Multihull Structure Thoughts

Part 2 of the CC 37 jpegs with 2 CC 40 study prints

 

Many years ago I bought the study prints to the CC26. The drawings are so beautiful I have them on the wall of the lounge. It is a spectacularly beautiful boat and by all accounts sails very well.

So why didn't I build it ?

Because the veneers it is designed for are not readily available in Australia. It would be incredibly expensive to build. Many times the cost of foam, even before I found the chinese suppliers.

Incidentally I think it's constant camber, not chamber. The hull sections have a constant double curve which makes them very strong and light. You are essentially making your own double curved plywood. This method made sense 30 years ago, but less so now unless you can find the timber sheets.

 

A request came for a comparison of the TR 36 and CC 37 “performance” and what would be the difference between to two tri’s. Please understand my simple performance calculators work on implied ratio’s not actual hull resistance and rig power. IOMR and EG Texel rating have closer performance parameters but do not match the VLPL type actual resistance calculators that take in every variation of hull shape for resistance and a calculation of the rig power based on EG wing mast, sail cloth, cord ratio etc. With that qualification I will give some general conclusions based on my simple calculator that works on things like displacement length ratio, sail area displacement ratio, waterline length, stability and rig overturning moment.

So, we will add 1000 lbs to the weight of each tri for its sailing displacement in “racing trim”. There will be 2 rig configurations investigated, the first is with main and genoa and the second configuration is with the main and screecher.

The basic CC 37 has a waterline length of 35 foot, a beam of 24 foot, a displacement of 8,500 lbs and a stability of 89,250 foot lbs. In main and genoa configuration the sail area is 807 square foot, with main and screecher the CC 37 sail area is 1180 square foot. The centre of effort of the rig is 22 foot.

The TR 36 has a waterline length of 36 foot, a beam of 27 foot, a displacement of 7,500 lbs and a stability of 93,750 foot lbs. In main and genoa configuration the sail area is 786 square foot, with main and screecher the TR 36 sail area is 998 square foot. The centre of effort of the rig is 24 foot due to the TR 36 taller mast.

The performance results theoretically are fairly close. With the main and genoa up the TR 36 is 4 % faster than the CC 37. With main and screecher up the CC 37 is theoretically 1 % faster than the TR 36 due to having 18% more sail area. But this is not the real truth. This may be the reality in winds below 10 knots but at higher wind speeds other factors start to come into play.

Factor 1. The TR 36 has 5% more righting moment but is 13% (1000 lbs) lighter than the CC 37. This is due to the TR 36 having 3 foot wider beam.

Factor 2. The TR 36 carries its smaller sail area higher up than the CC 37 in each rig configuration.

Factor 3. Hull shapes. This is the real difference between the 2 tris, in moderate to higher wind speeds (from about 10 knots above). The displacement length of the TR 36 is 23 % better than the CC 37 because the TR 36 is lighter on a longer waterline. Also, the length to beam on the main hull of the TR 36 is 10.2 to 1. The length to beam on the CC 37 main hull is 7.55 to 1. The float of the TR 36 length to beam is 16 to 1 versus the CC 37 float length to beam in heavy airs is 12 to1.

Factor 4. The prismatic coefficient of the TR 36 is higher than the CC 37. This means the TR36 has the buoyancy pushed more to the main hull ends which minimizes a deep or full mid section. Deep and or full midsections cause drag at higher speeds. The same logic applies to the floats. Also higher prismatic fuller ended hulls tend to pitch less if correctly designed.

A small confirmation of the above is provided by the owner of the CC 37 who said “When the lee float is immersed enough (when you push her hard till the deck of the lee float is almost under) she will accelerate because the main hull is lifted a bit and her speed comes in the much higher (15 plus knot) regions.” Translation, when the main hull lifts it reduces the relatively fat and deep main hull drag and put the sailing pressure on the relatively thinner float hull.

