Multihull Structure Thoughts

A short note on plywood thickness. The Buccanner 24 (displacement claimed 2000 lbs but some boats weighed over 2,000 lbs) tri plans specify 3/16 inch ply floats including float decks and 3/16 mainhull sides. The Farrier TT 720 (displacement claimed 2,720 lbs) plans specify 4 mm ply float skins and 4 mm ply on the main hull sides above the 6 mm ply bottoms. Woods 24 foot Strider (weight 1200 lbs displacement 2000 lbs) catamaran can have a 4 mm ply skins on stringers with glass outside according to a Woods post when asked.

The 28/30 foot Firebird tri (smaller Wild Thing) plans specify 2 layers of 2.4 mm ply on floats skin with stringer backing. Jamez said at least one Firebird was constructed, based on a GBE main hull. Demon Tri a 23 or 24 footer had tortured ply floats and main hull (Gary Baigent will need to confirm) Jpegs of both below.

“In 1972 Tennant designed Bamboo Bomber, a 9.8m open bridgedeck racing catamaran, featuring stepped, flared hull sides and blister cabins. Journalist Gary Bagient requested Tennant make changes to this design, including fairing up the hull sides to enable construction from stressed plywood. Two boats to this design were built, Supplejack and Superbird. Gary Baigent launched Supplejack in 1977, and with her rotating plywood and foam wing mast, proved devastatingly fast in certain conditions” From: file:///C:/Users/SysOp/Downloads/malcolm_tennant.pdf

If you have slightly compounded 4 mm ply it has lasted very well on hundreds of boats, if you compound (torture) 4 mm ply it is a lot stiffer in both directions and acts like 6 mm ply on the flat. If you put 200 gsm carbon fibre either side of tortured 4 mm ply you end up with a better solution.

Please tell us if you have any experience with tortured plywood boats or have sailed on one. It is an interesting mode of construction and real knowledge and experience of the method will help.

 

My understanding is the (1 and only) Firebird has spent most of its life disassembled and in storage. I seem to recall it for sale on our ebay equivalent within the last couple of years - cheap.
As built it was the early version with smaller rig and floats. I thought I had a picture of it sailing somewhere but can't find it now. Malcolm updated it with a Firebird 2 design with a bigger rig and floats but I would be surpirised if one was built. Malcolm told me once that most of his small tri plans (and many of the cat plans) only sold a handfull of plan sets each. He also designed an Formula28 tri called a Stinger which would probably have been faster but looks like it had even less interior. Bit pointless today when a fast tri can still have some room.

BTW, my 7.6m tri is 6mm ply floats, 6mm mainhull, with 9mm bottom panel and cockpit floor, all covered with 200gm boatcloth.

 

The firebird mentioned above is mine, it didn't sell, Jamez, not surprising really. It is still in storage.... I described the issues with the design in an much earlier post on this series of posts by Oldmulti.
To answer the floats were built using the stress ply technique from 4mm gaboon ply and glassed BOTH sides with 200g glass cloth. they are probably around 150kg each. The main hull was an early solid glass GBE hull with a whole raft of issues around that.
Cheers Tb

 

I'm not doing a blog on this part. I enjoyed doing that on my Gougeon 32 rebuild, so I should be here too. I did a little study many years ago that I made a blog post about: http://www.ptwatercraft.com/Begining%20Tortured%20Plywood%201-6.pdf
Funny that you mentioned the electric power cat. I think that low horsepower multihull powerboats are where some future could be for tortured plywood. Hulls with less rocker and freeboard seem like they should be easier to build. I had the freeboard hollows that you talked about and also had hollow near the keel, both of which were forced flat with lots of sticks and hot glue. I think the freeboard and rocker caused the hollows. If I finish the book, I'll need photos and quotes from people who have done it. Photos like the one that Oldmulti posted with the couple hugging in the background and some backstory. I'll say that learning that some 8.5 cats are built with 4 mm has restored my faith. I wouldn't recommend doing a boat that big with 4 mm but it makes me feel not so nuts.
Thanks again for the encouragement.

