Multihull Structure Thoughts

That's what I was doing today, making extensions to the cases, after reading this I might give the rest of each case a little more wriggle room, Lanolin grease is good...

 

Dick Newick was a very creative designer with superb trimarans and proa's to his name. But at times he did some other designs that were not well know but had some interesting aspects. EG Vaka Fanaua a 15m (49.2') "island freighter" trimaran for Pacific outer island communities. The boat is power by a Lug schooner rig. Lug rigs were used by british fishing boats up to 80 feet long some had over 2000 square foot of sail and were controlled by 2 men. Lug rigs only weakness is upwind where they can only point to about 50 degrees but on all other points of sail they are powerful and relatively cheap to build. Newick also did a 26 foot tri for a home builder using a lug rig. Sorry I cannot find an image on the web at the moment. Dick also had a personal 16 x 11.5 foot trimaran that he experimented with. The boat had the option of 2 lug rigs as well as a crab claw rig. Photo's below, Dick is the guy in yellow. Each of the rigs proved to be able to drive the tri to over 10 knots carrying 2 adults and a child. Newicks creativity was a loss to multihulls.

 

The 26 foot lug sail trimaran mention in the previous post was: "Dick Newick suggested a lug sail on his "Back to Basics" or B2 25' trimaran, for a really quick and easy setup, but I'm not aware that one was ever built. It did have a life with a conventional rig as the Argonauta transportable trimaran."

 

Older multihull designs often had more conventional monohull type rigs than modern 3 wire fractional multihull rigs. The older rigs could have tighter forestays and were more able to control mast bend than modern fractional rigs. Result they could point really high if they had good boards and had a stiff enough platform to mount the rig on. Modern fractional rigs can be mounted on more flexible tube cats, trimarans and if correctly setup with rotating masts etc will go to windward just as well as the older style of rigs. Both achieve the same thing but the monohull type rigs require more bits (like hydraulics, runners etc) to optimize them. Attached is an example of an older multihull rig.

 

Thanks for all the great info. I'm absorbing as mush as possible to use in my design.

 

One aspect of home building that is annoying is fitout equipment EG hatches, turning blocks, winches, sinks, heads etc. Do you buy equipment upfront and build to that equipment by installing the right size support blocks or inserts into foam sandwich decks etc. Some items like fridges, heads need fixed size enclosure etc. But boat builds often take years and equipment manufacturers constantly change products, so you build an area in a boat to fit sales brochure dimensions but years later you cannot buy the product. Most boat builders do not buy equipment up front, build a boat then add support blocks etc later which adds weight and may not be structurally good. Many people choose to build a boat and build equipment themselves to suit the boat. EG make there own block, hatches, windows, ice boxes etc. This allows the boat builder to design all the components and attachment points on the boat before they start to build which reduces additional work later. John Marples did a little book "Constant camber and seaclipper trimaran construction and outfitting" in 1981 that details many home built solutions to blocks, anchors, cleats, steps, draws spray hoods etc. Summary. If you are going to design/build a boat think about all the equipment the boat requires prior to commencing building. It will save a lot of fitout hassles later. An example of home built blocks below. You can make stainless steel versions just as easily.

 

Nick Cruz is a New Zealand designer that does some interesting smaller designs for home builders. I like this boat because it is simple should be easy to build and have just enough room for a head and maybe 1 bunk. The trimaran is 21 x 14.75 feet folding to 8.5 feet. It displaces 1100 lbs and carries 285 square foot of sail. The boat is basically ply and timber and a day cat should provide the rig for the boat. The cross arms are a 150 x 50 mm timber (or lamibeam if you cannot find quality timber) covered with glass as per attached study view. The "hinges" are stainless steel. The "hinges" are basically in compression as any tension load is taken by the deema water stays. A boat that appears to be simple but has some good design features.