Overall summary. I want a race between these two tris (both are in Holland). I very strongly suspect the TR 36 will be faster than the CC 37 around a race course and over a longer distance. The TR 36 hull shapes are less constrained by the build technique and have been designed 4 years ago. The CC 37 was designed over 25 years ago, designers have learnt a lot in the interim. Given a choice, I would cruise in a CC 37 without a problem. If I wanted to racing or just go fast, I would choose the TR 36. The CC 37 will average only slightly slower than the TR 36 in real cruising. But the TR 36 will go a lot faster when racing and especially in stronger winds.

PS I partially agree with Guzzi3 about the supply of suitable veneers to build a constant camber tri. But the big advantage of the technique is you can use lower quality veneers in the build process. The optimum is WRC veneers but EG 3 or 4 mm cheap exterior ply can be used in larger sizes of constant camber boats. There may be a slight weight disadvantage but you are still going to get a good strong hull that can hit a wharf or touch bottom without too many problems.

 

It is possible you could get kiri thin enough from china to build one. I've not seen the layups for the bigger boats so you might be able to use available thicknesses on them but teh 26 required stuff so thin is was hard to get and would have been incredibly expensive.

One thing I did consider was building the panels in strip plank. It might sound mad but if you think about it there need be little or no spiling, so the panels could be produced quickly. You lose multiaxial strength but it's easy to get that with the glass skins.

But then why not just do a strip design ? But then why not just go foam ?

Too many choices

 

I’ve often wondered why not CC panels from foam or balsa? It’s a viable as any of the other quick building methods. Being able to bag the laminates would be much easier than laying glass down on foam shaped over forms, especially the inside.

Marples has created a wonderful range of capable cruiser racers. I think there are a few more extreme racers out there too. Also, PAT’S is a CC design.

RonB.

 

Are there no multihulls built in strip planked Paulownia. Have just started using this nice wood and it is nice work with.

 

Guzzis3 spoke about the difficulty of obtaining materials for a Marples constant camber 26. He likes the design but it would have been expensive to build and his conclusion was foam glass may be cheaper. Guzzis3 also thought a possibility may be strip planking over a constant camber mould. Let’s look at the Marples CC 26 and DC3 folding trimarans. The Marples CC 26 is 25.3 x 18 foot and can fold to 8 foot for trailing. The weight is 2,000 lbs and displacement is 2,800 lbs. The 30 foot mast carries 308 square foot in the main and foretriangle. The DC 3 is 27 x 17.3 foot and can swing wing down to 8.5 foot for trailing. The weight is 2,400 lbs and the displacement is 3,000 lbs. The 32 foot mast carries 407square foot in the main and foretriangle.

The reason for doing this item is both these tris are John Marples designs and use constant camber hull panels as there base of the hull construction. The panels in the main hulls are 9 mm thick and preferably built from 3 mm veneers or thinner of WRC. The DC3 says it can be built from EG 3 layers of 3 mm plywood although this is not preferred. The bulkheads and frames are plywood with some timber backing. There is one major stringer fore and aft in each hull side. Floors, bunks, shelves etc are glued or taped to the hull side to reinforce the hull structure. The deck on the CC26 is also constant camber 9 mm thick. Due to the swing wing approach the decks on the DC 3 floats are plywood.

So now it comes down to how cheaply you can obtain the timber materials. Many years ago, I found an Australian company that imported 3 mm WRC veneers. The company found a US firm cutting the veneers for a “lightweight plywood”. It cost a lot but it was required for the cat that was being done. There are several sources of thin plywood and veneers around but the timber type, quality or glue lines may not be suitable. If you have access to a plywood manufacture talk to them about a “roll” of veneer, they may sell you one. Plywood manufactures use in effect a knife to cut a veneer of a rotating log, some manufacturers do a continuous cut and “roll” eg 100 foot of veneer. Some core veneers are 3 mm plus mm thick. Again, timber quality may not be good.

Did I ever tell you about a guy I knew who did a strip plank monohull in Pinus Radiata (junk pine) and sailed around the world without a problem. I am not recommending junk wood panels in larger sizes but for smaller boats the strength of the material is often of less importance than the stiffness of the panel. Stiffness is general developed by thickness of the panel and or curvature. Constant camber panels work by being “thick” and having a 2 dimensional curvature in them. As a comparison there are 28 foot coastal cats that have 4 mm plywood hull skins. These tris have 9 mm hull skins, relatively the tri hulls are thick.

