Multihull Structure Thoughts

Ross Turner designed and built his Coral Coaster 29 in 1983, and sailed her until 2005. The full bridge deck centre cockpit cat is 29 x 15.5 foot weighing 2700 lbs and displacing at full load 5400 lbs. The rig is a mast head with a 153 square foot main and 312 square foot genoa. The CC29 sailed between Australia and New Caledonia (915 miles of open ocean) and has done thousands of miles of coastal sailing. This is an ocean capable boat that is fast (8 knot averages 15 knots occasionally) if keep under the maximum displacement. All Ross’s designs (Jarcat 5, 6 and 7 as well as the CC 29) are based on the same plywood over timber technique which has proven to give a strong, fair hull at minimal weight, effort and cost. Epoxy glue and stainless steel screws are used with marine grade plywood and western red cedar stringers and frame edgings. Epoxy & thin fibreglass cloth and epoxy resin is applied over the entire exterior, and painted with epoxy undercoat and polyurethane top coat gives many years of maintenance free use. Unfortunately plans for this design are no longer available.

Ross Turner was a genius at wooden boat design and construction as his smaller Jarcat range shows. If you build this boat to plan you would get a very strong ocean capable boat. The single chine hulls topsides are 6 mm ply with 9 mm ply bottoms that are glued and screwed every 100 mm to chine logs and gunnels. The boat is covered by 85 gsm glass cloth and epoxy. The bulkheads are 9 mm ply with stringers and some intermediate frames. The sterns are 12 mm ply. The underwing and cockpit seats are 9 mm plywood. The deck cabin is 6 mm plywood with framing support. The side windows are 6 mm acrylic. The forward windows are 10 mm acrylic. Internal furniture is 4 mm ply for shelves, doors, seat fronts etc.

Catsketcher sat in a nice Coral Coaster 29 built by Jack Langois of Gosford. She was faster than she looked like she should be and had a lot of room for a 29 footer. The accommodation is out of the ordinary and needs a fair bit of calisthenics to get into the main table area (because it has sitting headroom only, you move between the hulls via the cockpit if you want full headroom). That said the boat worked well and Jack is happy with his boat.

 

The Grainger Alfresco 1060 is a sports cruiser semi bridge deck catamaran of 35 x 22.5 foot that displaces 6700 lbs carrying 764 square foot in the main and fore triangle. The first 2 of these boats that were built virtually side by side and had full headroom deck cabins built on them before they hit the water. These boats were the reason that Grainger decided to either full race cats or bridge deck cabin cruiser racers. One of the cats had a smooth curve joining the wing deck to the hull the other cat had a flat chamfer panel joining the wing deck to the hull. The flat chamfer panel cat was a bit faster and had a smoother ride in side by side sailing. Small things can make interesting differences.

This design had at least 2 iterations. The initial design had hulls have a 10.8:1 length to beam ratio. The second version had hulls with a 9:1 length to beam ratio. A more realistic assessment of what people put in boats probably made the designer widen the hulls. The hulls of the initial boats were 2 layers of 270 gsm cloth 15 mm WRC 2 layers of 270 gsm cloth in epoxy. The layup is doubled below the waterline. The bridge deck was 18 mm duracore and glass. The decks and subsequent deck cabin were foam glass. The main cross beam structure is a combination of 2 web bulkheads 1.2 meters apart with unidirectional glass top and bottom of the “large” box. The mast sits on a bulkhead between the 2 webs. This was changed with the main deck cabin modifications. I have more details but the boats I saw being built were “evolved” as they were built which leaves me in doubt as to what was designed versus what was created by the builders so I will refrain for further structural information. The boats sailed well but I suspect their displacements were heavier than designed. The first jpeg is of the narrower hull version and the other 2 jpegs are of the later wider hull versions.

 

I was very impressed with the Alresco 920s Twos Wild and Rum Tum Tugger. When we returned from cruising on our Twiggy trimaran we wanted something like the 920 or the Fastback 30. I corresponded with Grainger about a larger version of the 1060. He said he could stretch it for me and we thought very hard about it. I asked him about the lack of space in the dinette/galley area and stated that we could have much more room if the aft end of the cabin was made vertical and the accommodation extended aft. I was told that could not happen and we decided to not go ahead with a Grainger. Then I read an 8 year old mag (gotta keep your old mags) and saw an OSTAC ad for a Parallax 11 - a very lovely boat with no bridgedeck cabin. I rang the designer Robin Chamberlin and built an open bridgedeck 38 footer - which I still love 23 years later.

