Powercat 4.3m long

[cookiesa]

Hi everyone, please be gentle I'm a newbee!

I am working on a plan for a small powercat for coastal fishing. Cat because of stability when anchored and economy when running. Doesn't need to do 38knts but handle 1-1.5m swell and reasonable winds (around 15knt?)

I would like a twinfisher or devilcat but want to try and design/build one as a project. I have previously rebuilt/restored small sailing craft and converted a 16ft dory in to a small power cruiser (monohull) previously. That said I have lots to learn and plenty of mistakes to make but hopefully will end up with a boat that works!

I'm looking at glass over ply for construction and to help keep costs down a single outboard centre mounted. I'm hoping around 40-50hp will work but that may need to be changed when sea trials start!

After plenty of research I am looking at a V of around 14 degrees at stern carried 3/4 forward before finishing at the bow. (V for stability/ rough weather handling but keeping it minimal to reduce HP required. Bulkheads will be full width (2m). Most of cockpit will be self draining with a covered helm area and rocket launcher.

I would appreciate everyone's thoughts on a few of the following;

1) I feel my hulls should be narrower (currently .6m wide) and the bridgedeck made wider (currently same as hulls (.6m wide) (I am thinking in the vacinity of a hull width of .4m wide leaving a gap of .8m)
2) Currently the bridgedeck clearance is .4m I would like it higher but I am also trying to keep the cockpit floor to deck level around .6m for safety with smaller children on board and also whilst lifting fish in.
3) Currently the hulls are "box" like towards the stern rather than making them taper narrower, I have kept this to help offset the weight of the outboard (pod mounted on centreline between hulls)

I have some good info already on reducing cavatation with a centre motor so am not concerned with that.

Sorry if this has already been covered I have tried a few searches so hopefully this isn't to repetative!


I am at the model stage but think I could go with narrower hulls.

[tri - star]

Stability and buoyancy, are tricky with smaller cats.
I suggest you go for a few rides in sail cats, in the length you are
thinking about.
As you will find that the movement of the people aboard a smaller
vessel has a suprisingly, significantly greater effect, than on one
- only 20% longer.

The fineness of the hulls becomes a redundant issue, if you don't
have sufficient initial resistance - as the vessel heels over,
Even in moderate conditions.
You mention children. So I suggest, even greater caution than usual.

Go down to the beach. Notice how much fun the 16 ft cat guys are
having. Also how easy it is to " fly " a hull. - And flip the boat.
Go for a ride in a 40 ft power cat. It will, 99.999 % of the time, keep
both hulls firmly planted in the water.
( Which one, is the "family friendly" boat ?)

Before you go any further - I suggest you find out why, all the above is so.

Don't get me wrong I am a muti - hull guy !
But ALL hull forms have their limitatations.
A cats advantages come from having less boat in the water. At it's most basic.
This means, however, that is a poor load carrier. Also not very forgiving, of
being overloaded. Particularly as the cat gets shorter.

The vessel you describe will need to be a good load carrier and will be prone to
overloading. All this is somewhat contradictry. Sorry to say.

Cheers !

[cookiesa]

Thanks for the response.

Certainly have some design issues to work on however in Australia smaller powercats have been around for years and there are several manufacturers that have proven this design works well for coastal use.

Some even shorter. I am more or less restricted to around the 4m size. 5m would be maximum but of course increasing length also increases cost. Most of our fishing is done within about 10km of the local marina and a "long" trip would be around 30km.

Most of the boats are the usual fiberglass or alloy runabouts with planing hulls but the conditions here mean you are often bucked around at anchor and often running at speeds that mean you can't get the efficiency out of the hull as it is too rough on the plane.

My original boating was with sailing and the point you make about stability etc is not really a fair comparison. Very different use, as you no doubt already know cats run a higher aspect rig moving the COG higher up then even a similar sized monohull. Sailing places very different forces on the hull.

[tri - star]

O K cookiesea.

It looks like you have done your homework.
First, in my experience, a V of 14 degrees is a good choice for a planning cat.
You will get both a soft ride and good economy.
At 23' to 25' a tunnel of 33 % of the W.L. Beam will also work well.

However, it does not follow, necessarily - that this ratio will give you
adequet buoyancy in a shorter vessel.

