Seaknife based catamaran

I'm going to build a catamaran and have been reading through threads here etc. I'm fascinated by the sea knife concept. I'd be interested in people's opinions on making it into a multihull. Attached is a rough mock up for discussion. I used dimensional drawings of the sea knife, and riussian sea knife, as well as photos to generate the hull profiles.

Obviously unless the hull planes at low speed the big flat sections on the leading edge will be a problem. I also need to carve out the tunnel edge but you get the idea anyway. Dimensions are 25" deck to keel and another 30" deck to top rail.

Over all dimensions are 7'x14', which is the maximum I can fit in my little garage to build. The boat will be beach launch only, 2-3 people for fishing/diving. Seas where I live are normally very flat but can change very, very quickly. That's why I'm a little concerned about the displacement profile. I don't plan on going very fast, but unsure how many knots really.

 

Your drawing is puzzling. Why would you build something so impractical, with a needless degree of complexity? You want a small boat for fishing or picnicing? A simple flat bottomed skiff will work far better than the boat(?) that you have drawn.

Knosiseux, do not take offense at these remarks, none intended. You did ask for comments.

 

No offense taken. My only apprehension about a flat bottom skiff is if the seas come up it would be pretty unpleasant. Most boats here are v bottoms or, what is quite popular are v bottom alloy boats with side pontoons. They're popular mostly because they can take a good beating on the beach I think.

I don't think I'd build what I posted, it was more for discussion really, as I'm working through using ACAD 3D for the first time. It seems most cats or tunnel boats, and I don't really understand the difference, have fairly constant geometry, where as the sea knife didn't.

It's probably less confusing if I remove the extrusion on top which I was going to carve the bain area out of.

 

Oh, also I should mention that building it is more than half the fun for me. I don't really mind if it's overly complex. I'm not looking for something that is necessarily easy to build.

 

that design would have a lot of drag as well, those flat vertical surfaces at the back of each keel would make it impossible to get up to speed without a lot of power.

 

What you've drawn there represents probably the most non-linear and viciously reacting change in hull lines that I've yet seen. You simply cannot..or should I say, should not, try to reduce the buoyant restoring moments to the point of being a SWATH or even less than, with such an incredibly large angular excursion "limit" awaiting the unsuspecting passenger. You can work around that to a limited degree ....but Peter never really managed to do so, taking in to account all the operating compromises that inevitably enter in.

 

Good feeback Petro, thank you. As far as the flat portion on the rear of the keels, I liken it to the 80s and 90's Corvettes. The sharp change in shape created a pressure vortex that in turn actually reduce drag...but again this isn't water.

BMcF, I don't relaly follow you, I'm not that that familiar with naval design terms yet. I get the jist though...it's bad idea. It certainly struck me as having some limitations, even with my basic knowledge. So I suppose I should do some reading about planing hulls and specifically multihull designs.

 

Basically what BMcF is saying is that if you push down a hull, any hull, into the water and let it go, what occurs….it “pushes” itself back up. It does that because the amount of buoyancy, or “volume” in the hull is greater than the force which the hull has been “pushed” down with. Since buoyancy is simply the density x the volume. This is expressed as a force. The force, or weight, of the hull is balanced by the volume of the hull, so it floats and at a given waterline. This is basic Archimedes principal.

So when a boat pitches (think of the hull being pushed down) the volume in the hull above the water line helps to restore the hull back to its equilibrium position, as the buoyancy and the weight of the boat are equal; otherwise the hull doesn’t float.

So in your design, as the hull pitches, where is the “volume” in the hull to restore the hull …there isn’t any..well not enough to make any difference….until the waterline is up against the underside of the raft structure. So, as the boat pitches, it shall slam and slam badly and violently since the waterline needs to be up to the underside of raft before it gets any “volume” (buoyancy) to counter act the wave profile (the increase in wave amplitude) against the hull.

Most hulls have a flared hull shape above the DWL to allow a gradual increase in volume (for a gradual change in restoring force) so the pitching is “smoother” not sudden, as in the case of your design.

 

Ahhh, no I know what you were getting at. That's why I though it would have to plane from almost word go. The deck hitting the water will be a bone shaker. It doesn't seem any more extreme than most of the multi-hulls that come up under a google image search. Although my proportions are probably all wrong to make it work. I'd probably have to extend the hulls significantly to get the volume reuqired.....

 

Probably the only way that sea-knife idea would succeed, really. Two of them tied together. One on its own is hardly stable enough to ever be useful. Not to mention the lack of useable internal volume.

 

Ah... um... er, what will you name it?
May I suggest Flip, Flop and Fly or perhaps Crash Boom Bang?
Save your more creative juices for model boats.
Figure out a way to put a temporary extension on the garage and build a 16 foot V hull from plans.

 

Have a look over at this page and scroll down to item#52.

http://www.yachtforums.com/forums/general-catamaran-discussion/2701-trimarans-bladerunner-4.html#post16010