New Hull Design

Hi All, I'm brand new to this forum and looking to dive into boat design. I have an idea for a hull I'd like to build, but I'm unsure of how to start. How do you determine length, beam, angles, etc. for a catamaran? I'd like it to be river capable and powered by twin 25 hp outboard props (so that it's legal on my local river). The reason for the cat design is so that it's somewhat stable in up to 4' seas, as I'd also like to use it for near-coast halibut fishing. My initial thought is aluminum construction, 16-18' lenghth, 7' beam. Something roughly in that size category. I appreciate any and all help you can provide. I'm not looking for real specific answers; more of just direction to resources, as I don't think this is a very common hull design. Thanks!

 

A catamaran is not inherently more stable than a monohull. A narrow cat like you are suggesting may be less. What you are asking for is about three shelves worth of books. Considering the investment on building a boat, buying an existing design would be better. Particularly when you can find out how the design performs in actual conditions.

 

Welcome!

Your first step should be learn how to search these forums using keywords
specific to your project. There is a lot here that you will benefit from.

Gonzo has given you your first piece of sound advice.

If you are unable to determine the principal dimensions of the vessel you
want to build, then I'd guess that you are a very long way from being able to
design a sound safe vessel.

 

PBJely clearly says "I have an idea for a hull I'd like to build,". Therefore, it makes no sense advise him to buy an existing design.
But unknowingly, Gonzo, you've touched a topic that can open an interesting discussion : can a monohull be more stable than a catamaran?
Logically we must assume that both boats have the same length, the same total breadth and the same displacement. Since establish which is more stable or less is relative, we could establish as parameres to compare the stability:

• maximum value of GZ
• area under the GZ curve up to the angle of maximum GZ value

Accordingly what the experts say on this matter?

 

Certainly a cat will "feel" more stable in the size of vessel mentioned, with changes in lateral trim being somewhat less, as people move around. You can fish a bunch of people along one side even in a small cat, and not notice much change in the boat's stance. What's with the 2x25 hp, are you restricted to that ?

 

welcome to the forum,

what is described is more like a tunnel hull than a true catamaran. a catamaran usually has two separate hulls joined by beams or a deck (and intergral beams), and at 16-18 ft might have 8 to 9 ft beam.

Rather than get bogged down into details of convetional hull design, it might be faster, and more fun, to make a scaled down version of your hull, perhaps 10 ft long with 4 ft beam, out of inexpensive plywood and adhesive caulk and deck screws and go try it out on local waters. That will tell you more about how it will behave than a lot of expert opinions. but you may find out there there could be a good reason no other hulls are built this way.

good luck.

 

There is nothing "new" here, but I'd say 2x25 won't suffice. need at least 2x50 imo, assuming we are talking a 20 knot cruise speed.

 

Thanks for the replies. I'll try to add some details that will make this idea seem less crazy (hopefully). The Kenai River is limited to 50 total hp (one 50 or two 25s is legal) and 20' hull length. THe idea behind the cat is to maximize space available to fish. I would prefer an even wider boat but was worried about the ability for 50 hp to push it. Right now, there is basically one boat that is made specifically for the Kenai, and that is the Willies Predator. Great boat for the river, but it would be suicide in the Cook Inlet. I am trying to develop a boat with deeper gunnels that can handle moderate ocean conditions. I don't want to go 50 miles offshore or anything; maybe 5-10 miles in 4' water max. I would prefer to build a planing boat (perhaps with a foil design), but not sure if that's even feasible. I have quite a bit of experience with other styles of boats (racing and recreation jetboats and hydroplanes mostly), but I have never been in a whitewater river-capable cat. After exhaustive searches, I can't find anyone that has ever built one. I love the way cats handle in the ocean. I like cats with two outboards over one for handling reasons. I don't see any reason why a cat couldn't handle whitewater conditions. My biggest hesitation is whether or not this can be done under 50 horsepower. As for the design aspect, my other great concern is bouyancy. I need the thing to float and hold 4-5 fisherman when stopped, but if I put too much hull under it, the boat will never plane because of the surface area in contact with the water.

 

If bottoming out is an issue in white water, a cat with two engines has more draft, and two chances to cause expensive damage.