Guzzis3 also thought about using a constant camber and lay strip plank cedar strips over the top. Possible, but constant camber moulds have curves in 2 dimensions on them so some of the strips may have to be shaped to get the full panel layup. The idea is worth a thought as EG cedar wall panelling is 10 mm thick, fairly easy to obtain and by the time it was sanded down it would be about 9 mm thick. The only downside I can think of this approach is you can only glass one surface on the mould then you would have to build a support frame to support the panel of the mould then you would glass the other face. As Guzzis3 said it may be easier just to do a conventional strip plank hull.

The absolute beauty of constant camber construction is once the panels are made; the build is relatively fast because you have a minimum of stringers floors frames etc to build. Just build the hull shapes and glue/tape any bunks, shelves etc to the hull surface. The second advantage of Marples designs is he is a very practical designer EG on his 16 foot tris he use 9 mm plywood on decks to minimise the amount of structural framing to support someone to walk on the deck. Marples designs for a minimum of parts and those parts be strong/stiff enough to be installed without additional framing etc. The best example of Marples skills is the simple swing wing cross arm system he has done for a range of his tris. Simple planks of timber and bolts. Stuff you could buy at a local hardware store.

Constant camber is a good build system, it produces good strong long lasting boats if well designed and built. The jpegs are of the CC 26 and DC 3.

 

I remember people doing it years ago but couldn't point you at a specific case. Lots of small boats got built using it and I think it remains popular for surfboards.

Constant camber hull shapes are not ideal. They are ok but compromised somewhat to accommodate the method. They aren't terrible hull forms, but if you step away from the method it makes little sense to retain the compromises. The strength has a lot to do with double curvature and relatively thick skins.

As has been said so many times before people get way too hung up on hull skins. They make up a small part of most builds, so saving money or time here makes little difference to the whole build, but you are stuck with that decision because the hull is the base upon which the boat is built. You can start with a second hand rig and change it later. Cheap fridge or stove..swap it out later. Second hand rudder swap it. The one part you can't change is the hull, so this is the bit you have to consider carefully. It affects not only your use of the boat but resale also. Strip plank farrier tris usually get a little less money than the same boat in foam, for example.

Foam used to be prohibitively expensive, now you can get PVC foam cheap from China. I know I keep banging on about it, and I know the numbers vary around the world but being able to build full length panels and tape them together to make a pretty light hull in any size that will never rot or rust and even be UV proof provided you paint over epoxy or use vinylester or even polyester. You get a low maintenance light stiff hull, a great base upon which to build your dream boat that will outlast you.

Regarding CC foam, I thought about this. It has some appeal. If you scored the foam it'd take the shape and I'd infuse it. You might need to do some fairing. The trouble is if you step back and think about what's involved compared to for example vertical strip foam, the Farrier method, the build would not be much slower and no constraints on hull form.

Richard Woods has a whole range of cats that can be all flat or single curve foam down to the chine, then you usually have a choice of dory, multichine or round bottom hulls. This means that even if you go rounded you only have to make one reasonably straightforward mold. Some of the "cuddy cabin" tops are double curved but you could glass one side, score, position the roof sticky tape in position, fillet an glass tape inside then glass over the outside draping it to attach to the sides. They are very practical boats to build.

 

I was always under the impression most CC builds were done in plywood ?
I don’t think veneer has ever been cheap even when it was more available, at that time plywood and good quality plywood at that was cheap.

 

This is not so much about the tri but about the build blog of a specific F-28SR. A F-28SR is 28.4 x 19.7 foot and can be folded to 8.25 foot for trailering. The weight is 2690 lbs. The sail area is 495 square foot. The daggerboard draft is 5 foot. This version has C foils in the floats. The outboard is 8HP. The main hull shape has a flat rocker throughout the midsections. Designer Ian Farrier says this promotes early planing. Farrier says the steep buttock-run aft helps keep the bow from burying. The folding amas of the 28 have more volume than those of the 27. This increased ama volume gives the 28 more stability and keeps the akas higher above the DWL. This results in less spray hitting the akas, making the 28 dryer and faster.