Open bridgedeck cats seem like a good idea but in reality they are a bit of a pain. If it is wet, you want cover from the rain or spray, if sunny, you need cover from the sun, if the toilet is in one hull, you need to get dressed to go from your bunk to the dunny. If your son sleepwalks (one of ours did we found out) you have to lock them inside their hull at night. Our semi bridgdeck cat had two different solid cockpit covers before I eventually built a foam cabin. Now she looks pretty similar to most other cats but you can see well from inside and so we usually sit inside when voyaging, a vestige left over from her open bridgedeck days.

 

YoungGrumpy. I have looked at the Fish and Chips plan and agree you could do dagger boards BUT it will effect the seating and accommodation plan available. Dagger boards are easier in theory but it depends on the dagger board case you make and how you install it. Having built a glass dagger case and a centerboard case there is not much in it. As the jpeg below will give you an idea of the work involved in just glassing in position the top of a dagger case let alone even more work required to do the bottom of a dagger board case you can get the idea why it may be easier to stick with the standard plan. A Fish and Chips is going to be a fast cruiser but will not be a racer and as a result the designed boards will be satisfactory for the boat. If you want all out speed up wind a really well shaped board will have more effect than brute area. Good luck on your building bug.

 

Catsketcher mentioned he was looking for another design, he liked the Alfresco 920 design (30 x 19 foot displacing 5700 lbs with 585 square foot of sail, structural details on page 20 number 292) and then asked for a larger version of the Alfresco 1060 from Grainger. Tony Grainger was reluctant to make the changes requested but Grainger eventually drew a larger version for a Belgium client called the Alfresco 430. The 430 is 43 x 26 foot displacing 15400 lbs and carrying a 573 square foot main and 418 square foot genoa. The hull length to beam is 9.5:1. The Alfresco 430, Bossanova, built in NZ, gained a reputation as a fast cruiser but occasionally was wet, as water went over the bow, pass the main crossbeam and down into open entrance hatches. Tony Grainger has lost the plans for this boat but basically it is a WRC glass hulls with plywood bulkheads and under wing. The decks were ply foam ply which was the fashion in NZ in the late 90's. During the initial test sail in NZ in 1997 Bossanover did 17 knots reaching in 25 knots of wind and 8.5 knots upwind under a full rig with 9 people on board. The accommodation has a wrap around seat that can sit 10 people and the galley etc is large and practical. The Alfresco 430 then morphed into the G430 full bridge deck cabin cat that basically had the same hulls but more bridge accommodation, slightly more displacement and sail area.

Due to the 920’s excellent sailing ability and good design I suspect other designers were inspired. At the same stage Robin Chamberlin after having designed the Parallax 11 designed a smaller version called the Parallax 9. The 9 was 29.5 x 18 foot weighing 4000 lbs displacing 5200 lbs carrying a 305 square foot main and a 310 square foot drifter reacher on a 40 foot aluminium fractional rig mast. The boat was foam glass with triaxial, biaxial cloths and vinylester resins throughout as it was built in production moulds. Having walked aboard both the 920 and Parallax 9, the 920 suited my style better having a bit more room where required. In a race between the 920 and Parallax 9, both with dagger boards and similar rigs, I would suspect it would depend on the skipper more than the boat as to which was faster. We will discuss the Parallax 11 (which is also the Corsair 3600 and extended version Parallax 11.6) in the next item.

 

Thanks for the info on the Alfresco's OM. My collection of material begins in the early 2000s so that fills in a lot of gaps. You can see how the Mystery Cove boats (with designed in BD cabins) evolved from them. Tony Grainger must have been pretty pissed with people adding their own cabins because he still mentions it on his website. TG seems to have designed other series' that were relatively short lived such as the single step cabin 10, 10.6, 11.8 and 12.2 which were available in the late 2000s. More recently several other types have appeared on his site only to quickly disappear such as the Barefoot and Flying Fish. His current focus seems to be the Raku series which look pretty nice.

 

Jamez. I thought the attached PDF would be of interest. I will be getting back to some older Grainger designs soon.