 

The following is about a development of a catamaran with a step planning hull. The designer is Alan Alder, an engineer/inventor with 40 patents and has designed Etosha, a 60 foot aluminium ketch that was the first-to-finish monohull in the Transpac Race. After several years of development and testing a 14 foot 150 lbs catamaran was professionally built with a biplane rig. The main feature was a step in the hull about 50% from the bow that is ventilated by air drawn from above the waterline and feed by tubes to the step point.

The result of this is speed. When the cat is planning only about 20% of the hulls wetted surface is actually touching the water reducing drag significantly. Initial test sailing of the cat in 1992 showed it could keep up with windsurfers in stronger wind conditions. We are talking 20 plus knots. The downside in light winds there is a lot of wetted surface and the step causes drag which means a conventional cat hull can sail faster. On the cat the vent holes in the step and the air channel allow the step to rise free at much lower speed than is possible without them. Air is fed to the vents through a “trunk” just forward of the step. The trunk is sealed from the rest of the hull by bulkheads but is open at the deck. Air rushes in at the deck opening and down to the vents behind the step. The kink in the hull bottom slope behind the step works in conjunction with the upward slope of the wake at this point to achieve the optimum planning angle of about 4°. The lift/drag (L/D) ratio of the hull is about 10:1. When planning, the drag is mostly friction, and the faster the speed, the smaller the wetted surface as the hull lifts out of the water. This maintains the 10:1 L/D over a wide range of speed.

The development of the step planning hull was not a simple task. The original idea was initially drawn up and computer modelled (in 1990’s this was an evolving art form) then a 12 foot single hull test model was built. The model was made in two 6 foot sections. The aft section could be altered in height in relation to the forward section allowing varying heights of the step. The aft section also had windows near the step to view the water flow and turbulence. After tow tests the 12 foot model indicated the best step height for high speed and lower speed to allow the initial take off from displacement speeds to planning. The software was updated to get more accurate predictions compared to actual results. When satisfied with the results he applied for and was granted a U.S. Patent 5,191,853 (attached). The patent has expired and Alan Alder is enthusiastic for someone else to develop the idea.

Why the reluctance by others to develop the idea? Number one, was a French 60 foot 11,200 lbs biplane racing catamaran developed by Yves Parlier’s that sailed under various names. It performed very well under some conditions but was beaten by conventional racing tri’s overall across a range of conditions. Number two was the development of foiling cats and tris. The foilers ultimately can go faster across a range of conditions and are not as limited in light air conditions.

The hull shape is relatively simple for a fun day sailor but the various people who have tried to develop planning cats have not had great success. The main problem with other designs was high wetted surface and a hull shape that only really worked in stronger winds. The advantage of Alan Alder 14 foot cat is the vented step that improves planning speed but the other issues still remain.

The jpegs give the idea. The last 2 jpegs are of the French 60 foot biplane planning racing cat. The PDF is of the expired patent.

 

I seem to recall this stepped idea has been done before in cats but can’t think where, it’s almost old school in power boating.
While it’s all good fun in record chaser’s I can’t see it being much chop for ordinary off the beach sailing unless that beach is in Namibia.

 

This is an interesting concept of a 28 to 30 foot home designed cat. The conceptual designer had already owned and sailed several monohulls and 2 Wharram cats (22 foot and 26 foot). The Wharrams inspired his desire for a larger multihull. He liked the stability with reasonable performance and its ability to sail in shallow water. What he disliked about the Wharrams was its flexibility that Wharram values so highly. “The flexible boat suffers much more chafe and wear, cannot have a taut jib luff and consequently is compromised as a sailing boat.” The next thing he disliked was the accommodation layout for a 26 foot cat. “On my 26 foot Wharram, the galley has to be removed to access stores in the aft end of the port hull, and the navigation station has to be disassembled to access the berth in the starboard hull.”

So the guy started to conceive a new catamaran that addressed some of these problems. The first approach was to design a hull shape that suited his need for a sailing in shallow water. He conceived a Wharram inspired asymmetric chined hull shape that did not require boards. It was very thin. Next, he analyzed how to build the structure and found it was going to be a lot of parts if built in plywood and timber. He searched for alternate build approaches and concluded that tortured plywood or cylinder moulding would be easier after a learning curve. Using tortured ply needed a change to the hull shape and as a result it became a long thin symmetrical hull. There was not enough living space so “pods” were designed above the waterline. Logical so far.