Now we get to the main interesting in this build. I will quote the airline pilot part time builder “Building a boat is a great experience. In hindsight it took me quite a bit more time than I expected. In the planning stage I thought it would take me about 5-7 years, based on the 2500 building hours that it would take according to the designer. It turned out to take more than 4000. Also, working alone is not very efficient, I found out. Designers are generally quite optimistic in their build time allocation. The extra time is not a big problem, as I had not set myself a launch date goal and I enjoyed the building process.

Working with vacuum is also time consuming, but does give a stronger laminate with a more constant quality. This project is on the edge, size wise, unless you can employ extra people for the bigger parts. Those bigger parts sometimes had me working 8 hours in a row. Vacuum infusion will be a bit more relaxed as you’re not chasing vacuum leaks with the clock ticking.

Often, I worked on several projects at the same time, so I would not be idle if I ran into problems on one project. I also changed the build order. The (excellent) build book continues work on the main hull when the floats are ready. I left this to the end. When the main hull is ready, it looks like the boat is almost finished, while there still is a lot to do. This can be hard on your motivation. That is why I first build all parts, like the rudder, daggerboard, beams etc, before I started work on the main hull. That worked out well. When you need the daggerboard case, while working on the main hull, you just pick it from storage. this way you finish intermediate stages and have a new goal to work to each time.”

The amount of education in this blog is very good. Vacuum bag foam glass and some infusion is used in the construction. The story is the same as other builders preparation to build a part is many time the actual manufacture of the final boat part. EG He had to drill 3000 holes in the male mould to attach the corecell foam for EG a float half. He found certain types of tape to seal the edges of vacuum bags did not mix with some resins etc. There is a lot more learnings. The jpegs give some idea.

This blog will take an hour plus to read and absorb. Have a good day. The home page is: nyker.nl http://www.nyker.nl/index2.html The first build page is: nyker.nl http://www.nyker.nl/2011/EN2011.html

 

A small correction: Nykers boat is an F85 SR, which Ian initially developed from the F82R to fit the NZ 8.5 box rule. Coincidently it was released following the build here of a stretched F82 (to 8.5) which utilised the floats off another similar sized racing tri, and retained the folding etc. Ian came on our local sailing forum where it was being discussed and while complimentary about the boat made the point of telling the builder he couldn't call it a Farrier. I think the handful of F85SR's that have been built have all used the optional taller mast than the '8.5 box' spec rig. Other than a general family resemblance its nothing to do with the production Corsair 28.

 

Chris White Juniper (and Explorer 34) was also constant camber. IIRC White built two sets of panels off the same mold. One set went into Juniper and the other set went to Dick Newick for the build of Pats.

 

Jamez. Thank you for the corrections on the F85. My apologies for posting some incorrect diagrams of the F 28.

 

Agreed, the CC 26 is a very pretty boat. I costed it back in 2003. Veneers were difficult to source and expensive but I found I could get 3mm Gaboon ply for $25.00 a sheet If I bought 100 sheets ( I worked out one would need 95 sheets cut into strips to build the panels). All of the ply (including some 6, 9 and 12mm) required came to about 3.5k. On top of that there would have been a 200l drum of resin, hardener, powders, glass cloth etc. and a bit of solid timber here and there and materials for the mold - say 7-8k all up to build the platform. I'd corresponded with a guy in Aus who had built one there, had a carport big enough to set up the mold and space to store the panels. But something kept holding me back, well two things actually. One was that the boat is tiny inside for its length and has little headroom. Two, I couldn't commit myself to the idiocy of slicing up a lot of very nice gaboon ply into strips and then essentially making it back into plywood panels. Marples has some nice looking designs but they are very much of their time. Relatively small rigs, small volume floats and cramped interiors (in the smaller ones anyway) don't stop a boat being a good cruiser, but things have moved on from there.

 

Happy to help, there are a lot of models around the same size to keep track of. We had a broker here advertising an F82 as an F28 for months a few years back.
Thanks for posting the link to Nykers blog, I hadn't seen that before. My own boat was well under way by the time the F85SR plans were available, would have been tempting otherwise.