 

The Parallax 11 started as a semi bridge deck catamaran similar to its daughter the Parallax 9. But the Parallax 11 morphed very quickly into a full cabin bridge deck catamaran. The 11 was 36 x 20 foot initially weighing 8500 lbs with a 48 foot mast carrying a 456 square foot main and a 383 square foot genoa. The Parallax is composite sandwich construction, Divinycell core vacuum-bagged with Triaxial and Biaxial composites; even the bulkheads and kitchen counters are foam cored to save weight. The structure of the female molded cat hulls were 225 CSM, 576 gsm triax 15 mm divinycell, 410 gsm biax in vinylester resin. The layup was doubled below the waterline. The deck/cab layup 225 CSM, 576 gsm triax 12 mm divinycell, 410 gsm biax in vinylester resin. The internal furniture is 225 csm, 410 gsm biax, 8 mm divinycell 410 gsm biax. The Parallax 11 are fully integrated structurally sound cats.

According to one owner “The boat can be a very good performer when lightly loaded. On a recent trip to Bermuda in a little over three days, which is very good. The Parallax is only happy if left to her anorexic tendencies. Heavily laden as she will be for the trip to the Caribbean, she will be if not slow, laboured.”

The Parallax 11 morphed into the Corsair 3600 about 1992 with slightly widen hulls to take a greater payload. The claimed displacement of the 3600 varied between 9900 lbs and 11200 lbs, so the wider hulls were needed. An extended version called the Corsair 4100 was proposed being a Corsair 3600 having an additional 4 feet inserted in the middle to improve the space in the main deck cabin and a foot added to the stern for an overall length of 41 foot. A Corsair 3600 has circumnavigated doing over 25,000 plus miles surviving many storms. The cat averaged 7 to 8 knots in reasonable winds and peaked at 18 knots when surfing or lightly loaded. Finally there was a Parallax 11.6 which was 2 foot longer and 1 foot wider but slightly lighter at 9000 lbs.

 

The Mystery Cove 380 was one of Tony Graingers more popular full bridge deck cruising catamarans. The cat is 38 x 23 foot displacing at the dead water line 15800 lbs. The rig is fractional carrying a 513 square foot main and a 367 square foot number 1. The main and foretriangle is 798 square foot. The hull length to beam is 10:1. This boat can sail well and handle a reasonable payload. The structure of the boat is mainly foam glass. The hulls are 850 gsm quadax, 450 gsm 45/45 biax, 20 mm divinycell foam, 600 gsm quadax, 450 gsm 45/45 biax. The underwing is 2 layers of 600 gsm 45/45 biax, 25 mm divinycell, 2 layers 600 gsm 45/45 biax. The cabin top is 600 gsm quadax, 450 gsm 45/45 biax, 25 mm divinycell foam, 600 gsm quadax, 450 gsm 45/45 biax. Intermediate bulkheads are 2 layers of 450 gsm biax either side of 15 mm divinycell foam. The webs on the main bulkhead 2 layers of 600 gsm biax either side of 20 mm divinycell foam with a top and bottom flange of unidirectional glass.

There were at least 3 variants of this boat. One variation is a strip plank cedar hull home builders version with some plywood bulkheads. The “production” version as above and a narrower beam version of 21 foot that displaced 13200 lbs.

 

Frantic Drift is a one off tri built in the late 90’s by a NZ guy who wanted fast tri for the 8.5 meter class. The tri was extended to 30 foot to fit an outboard. The tri is 30 x 24.6 foot weighing 1570 lbs (including rig). The carbon mast is 45 foot high with a cloud of sail. This sounds very light and what’s with the sail area. This tri on a boat test in 10 knots of wind recorded a maximum speed of 19.8 knots using a reacher. The owner claims the tri is capable of 20 plus knots, really? This is a pure racing machine with effectively 3 limited berths and not much else. The sail area is Kevlar, spectra with mast head kites and screachers etc. The main hull is from the outside 2 layers of unidirectional 200 gsm Kevlar, 10 mm foam, 200 gsm carbon cloth inside. The main hull bottom has an additional Kevlar 45/45 on the bottom all one in epoxy. The floats are 175 gsm Kevlar cloth, 200 gsm e-glass cloth, 10 mm foam, 200 gsm carbon cloth inside. The cross beams are ply, cedar, fiberglass with carbon flanges. The foils are fiberglass and carbon. All this adds up to a boat that accelerates like hell and responds to any puffs etc. needing the helms person and crew to be very aware of what is going on as the first jpeg shows. Good main hull shape.