The crossbeams and rig came next. The rig started out as a sloop with an A frame type cross beam structure. The 2 sloop jpegs give the idea of the proposed crossbeam structure. After an analysis of the concept, he found that it was a high load approach in both the crossbeams and rig that was going to be difficult to build and maintain. “The weak point of the modern catamaran, with the mast mounted on a beam between the two hulls, is the main beam. Any failure of the beam not only means the loss of the rig, it seriously compromises the structural integrity of the whole boat. The beam is horrendously stressed, and takes an enormous load at precisely the worst point. This cannot be a good thing, and one of the main things I would want to do in building my own catamaran would be to get rid of the mainbeam.” He decided to reduce the loads on the cross beams and rig by using a biplane rig on each hull.

Each mast of the biplane rig would have a 28 foot mast placed on a stub that is attached to the hull on the inner side to minimise intrusion into the hull. This will allow the masts to rotate or be able to “reef” the main by rolling it around the mast. This is the final concept. The internal accommodation is basically 4 berths, a small galley and toilet in the hulls and there is reasonable headroom which provides more of the accommodation wanted.

The final jpeg is of the proposed hull structure. Initially, he was going to try tortured plywood on the hulls but would change to cylinder moulding if he could not make the tortured ply approach work.

The jpegs give the idea. One person’s thoughts. He did not claim to be a structural designer and was willing to experiment with the build so be careful about the structure jpeg. No indication if it was built.

 

This is about an electric foiling catamaran. It is also evidence that you do not have to spend a large amount of money to achieve a viable boat. Chris Maas wanted a short distance commuter boat to buy groceries at a local store across the water. He went for a ride on a friend’s outboard powered 24 foot foiler catamaran and was inspired. He found a used Hobie cat 18 whose rig was little value, cost $200. The next decision was the motive power, Chris wanted to be environmentally friendly and choose electric power. Next was the decision about foiling, now the fun begins. Chris then said “I did have a basic design in mind before I started talking to Paul.” Paul was Paul Bieker of America Cup fame etc. It helps to have a guy down the road who knows a little about foiling.

Chris being a retired boatbuilder started to work on his commuter foiler. The cat hulls had a basic 6 mm ply seating pod built in between the hulls on the cross beams. Next came the foils. Now here is where Paul Bieker came to the fore. Paul had experience of immersed Moth T foils and as the cat foiler was likely to travel below 20 knots. So the Moth T foil shape was used for the main foil and is close to a NACA 63-214 profile and was modified from Bieker’s original dimensions by Maas to give the flaps on the trailing edge at the wingtips more authority in roll control. The foil measures 244cm (96”) span, 20cm (7.9”) chord, 28mm (1.1”) thickness. The flaps are 25% of the chord and 40cm (15.75”) long. The vertical struts are Eppler E836 hydrofoils with a maximum thickness of 25mm (0.98”) at 42.8% chord. All foils used were molded in female MDF molds, but for the main foil, Maas used a simple solid douglas fir core he covered with 15 layers of standard-modulus 200-gm2 (5.9-oz/sq-yd) unidirectional carbon fiber and one layer of ±45° plain-weave cloth carbon on the outside—all surplus from Boeing. The foils resin was epoxy initially room temperature cured then post-cured at 140˚F (60˚C) for eight hours.

For the aft foil, Maas repurposed a second hand Moth main foil with an MH 32 airfoil’s profile. That foil is made from solid unidirectional intermediate-modulus carbon with a layer of ±45° in the outer skin and has a flap that takes up 30% of the chord length. Both the main foil and the outboard foil are controlled by surface-sensing wands connected to pushrods made from his brother’s surplus carbon fiber tiller extensions. The wands are built from old carbon fiber windsurfer battens with attached pieces of PVC sewer pipe as paddles to keep the end of the wands at the water surface. The wands control the flaps on the foils which controls the cats flight height.

The motive power was going to be an electric outboard but as with the rest of this build Chris found an old two-cylinder Tohatsu outboard, with corroded engine innards. Maas didn’t care, because he was going to replace the combustion engine with a 10-kW permanent-magnet AC (PMAC) motor before extending the shaft by 18.1” (46cm) and attaching a 9.25” x 11” (23.49cm x 27.94cm) prop to suit his purpose. This was a step learning curve as “just” replacing the old engine with an electric motor required a lot of electrical control knowledge. The next second hand online acquisition was two 12-kWh Li-ion batteries destined for service in a Chevy Bolt electric car. After 400 hours there was enough of a foiling cat for testing. Initial testing indicated some flight height and control issues which with refinement were resolved. After the performance issues were resolved a formal canopy was added to the temporary seating and control pod.

