Catamaran design help

(Also posted in Sailboats/Construction forums)

I'm an amateur enthusiast, working on designing my own cruising cat, and I've got afew questions...

Having been considering several construction materials, I'm still trying to work out which might be most cost-effective while also being reasonably sturdy and safe for blue-water extended cruising. What can you tell me about the benefits or disadvantages of...marine plywood with fiberglass over, fiberglass alone, composite?

Using a tape measure and my own home as a rough guide, I've been attempting to plot out exactly how much cabin space I'd need in the hulls to be comfortable. Some friends say 8' wide by 15' long, others say 6' wide and 20' long. I'm expecting to use just enough space for a double-berth, a small sofa, head with shower. What dimensions might be "ideal", without being too big? Trying to keep this cruiser at or under 50' if possible.

I've been trying to work out a retractable daggerboard design for each hull, but obviously the cabin space might get in the way of this. Is there any way the daggerboards might be inward in the cross-section rather than dead center of each hull? How might that effect overall performance? Trying for a reasonably efficient design for offshoring that can also retract some for shallow water or for hauling.

Also, just any design tips or advice would be greatly appreciated. This will be my first multi-hull, if that matters.

 

If you intend on building the boat you are about to design, then I would be concerned with the problems you don't know about, and even more concerned with the problems you don't know that you don't know about...

I know its not multihull specific, but a book like "Principles of Yacht Design" by Larsson & Eliasson will give you some insight into many of the compromises that make a boat, including the questions you have now.

Designing a boat is an admirable way to spend ones time.
Building it just compounds the problem

Best of luck to you, enjoy it.
Rob

 

There is nothing better than building your own boat. Your description of what you want reminds me of a boat I saw up on Lake Erie. It was a 22ft. Hobbie cat. I was looking at it and sure enough the hulls had the asymmetric shape if the good old hobbie 16, but this was a bigger cruising version. It was over 20 years old and made of 1/4 in plywood with 6 oz. fiberglass skins. The interior had longitudinal stringers about 1 inch thick every 6 in. and transverse stiffeners every foot and a half. Seemed like a lot of wood work but I believe that is probably the most cost effective. For something under 30ft. All fiber glass you need a mold. Foam core is nice but much more expensive, More for the performance oriented boat.
Dagger boards at an angle… no problem. Putting them on the inboard edge is common, easier to move them up and down there. The boat I mentioned above had them on the outside due to the Asymmetric hull shape.
As for the proper size, length, beam, rig, freeboard…I suggest look at what is available for sale. Find some boats that are kind or what you are looking for. You will want to keep track of their length, beam, height of hulls, mast & sail area, and displacement. Find 5 or so boats. There are some ratios you will find consistent. Such as beam (whole boat) /length , mast height /length, displacement/ length. Using the same ratios you can get a good starting point for your boat.