Now the part that really worries me. This boat is crewed by 2 Olympians, so no lack of talent. The results of the 2011 Coastal Classic 125 nautical mile race. A 60 foot OMRA racing tri won in 5 hrs 44 mins, a 50 foot all carbon “cruising” cat second in 7 hrs 24 mins, a 40 foot Grainger racing cat third in 7 hrs 52 mins, followed by 3 x 8.5 meter cats finishing 8 hrs 30 mins to 9 hrs 48 mins etc. Frantic Drift came in 10 th in 10 hrs 14 mins. It obvious that this tri is too much of “cruiser” for NZ. No wonder NZ thinks that if you’re not up to America’s Cup standard in sailing, your not trying hard enough. Frantic Drift only averaged 12.2 knots, Attitude 13.8 knots, Dirty Deeds a 8.5 meter averaged 14.7 knots, the OMRA 60 racing tri averaged 21.7 knots over 125 miles. Frantic Drift would be a winner in most fleets around the world but in NZ it’s just a fast boat that’s beaten by 4 mm tortured plywood 28 foot cats like Attitude.

 

Oldmulti,
appreciate you answering.
My only concerns now with the centerboard would be a) fouling/marine growth - daggerboard slots on my Seawind 24 here require regular cleaning, or I have a hard time moving them in or out
b) control lines access - but that is more of a search for a better solution.
Then it is to the holy grail of a small cat - "one pin, two bolts from trailer to water"- search. What Ray is offering is probably better that what I have, but should be improved.

 

YoungGrumpy. Please look at page 29 number 426 on this thread for ideas on folding cats. Fish and Chips hulls are good but the one pin 2 bolt search is every ones holy grail. Takeaway is one option but there are others. Even Seawind 24 builders built a one of folding version that folded like Thomas Firth Jones 23 foot Brine Shrimp. The standard cross beams were cut in half, aluminum plates welded on the cut ends and a hinge arrangement attached. It worked OK. The boats performance was unaffected. The jpegs are of Simpsons Takeaway.

 

Tom Speer wrote this in 2013 to explain how a tri float should be designed in relation to the main hull at various wind speed strengths. A foiling tri implied in the design which is interesting as Tom got a little side lined from his aerospace career, where he did aircraft foil design, into doing work for Oracle’s first big wing mast America’s Cup tri USA 17. He said he did the initial calculations of the wing mast foil at his daughter’s wedding! The blue lines in the diagrams below are the heel and trim angles as the wind speed increases, the black lines are the apparent wind speed. What it shows is as the wind speed increase the heel angle increases and the bow trim angle is increasingly negative. This implies either the float centre of buoyancy has to move forward or the float is moved forward.

Tom Speers words from here “When designing the size and position of the amas, you should construct a mulithull footprint diagram like this:


It shows the virtual center of gravity location (the c.g. location that would provide the same pitch and roll moments as the sail rig) (black lines). It also shows the combined center of buoyancy of the two hulls as a function of pitch and heel angles, with the total displacement equal to the weight (blue lines). (You can include the down-force from the rig if you want to get really fancy.) Since the virtual c.g. and the center of buoyancy have to coincide, you can read off the heel and trim angles for each sailing condition.

You can also convert stability indices, such as from Shuttleworth, and do the same thing.


Diagonal capsize is generally the most critical condition for a trimaran. The footprint plot helps to strike the right balance between pitchpole, diagonal, and sideways stability.”

Next we will discuss one solution to the dynamics of tri designs The jpeg is USA 17.

 

The way that Malcolm Tennant overcame the problem of diagonal stability in a trimaran was aggressive thinking for its day. He shifted the floats forward in relation to the main hull. The float bows were up to 4 feet in front of the main hull bow. Modern trimarans achieve increased diagonal stability by shifting the centre of float buoyancy forward and moving the centre of gravity of the main hull (entire boat) and rig aft.