With a lightship displacement of 990 lbs (450 kg) and a maximum payload is 550 lbs (250 kg), including the pilot the take off speed is 8.5 knots, which draws 9 kW. Cruising speed is 12 knots at 5.8 kW. Without foils, the cat manages 8.5 knots at the same power draw, which constitutes a 30% reduction of efficiency. Currently, GPS-recorded top speed (foiling) is 16 knots at 10 kW. Chris Maas says a maximum runtime of two hours at cruising speed before he has to return to the dock and plug in.

The result of this work, an excellent small commuter craft to pick up groceries. Chris use of second hand parts, some good design help and a lot of creative work produced this craft at minimal cost. It has achieved all his initial aims. A lot of fun. The jpegs give the idea of the foiling cat. The main information for this item came from Professional Boatbuilder magazine.

 

A bit more and a few additional shots of Gizmo here:

Two Unusual Multi-hulls https://www.boatdesign.net/threads/two-unusual-multi-hulls.66456/#post-922485

 

A simple item about a slide on camper company. A utility vehicle (pickup or SUV depending on where you live) can have a camper unit put the back of a 4 wheel drive for camping in remote locations. A slide on camper is a walled box that contains EG a double bed, kitchen and seating unit etc. Trayon an Australian company was asked by a few of its clients for “boating” version of the slide on camper so they could take it away for the weekend and float down a river.

The catamaran is 21 x 8.2 foot with a displacement of 3300 lbs. The length to beam of the slightly asymmetric hulls is 10 to 1. The draft is 1.6 foot. The power is Torqeedo electric 10.8 KW propulsion using four 26-104 Lithium-Ion batteries. The range is 80 kilometers at 5 kilometers/hour (16 hours duration).

The “accommodation” is basically a flat platform with a steering seat forward and protection panel. Aft of the steering seat is a 12 x 8 foot space that you can put a tent for the accommodation. With the usual camping equipment you can have camp beds, stove, portapotti, wash basin etc. Enough comfort for a weekend trip down a river etc.

The build is plywood, timber and covered with fiberglass. Originally it was going to be a production item but ended up as a set of plans for home builders.

The jpegs are of the camper catamaran and 3 of the “slide on camper concept”.

 

This tri has been featured before but this has further detail. The tri is the Cross 18, which was designed in the early 70’s by Norman Cross. The tri is 18 x 11.75 foot (which can be folded to 6.5 foot for trailering) with a weight of 400 lbs if built with thinner plywood specifications, about 500lbs if built with heavier optional plywood specifications. The 24 foot wooden or aluminium mast carries a 65 square foot jib and a 110 square foot mainsail. There have been builds that have carried larger Hobie 16 rigs of (26.5 foot mast, 215 square foot). The length to beam on the main hull at the waterline is 9.4 to 1. The length to beam on the floats is 12 to 1. Draft board down is 3.5 foot, draft with board up is 1.2 foot.

The open cockpit can sit 6 people but for fun sailing 1 or 2 is all that is required. Various sailors have made performance claims but the most realistic suggest 16 knots peaks and near wind speed in lighter wind ranges if you have good sails and a light boat. This boat is about fun and good performance not outright speed.

The build is basically plywood and timber on bulkheads and frames. The original design specified 1/8 inch (3.2 mm) plywood skins, glued and bronze anchorfast nails onto the frame. Many builders choose 4 mm plywood but others opted for the optional ¼ inch (6.4 mm) ply on the main hull skin. The bulkheads throughout are 6 mm plywood with minimal 18 x 18 mm framing support. There were timber stringers at the chines internally with a glass tape on the chines externally. The decks are 6 mm plywood throughout. The entire build on most boats were West epoxy saturated with a light (eg 200 gsm cloth layer) on the exterior. The main cross beams were laminated Douglas fir to about 150 x 65 mm tapering towards the ends of the beams. The hinge arrangements were originally 6 mm aluminium plates with four 8 mm bolts through the cross beams and a 10 mm bolt for pivoting with a 10 mm locking bolt. Some builders made the hinge plates out of 3 mm stainless steel. The foils (dagger board rudder) were made from plywood or Western red cedar wrapped in fiberglass.