 

I have spoken about rudders before and its a builders choice as to what purpose a boat is being used. Shallow waters a kickup rudder, deeper waters a spade rudder. Spade rudders are more efficient and often lighter. Only in small boats are kickup rudders easier to build. Both types of rudders, if correctly designed, give equal control. I have sailed on several boats of the attached design and found no difference in control between a kickup rudder and a spade rudder boats. Modern spade rudders are built of lighter materials and simpler top bearing arrangements. But the major forces on spade rudders is two fold, high speed surfing down the face of a wave and the more problematic drying out when sailing into a beach or sitting on ground due to mooring etc. When drying out spade rudders don't just touch down once, they often hit bottom many times and are dragged from side to side by wind or waves etc. as the boat settles. Older materials take abuse netter than carbon fibre or foam glass rudders.

 

Foam fiberglass is often the preferred building material for a multihull. Foam glass can be easy. Hand laid fiberglass over a panel of flat foam is easy to do, but hard to do well. The choice of resin needs to suit the type of fiberglass you are choosing to use. The foam needs to suit the resin. Does the foam need to be sealed with a slurry to minimize weight gain as some closed cell foams have large cells when cut open. Once you have the materials sorted then you have to make the part. I will assume a simple flat bulkhead (BH). The choice is make a large panel then cut out many smaller components or build a panels to size. If its a BH do you want to trim around the edge of the foam to allow any taping to be "hidden" when built. Are any inserts required in the BH to take equipment or provide an opening. You then have to ensure that all components of the BH are smooth, flat and dust free. Next cut out the fiberglass to slightly oversize including any strengthening layers etc. Label the glass and store it in order of use preferably with EG plastic separation layers. Now the important part. Calculate how much resin is required to theoretically make the part and add 10%. A good hand layup will be 50% glass, 50% resin. An excellent layup will be more glass than resin. A average layup will be more resin than glass. Next decision is, do you put resin down first then lay the glass over the top or if its only EG 1 layer put the glass on first and push the resin thru. In either case buy good compression rollers and learn how to use them. Also have extension handles for compression rollers for the hard to reach parts. learning how to hand lay fiberglass with good resin to glass ratio's is not hard but takes experience. The better the glass to resin ratio is the stronger the layup, as long as all the glass is wetout. Given a choice between resin rich and inconsistent wetout take resin rich. Finally please do layups in the correct temperatures and humidity ranges. Next time we will discuss vacuum bagging followed by resin infusion.

 

Vacuum bagging comes in 2 forms: Cheap and occasionally effective and properly done. Probably done vacuum bagging information is available across the web. The document below describes it in detail. I will describe the cheap process that you can use for small parts or experiments. If your going to do a bulkhead or a hull do the proper process. Cheap needs a vacuum pump, a flat surface or a mold, a resin trap, some tubing (strong garden hose), some hose connectors, agricultural thickness plastic, some bubble wrap and duct tape. The vacuum pump can be an industrial vacuum cleaner (cheap as it is very likely to burn out) or if your can find a real vacuum pump great. If you have a used shipping container company near you, the refrigeration units on old containers work very well as vacuum pumps. Build a resin trap, a piece of 100 mm diameter plastic water pipe about 150 mm long with screw caps on either end. At the top end glue/screw a fitting that will draw air out of the resin trap to the pump. At the bottom end glue/screw a fitting that will draw air/resin in from the vacuum bag. Cut tubing to go from the bag to the resin trap and from the resin trap to the pump. Tape a fitting to the center of the bag. The flat surface or mold needs to be clean and waxed several times. Cut out a plastic bag larger than the mold. Cut duct tapes to length to stick the plastic bag to a surface to seal the bag. lay down EG your foam then put a premeasured amount of resin on it. Lay your fiberglass over the top. Lay some bubble wrap over the top. Put the bag material over the materials, tape the edges and start you vacuum pump. If its polyester resin leave it running for 2 plus hours but check you cannot hear any leaks in the bag or around the edges, apply more tape if required. The up side of the cheap technique is you should get a good layup that will have excess resin on the surface. The down side is if it has worked well the surface will have the pattern of the breather material require some sanding to finish the part. Alternatively with the cheap method you can put the glass on the bottom surface followed by foam on top and vacuum it down. Upside smooth finish surface, downside you may draw resin onto the top foam surface that will have to be sanded off. Finally if you have no vacuum gauge you are only guessing at the pressure being pulled and cheap pumps often cannot be controlled. As long its pulls a 1 PSI vacuum it will work. Do not underestimate the clean up of equipment after each run or you will be replacing a lot of gear. Do not use cheap nylon suit lining material as a "peal ply" layer it rarely works and can cause a real mess.