I don’t know the chronological order of the designs but 3 Tennant trimaran designs appeared in a short period. ‘s The 23 foot Demon Tricycle, the 31 or 28.5 foot Firebird and the 39 foot Wild Thing. Wild Thing was one of the first strip plank cedar multihulls built. Firebird was basically a Great Barrier Express catamaran hull used as a main hull with floats attached and Demon Tricycle main hull was limited by the length of 3 sheets of 8 x 4 foot plywood.

The build of each varies. Demon Tricycle is 26 x 21 foot overall with a 23 foot main hull and 30 foot mast and about 260 square foot sail area. DT is a 4.8 mm tortured plywood for all 3 hulls with aluminium cross beams. The builder initially wanted an open cockpit boat but the design morphed into a cabin boat after initial sailings The design then was formalised by Tennant to be a full cabin boat shown in the jpegs. This boat was/possible still is fast. It could out sail standard Buccaneer 24’s and in racing has given a few highly modified Buccaneer 24’s a hard time.

Next came Firebird a 31 x 23.5 foot overall with a 28 foot main hull displacing 2000 lbs with a 36 foot 149 x 99 mm mast carrying 395 square foot of sail. This is the most well known forward float designs. The main hull was 2 layers of 4.5 mm ply with 8 mm ply bulkheads and 12 mm ply main crossbeam bulkhead. The main hull keel is 25 x 100 mm timber, the gunnels are 25 x 50 mm. The floats are 2 layers of 2.4 mm plywood with 8 mm bulkheads. The float keel is 30 x 60 mm. The main hull and float decks are 2 layers of 4.5 mm ply. Bunks and cabin soles are 8 mm ply. The fore and aft crossbeams are 154 x 105 mm 2.6 lbs/foot mast tubes. The dolphin striker goes from float gunnel to float gunnel and is a stainless steel strap.

We will discuss Wild Thing in the next item along with Oracle 17. These boats have more in common than you would think. The jpegs are of Demon Tricycle and an owners report and Firebird study and photo. The final 2 jpegs are Malcolm Tennants explanation of the float forward thinking.

 

Malcolm Tennant’s Wild Thing was a radical boat for its day being both a float forward concept and one of the first multihulls publicly built with strip plank cedar unidirectional glass construction. Wild Thing is 38.4 x 29.5 foot displacing 4000 lbs carrying 675 square foot of sail area. The tri was designed for sheltered water sailing with low freeboard and minimal accommodation. By shifting the floats forward in relation to the main hull Tennant gained diagonal stability for hard sailing. The tri could carry a full rig in stronger winds without lee bowing. This tri was lightly built with longitudinal wrc strips and unidirectional e glass at 90 degrees inside and out. The cross arms were aluminium with stainless steel dolphin striker.

What’s the relationship between this 40 year old design and Oracle 17 AC tri? Look at Oracle’s hull and float configuration. Oracle 17 was designed to beat a rule of a 90 foot waterline. So, the main hull is 90 foot long. The floats were 113 foot long, the tris beam was 90 foot and it displaced 38000 lbs. The solid 2 part wing mast ended up being 226 foot high. Now Oracle 17 could place its floats in any position it wanted but choose to have float bows ahead of the main hull bow. Tom Speer diagrams on page 43 indicates the thinking that was going on inside Oracle camp. With a rig that big that continuously brought the wind forward of the beam (USA 17 could do 28 knots boat speed in 10 knots of wind speed) the boat was always going upwind and could easily be overpowered with slight gusts. It was better to have the floats centre of buoyancy ahead of the main hulls centre of buoyancy to help hold the bows up in stronger gusts.

The next generation of AC cats that foiled have a similar design dilemma; how much lift do you have the forward foils handle versus the lift generated by the rear foils. As the rig powers up the forward overturning or tripping component increases. Do you add “spare” foil capacity to provide additional lift but accept extra foil drag at lower speeds or do you optimise and accept you will have a very sensitive foiling boat to handle in stronger winds. In the first iteration of foiling cats they were not allowed to have moving foils but later AC boats had foils that could move a couple of degrees to generate additional lift. Result smaller boats capable of higher speed but tricker to handle.

Wild Thing was a precursor solution to the same issues that Oracle 17 faced. The jpegs are of a couple different Wild Things and of the initial WRC Wild Thing being built. Oracle USA 17 jpegs next.