An interesting option was Sailcraft sail kits that allowed you to sew your own sails at home. At the time the Cross 18 was designed it was a realistic option but now there are so many second hand beach cat rigs and sails available there would be little point.

These tri were fast and fun for family river/bay sailing or cruising. If anyone has access to the plans the thread would be interested. The jpegs give the idea of the build and the boat.

 

Cross 18 part 2 pics with a confused cat saying this boat is not a cat.

 

This is to give a small update on a later version of the Seaclipper 20 but also to show the differences in design approach between 2 well respected designers. Yesterday we saw the Cross 18 trailable wood trimaran. Today we will look at the Seaclipper 20. The Seaclipper 20 is 20 x 15.5 foot (can be folded to 8.5 foot) has a weight of 800 lbs with a load capacity of 600 lbs. The rig can be EG a Hobie 16 rig of 218 square foot. The main hull length to beam at the waterline is 10.3 to 1 with floats varying between 11 to 1 and 15 to 1 depending on the heal. The draft with board up is .9 foot and board down 3.2 foot.

Again the Seaclipper 20 is a plywood and timber construction with plywood bulkheads and timber crossbeams. The mainhull and floats have dory bottoms. And are designed for easy construction. The mainhull bottom is 10 mm ply, main and float hull sides 6 mm ply, main hull transom 18 mm ply with an 18 mm ply false transom mounting the rudder, bulkheads 6 mm ply with some timber framing. The hulls have a stringer, chine and gunnel timber. Main hull decks are 9 mm ply. The folding component of the forward cross beams are 250 x 50 mm timber on the flat. They could be laminated from 2 layers of 25 mm for extra stiffness. The mainhull beam support assembly is 2 layers of 250 x 50 mm timber. The rear beam components are 200 x 50 mm timber. Look at the PDF’s on the net to get the idea. The folding crossbeams use 12 mm stainless steel bolts as pivots and lock bolts. The float decks are 12 mm ply. There are additional 12mm ply discs on the float deck and support timbers on bulkhead under the deck where the cross arm attaches to the float. The float also has vertical ribs about every 500 mm for further hull support.

So Marples has hull skin thickness 6 and 10 mm compared to the Cross 18 3.2 mm or optional 4 mm or 6 mm. The Seaclipper decks that are 9 to 12 mm thick versus the Cross 18 6 mm thick. The main hull reinforcement in the Seaclipper is bulkheads and some longitudinal gunnel chine support but mainly vertical ribs versus the Cross 18 frames and longitudinal stringers, chines, gunnels. The crossbeams on the Seaclipper 20 are 50 x 250 mm or 50 x 200 mm planks on the flat versus the Cross 18 cross beams 150 x 65 mm on the vertical. Same concept of tri but very different structural approaches.

John Marples tends to design strong boats capable of absorbing a few knocks. He also thinks that the work involved in doing a EG light deck structure with deck stringers and many deck beams is not worth the effort compared to a heavier deck plywood with less supporting structure. The weight gain is minimal and people have more confidence walking on the deck. Both Norm Cross and John Marples are excellent designers and both the Seaclipper 20 and Cross 18 have done thousands of sailing miles and lasted well. Both these tris perform very well. Each designer has their own approach to design and there is no “one” answer in design of any boat. You have to match your statement of requirements to what a designer can or wants to provide. As Dick Newick wife said “Why don’t you give the customer what they want?” Dicks response “Because it is not fast”.

The update on the Seaclipper 20. John Marples has designed a SC20 ‘Sport’ version. The “Sport” has comfy seats, a small dry cuddy cabin to put your picnic kit, cloths and other stuff in. The cuddy cabin also provides wind and spray protection. The seats accommodate four persons with plenty of room and they are long enough to sleep on. When operating the boat, nobody has to move when you tack, and the motor can be operated while steering in the seated position. The jib sheets come to the centre of the cuddy cabin top so either of the forward seated crew can make adjustments, or if sailing alone, all the strings are within an arm’s reach.

