New power cat - help please!

Here are a few pics with a bit of artistic licence
I have cut away all the top deck structure & 900mm off the rear, this now gives me a length of 5.4mt x 1.8mt beam. I intend to add a 600mm tunnel to ultimately give a 5.4 x 2.4 finished boat?

 

Very nice.
I think that most of the tunnels are wider and the sponsons narrower.
you can verify at the test papers on jimboat site of a cougar and a stv.
Just a thought.

 

I am not expecting to design the ultimate performance cat just a functional boat, however I really appreciate any input especially at this point!
I plan to power the boat with two 50 hp motors & would be happy with 25mph top speed?
I guess this project to take a year to complete so it is quite important to at least believe it will work?

 

You are going to need to come down from the underside of the tunnel to the cut edge of the sponson at almost 90* to narrow up the sponsons or you will be bobbing like a cork in the water. I would make the beam at almost maximum for trailering to help in stability.

 

Why did you cut 900mm off'n it ?

 

Good question........did you keep the 3' you wacked off the rear and do a neat job of cutting it off?

 

I could try to make the tunnel a bit wider but as the boat will be kept on a marina mooring the beam & length makes a big difference to the square footage cost, not cheap in our marinas!
I live on a very small island with very narrow roads, towing boats is not a very practical option & a police escort is required for any boat over 2.7 wide.
I have been looking at a couple of cats, one has a 350mm x 1000 tunnel the other only 300mm x 800.
I was thinking about making my tunnel 400mm deep x 600 but I could go 400 x 700? the other reason to keep the tunnel at 600 was because the plastic coated board I was going to use for the mold is 600mm wide.

 

a tunnel of 400mm (15") deep X 600mm (23") wide for chord length of 5.4m (18ft) is very narrow, and extremely low aspect ratio. results will likely be marginal.

sizing a hull by simple scaling sometimes gets unexpected results.

 

Have you thoughjt about making it wider and to make it trailable you can make it to fold ?

You could also try and find someone close to the water to leave the boat at so you won't need a police escort, pay them an affordable amount instead, almost like you hire a little piece of their land.

Also ask your authorities if they have a patch of land or an area they cannot do anything with and leave the boat here.

 

I must say that I don't approve personally to your's cutting that boat up. Oh, it's your's, you can do with it what you want, but I have doubts about the new boat's hull in the making. It would have been much better and safer to make new hulls from scratch. I haven't ever seen it work right structurally for what you are about to do - unless you plan to make a mould off'n those two halves.

 

OK plenty of negative replies but how about helping me work out a good ratio for the tunnels width & depth?
What are the disadvantages of a narrow tunnel? would I get cavitation or aeration on the props?
I respect you guys advice & recommendations but I don't understand the problems, are you saying the boat wont plane, wont turn, or what?

 

I think tunnel 900mm sponsons each 750mm en 250 to 300 mm deep .
this wood improve stability.
How deeper the tunnel how better it wil thake ruf water without slaming on the roof of the tunnel.

 

As f250 said, the higher the bridgedeck the better.
The water that gets pushed sideways by the hull also accumilates between the hulls, thus pushing it up a bit (it has to go somewhere). The further the hulls are apart the less the effect, but the better the chance of a lump of water to go between the hulls. Imo the further apart the more deck space you would have and the higher the bridgedeck the more pleasant the boat should be in choppy water.

I wouldn't worry too much about not enough draft, you will find plenty of weighty stuff you will want to pickle along when you go out. Boats are like females They gain weight quickly after that inital shallow draft launch.

The intent is not to give you negative replies, but to give the right information. I am really worried about your joining the hulls the way it looks like you are going to do. The hull of a powerboat takes a hell of a bashing while you are underway at speed. In roughish water, go out with someone else and jump into the water a couple of times when speeding. You will get the idea. Even though a hull has a shape, the water doesn't just flow around it or underneath it. A lump of water that you crash through is similar to riding into a wall with your car. If the hull is not made seamless out of a one piece integrated form, you may encounter serious problems. The forces at speed due to the hulls encountering water at different angles are huge huge.

You can also hold your hand on the inside of a hull when it planes. I think you are going to be shocked at the force of these little bumps it makes.

I see Frosty made a comment on buying a hull. If you do, you will still have plenty of work to do on it (trust me) but you will have a proper hull that is strong enough IF you do the structures inside it right. If you don't, and the hull twists you have a good chance of things breaking loose inside it.

I'm not sure why people think a boat is just an overpriced piece of fiberglass they can just slap together themselves for cheap. I bet if it was an aeroplane you are building you would think differently. Not much difference between them, you can fall just as hard with a boat and drown, together with passengers whom you as skipper is responsible for.

If I knew this is what you are going to do my response from the start would have been different.

Reconsider what you are going to do !

 

Furthermore, I would suggest you first learn a bit more about hulls and cat hull shapes since that is where you're interest is. Here is something to get you started.

http://www.multihullsmag.com/magazine/articles/cathulls/cat hulls.htm

 

O sorry Fanie, I dont want to cause any hassle on your site, I just kinder thought some of your members might want to follow me building my boat?
I haven't started any building as yet because I want to make a good & proper job.
I appreciate all the answers so far but if you think it is inappropriate to continue posting about my idea of course I will stop?

However I still think I can utilize the hull sides & get a wider tunnel, it will mean cutting off more fiberglass down the length of each half but I think I can still keep the beam at 2.4?
I was hoping that by glassing in the tunnel right across the width of the hull & glassing in multiple bulk heads plus the deck would give a pretty secure structure?
I dont know if my idea will ever work out work but I feel very confident about the integrity & quality of my work.