. Welcome and good to hear from you. I am making an assumption we are talking about composite chain plates. Thread entries are reported to me in local Australian time, result an entry you see as the 05/26 may have been made by me on 05/25 in my time zone. UD from the thimble need to kept as near perpendicular as practical and be spread over the biggest surface area possible. The problem is that the loads are large from a rig and in foam glass boats the UD attaching to an inner skin is depending on the inner skin glass foam shear capacity. Foam glass does not support localized loads well which is the reason timber or solid glass inserts are done under winches etc. Result you have to spread the UD load over a large area and place it in the direction the load is coming from. I hope I have answered the question please contact again if I have misunderstood the post.

 

Hull shapes vary depending on the task. Some of the latest research into foiling trimaran hull shapes is that for takeoff mode, people were assuming a flat planning hull would be best but the tank testing and real world results are showing elliptical hull shapes provide the best transition from sailing mode to foiling mode. On real world boats hull shapes are often dictated by the type of materials you have to work with. EG plywood boats often have chine hulls, foam glass boats have round bilge hulls etc. Early multihulls favored fine ends EG Wharram boats but over time designers put asymmetric shapes for and aft to minimize pitching eventually flattening the aft end of hulls to gain speed across the range. Hull shapes have now more refined getting a balance between speed and comfort. Below are 3 different hull shapes each works very well for there purpose. The Echo trimaran is an older design by newick but is fast and seakindly, later newicks had wider sterns. Klis is the 3rd generation of a small tri which had a simple but fast (for its size) hull shape. The 226 hull is from a 42 foot cruiser racer catamaran that has evolved from 30 years of design work. The shape is relatively fuller in the bow and stern than earlier designs and is a seakindly fast boat.

 

Oldmulti; you did indeed answer the question, thanks. It was in regards to the modified trapezoid shown in Composite chainplate4.jpg. I've got some BH re-tabbing in my near future, which then brings me face-to-face with the inner hull where the large vertical exterior chain plate is bolted to the side hull of my production cat. Been investigating how to provide additional factor of safety for this component.

Didn't think about the time zones showing up differently in the imbedded posts Learn something new everyday, I say!

Regards - Coastal Ogre

 

A second format for daggerboard cases is fiberglass or partial foam glass daggerboard cases. First you create your daggerboard EG a foil shaped western red cedar board wrapped in glass with some vertical uni directional glass for added strength. When boards get big a strip of aluminum down the back edge strengthens the back edge and minimizes back edge dings. A layer of cardboard or any smooth plyable non porous material is wrapped around board then a glass case is molded over it. A gelcoat or internal paint layer should be put in side the case. The cured case is removed and placed in the hull with a lot of reinforcement around the base of the board case. That means if its a foam glass hull a solid filler is put between the hull skins where the case is to be placed. Also plywood floors and partial bulkheads are placed around the base of the case to reinforce the area. The case is taped strongly to the hull and bulkheads. Finally the real trick is to do a layer of glass from inside the case to the outside skin to stop water access between a constantly flexing case and hull. Also remember you will have to put antifoul up inside the case to waterline level if the boat is moored. Finally the uphaul rope to lift the board need a snug fit in a vertical slot in the board or the case will have to have a rope slot round in it. If the uphaul rope can move from its slot it can jamb the board in its case. Can be a small problem. The plan below gives details of 40 foot cats case and boards.