The jpegs show the original Seaclipper 20 study plans, some build jpegs and a jpeg of the new “Sport” version.

 

The final for now of 18 to 20 foot tris and different designers approaches to the same size of boat. Erik Henseval designed the Sardine run 5.5 tri which is 18 x 14 foot that can be disassembled to 8 foot wide. The weight is 660 lbs and displacement 1200 lbs. The main sail is 150 square foot, self tacking jib is 64 square foot and Gennaker is 173 square foot.. Again, a Hobie 16 rig or larger depending on your courage. The main hull has a length to beam at the waterline of 10.5 to 1 and the floats are 15 to 1. The draft is 1.1 foot to 2.5 foot boards down (as the dagger boards are in the floats the draft can be deeper when heeled). Outboard is 2 to 4 HP.

The 18 foot Sardine Run is slightly different to the Seaclipper 20 and Cross 18 in that it has a minimal cabin space (maximum width 2.8 foot, floor width 1.1 foot). The “cabin” can have 2 bunks (slim people only) and storage space for a camp stove, portapotti etc. The cabin has 4.3 foot of headroom. This is a space to get out of the rain or contain the child while sailing. The downside is that you have no formal cockpit. The trampolines are the best option.

But the main feature of this design is its performance capability. It will out sail a Seaclipper 20 and Cross 18 under most conditions due to its better hull shapes. Eric Henseval used to work for Van-Peteghem Lauriot-Prevost (VPLP). Among other things, he was in charge of the shapes of the Groupama 2, Franck Cammas' 60 foot trimarans. He adapted his knowledge of fast trimaran shapes into a simple to build tri for the general public. As a guide the Sardine Run 18 has touched 20 knots and has a Bruce number of 1.43. It can handle full sail up to 22 knots at 1200 lbs displacement and the standard rig.

And now we get to the real issue. The Sardine Run 18 foot tri is brilliantly designed for home building in plywood. Henseval thicken the plywood skin and minimised the framing to eliminate a lot of the secondary framing structure required in a lot of other designs. The cross beams are just straight aluminum tubes bolted on to the hulls. The build time required for this tri would be 50% of the build time for a Seaclipper 20, Cross 18 or Trailer Tri 18. But please be warned, the hull and deck shape of the Sardine Run 18 is very important in the strength of the overall structure. You can only go so far in not having a frame or stringer support on a panel of plywood. Each panel twist and corner shape add to the tris strength.

The actual structure is a dory main hull that has a flat bottom. The hulls are built from 8 mm plywood with tapped seams. The shape of the hull and thickness of plywood provides most of the longitudinal stiffness. There are 3 bulkheads or partial bulkheads to provide shape to the main hull. The chines and seams are 450 gsm double bias in epoxy and the exterior of the hull and floats are 300 gsm satin cloth in epoxy. West epoxy saturation is used throughout. The full length cross beams are 100 x 3 mm aluminum tube with waterstays. The beams are held onto the main hull by stainless steel saddles or 10 mm bolts directly to the hull. The floats are bolted with two 10 mm bolts at each float joint. The cross beam structure is lighter than any of the other tri’s mentioned and can be built in half the time.

Now the big downside. This tri is transportable but not as easy to put into the water. You have to assemble it at your new location. A very nice tri that performs well and is very simple to build.

 

Part 2 of Sardine Run 18 foot. This section is about a sliding beam option built by a Canadian. I do not know if this is approved by Eric Henseval. Canadian built Sardine Run 18 sliding aluminium beams arrangements are seen in the first 4 jpegs. The sliding beam concept works as long as you have water stays to handle the bending loads. The beams themselves then only have to handle the compression loads and can have a minimum joint overlap (about 200 mm) in this case). Also sliding beams have another problem of binding if you do not expand the tri with the hulls remaining parallel.

There are some internal jpegs of the main hull accommodation.

Also, I have included a jpeg of one person’s attempt to add more space to a Sardine Run 18. A nice attempt but as I said the shape of the main hull of this tri gives a lot of strength. Any change to the cabin top will require additional strengthening and reinforcements which will add weight and complexity of the build. The space gain is marginal. Also the offset cross beams to allow compressing the boat for trailing is a good idea but again adds weight and build time. This is one design that should be built as designed.