24ft power cat design

http://www.africanangler.com/sb_boat_tests_view.asp?id=95

 

Hi Richard,

I like your designs and if one suited what I am after I would certainly buy one of your plans, a lot easier than trying to design one myself.

Sailhand,

I know what you mean about the whole trailer thing, my current boat is a 19ft Hamilton Jet 83 with a big heavy trailer and the boat isn't light either.
It had a 350 chev in it originally but I repowered it with a Holden ecotec V6 out of a VT Commodore, similar performance with less fuel.

I am looking into foam construction and may go that way.

Shultzfactor - I like the Sliver 29 and my original design ideas were along similar lines but have changed slightly now towards more like the Leech 825

 

Quite amazing how many different power cat models are produced in South Africa, as shown under "boat tests" in that link schultzfactor provided above. Trailable planing cats mainly, hard to imagine how such a small market can support so many makes and models.

 

The Leech is pretty! I can't believe they can push that big cat around with two 40s!

 

there is an interference drag with cat hulls, where the vectors of the wake interfere with each other. I dont know of any way to estimate it, just to say look for an increase in drag around 35%. All the same with twin 30s it ought to scoot. 13:1 should be fine too, but as they wont accommodate anything much, you might consider even skinnier?

You dont say (in what I read) what the deck layout is, if its minimal you could utilise some form of folding?

 

The attached chart shows the resistance increase due to interference depending of the distance between the symmetrical hulls. Asymmetric hulls does not have this resistance growth. That is why all the racing catamarans have basically parallel tunnel sides.

js

 

I'd say it has almost nothing to do with wave interference with racing tunnels, the speed is such and the wave-making so little, it is hardly a factor.

 

I've decided to ditch the 2.5 & 2.9 version and go with a 3.6m beam, demountable and fit on a 2.5m wide trailer, similar trailer style to the Elf 26.

 

one of the circumstances to conquer is getting it on and off the trailer if you have to do that all the time. As you are only trying to lose around 4ft of beam some work in the ingenuity of that would be worthwhile. I recall a homebrew wood version of the Tornado named 'Shark' that folded, but I cant remember how, save to say it was simple to do. And there are systems like this below, and Rob Denney's 40ft proas do something similar

40ft Proa
https://vimeo.com/165111969

Large Folding Catamaran

 

A sliding beam setup or removable beam would work not sure about a folding beam, outboards will be mounted on the hulls.
Beams will most likely be aluminum round or oval section or box beams(timber /ply/epoxy)
The trailer will support the hulls and central section, remove beams or slide then lift central section and slide or swing hulls together under central section.
That's what I've got so far, open to new ideas though.

 

I think the motion in a 24/25 foot, 12 foot wide power cat won't be ideal. You will get some snappy gyrations. You notice that the tourist cats are not super beamy, and they have fairly high centre of gravity, to lengthen the roll period.

 

That's why I prefer modest width and thus narrow parallel tunnel without interference, a split monohull. In this way it is possible to get the good properties of a catamaran without getting its negative properties.
In some areas, as in the Baltic, it has been found that catamarans does not work because the passengers becomes very seasick.

JS

 

HJS
I am not after a sharkcat or powercat that needs big HP to get it to plane, that type of design will be super slow with low HP.

Mr Efficiency
I have owned a few sailing beach cats, a racing cat and chartered a 36ft powercat so I understand the motion is different. The power cat will be used 99% of the time in sheltered waters with no swell.
Most of the displacement type cats I have seen have wide beams some very wide.
If the worst should happen and it's motion is horrible I can always narrow the central section to narrow the beam.
It's only on paper at the moment so nothing lost so far.

 

They do in many cases, but 12 feet is still a lot for a 24 or 25 footer. Of course a sailboat behaves differently with a high rig that damps motions.

 

just to allay any possible confusion, the folding Shark catamaran once featured in Wooden Boat I mentioned earlier was a Macalpine-Downie design for sailing. Wooden, with a solid deck, it strongly resembled the Olympic class Tornado.