 

Rudder attachment systems vary from store brought bits and pieces to specially made attachment points. If you break an attachment component is a remote location you have a problem. But Wharram's designers did a great bit of thinking with a simple rope attachment technique that works on boats up to about 40 foot. The attached is for a 30 foot boat. I have heard of no complaints with this system and it can be replaced anywhere. Some boats have 3 rope hinges others have 4. Study the plan details especially how not to have the rope rubbing against each other and the need to silicone some parts of the rope to the skeg. Simple, effective and maintainable. The other plan is of a more conventional outboard rudder attachment for a 33 foot tri. Finding the bits to buy for the tri rudder is getting harder.

 

Whilst we have mentioned Wharram simple system for rudders lets look at his evolution of cross beams. The beams construction started simple, went complex then simplified again. We will stick with boats about 24 to 30 feet. The Tane 28 had 4 solid timber beams of 75 x 150 mm timber built from 50 mm laminations. They were lashed to the hulls with low stretch rope. Next came the Tiki 26/30 cross beams which were complex in shape (triangular and I beam) and build as per attached. Then followed an upgrade cross arm plan for older classic designs, this is an I beam design used on many wharram designs from 26 foot up to 60 plus feet (more timber as boats get larger) attached 31 foot plan. Now with the Mana 24 the crossbeams have become simple plywood boxes with reinforcing timbers at the corners. Each beam type carries the boat forces as required but each beam type has become progressively lighter for a given strength. Also the ply box beams are as simple to build as the first original solid beams but a lot lighter. Also later beams are being lashed on with webbing which stretches far less. PS on bigger wharrams there is a large bolt thru the cross beams at the hull centreline attaching the beams to BH's. Rope lashings are added at gunnel edges for additional support.

 

Kurt Hughes is a talented designer who understands engineering, has built many personal boats and will ask experts if he doesn't understand something. A study of his designs often gives insights into good approaches to building structures. His blog is also worth a read as he often comments on new materials and occasionally why structures worked or failed. Below is 2 samples from his web sight of how he is doing some popular trimaran structures. The 40 foot SH tri can be done in ply or glass and at one point was the design of his personal boat. Please do not assume they are the exact design specs but they give a guide. A look thru his web site is useful.

 

Small cats designs vary from simple plywood boats to very well engineered structures. The following shuttleworth design is in the very well engineered class. The basis of this boat was designed in the 80's and was advanced for its time. These boats require a good builder who is prepared to build the boat as designed. The designer specified the direction of glass layers through out the structure and expected good fiberglass resin wet out ratio's. If it could be vacuum bagged all the better. If good rigs were put on these boats they were very fast. An open bridge deck version was real good. Study the material list it will tell you a lot about this boat. 30 years later many modern designs are not to this standard of design.

 

The following 20 x 14 foot weighting 1100 lbs displacing 1700 lbs with 320 square feet of sail cruising cat is an excellent example of alternative thinking. It was designed by an Italian couple who become architects. It was intended to be fun and cheap. Why is it original? Its 6 mm ply tortured hull gives room for a double birth in each hull. The plywood for the hull was soaked and dried many times to bend a shape into the ply before torturing. The freestanding biplane masts are 30 foot long with the lower portion 100 x 4 mm tubes and the top 7 foot a 92 mm tube inserted in the top of the 100 mm tube. The boat can be trailed (with a bit of work). The deck is also 6 mm ply. Internal fitout does a lot of the strengthening. There is an internal 75 x 20 mm gunnel full length with an external gunnel of 75 x 20 mm between the beams. The mast supports at the keel and deck is 15 mm ply across the hull. The aluminum cross beams are 180 mm diameter tubes attached with reinforcing straps. The length to beam ratio is nearly 12 to 1 and the prismatic is 0.62. These are good numbers. Sorry about the word document but I cannot find this on the web anymore.