I am still concerned that you look carefully at stability issues.
The actual cost of the hull proper, may only be 20 % of overall expenses.
So adding length is not always as costly, as is often presumed.

Cheers !

[cookiesa]

Thanks for the feedback, My main reason for the length issue is storage. We already have a 4x4, sedan, camper trailer, sailing dingy and 6x4 trailer and the boat will also be stored at home. The sailing dingy will probably have to go as it is.

That said I think you have answered another niggling thought I have been having and that is should the hulls be narrower to help with efficiency and chop handling. I'm thinking I'll stick with the widths.

I'd appreciate your thoughts on strength of the structure. I'm looking at bulkheads 9mm ply (filleted), stringers 20x40mm hull skin 6mm ply with 10oz epoxy/cloth to outside and decks, epoxied interior for rot resistence.

I'm hoping for aroun 20knts top from around 50hp but this I think may be a little ambitious!

Thanks again

[cookiesa]

In line with your thoughts I'm thinking of going to 5m

[tri - star]

Sounds good to me, generaly.
Although, with me you can never have to much epoxy.
Inside and out. However, you are on the right track.
Often it's the inside where the trouble often begins.

A trick that gives you a better glass to resin ratio is
to use two layers of 5 oz instead of one layer of 10 oz.
Also this makes it easier to cover hard to get at places.

There is a great book on epoxy / wood construction
put out by the WEST system people. That I found
useful. You dont have to use their epoxy necessarily.
Just check out the their book.

Signing off now.

Cheers !

[northerncat]

i think this is the kind of thing you are looking for, just scale it up to the size you want
http://www.mahnamahna.com.au/January%202007.html
sean

[cookiesa]

Thanks for the link, quite interesting!

[northerncat]

so tell me what can you do to reduce cavitation issues with a single motor?

i also agree with the length thing an extra m might cost you all of 150$ but prove invaluable as you head out fishing in the chop

[northerncat]

http://www.multihullsmag.com/magazine/articles/cat%20hulls/performance_of_cat%20hulls.htm
this an interesting link i just found that has already been posted on this site about hull design
sean

[cookiesa]

Thanks for the link. I had seen that. It is interesting when you go back over things you have already looked at though, helps you to either stay on track or reavaluate your plans!

[northerncat]

i thought it was interesting to, a little biased too biased towards the hysucat thing but still interesting
sean

[cookiesa]

They were really quite dissmissive of the type "A" yet Australians appear to be leading the area in smaller (and larger if you look at Incat etc) power cat design. Certainly there seems to be, on the net at least more Australian powercat designers than in other parts of the world. Or have I just missed them?

[northerncat]

one other reason im looking at assymetric hulls is that apparently the water in the tunnel is forced into a bulge(like the tunnel hull boats) and this allows you to lift your motor up a bit which means less drag= more speed
i agree with what you say about his claims re type a cats however there are a quite a few aussie builders making assymetrics westcat markham and dominator cats being a few

i quite like the look of these
http://www.porterieboats.com/story.htm

another interesting article, i must admit the idea of fitting a foil down the track would intrigue me
http://www.marinews.com/details.php?recordid=514

[northerncat]

there are a few aussie manufacturers that make assymetrics westcat are one of them
sean

[northerncat]

heres an interesting approach to working out the hp needed for propulsion

http://www.boatdesign.net/forums/showthread.php?t=16863
sean

[northerncat]

http://members.optusnet.com.au/~rhturner1/index.html
have you considered one of these 20kn from 25 horsepower
so it can be done, id oplay around with the styling a little however

sean

[cookiesa]

Yeah I have, think the plans are fairly $$$ for a fairly crude design. (Especially when you compare to plans like those from Mike Waller Design.... Have the study plans for his TC670.... Power or sail ?????? (Way the fishing is at the moment I'm weaking towards the sail;!)

[northerncat]

he reckons that his powerboat j6 gives you 18kn from a 25hp so his hull design must be alright, i would just use his hull design and build what you want up from the deck9styling is all about what you want a departure from the plans to give you this is fairly reasonable as long as you arent altering anything structural, pity theyre not assymetric otherwise id be interested, also mike wallers boat has more materials which means it will be a heavier boat plus they havent been designed for bigger outboards( not that im sure that the j6 is though)
sean