Low horsepower proa or motor canoe hull shape?

Sorry for the disjointed posts. I don't have a computer and am clumsy with the smartphone. Thanks for your patience.

 

If you are only after a small dinghy, there a dozens of much better designs ( many of them free ) than the one in the photo. Then you can consider comments on their actual performance, rather than experimenting from scratch. eg Sunny skiff 14ft plywood flattie plans http://intheboatshed.net/2009/06/10/sunny-skiff-14ft-flattie-skiff-plans/

 

Thanks Rwatson. I don't want a flat bottom, I've built a 16 foot banks dory and don't like the pounding. If I were to follow a design I'd probably build Ross lillistone's fleet or flint. The boat in the photo interests me because I could build cheaply it in a few days and I like experimenting. Thanks for the feedback and links.

 

PS is it the lack of volume forward and lack of flare aft that make you think my design looks poor?

 

http://sassdesign.net/Keyhaven Skiff, a semiplaning lightweight skiff.pdf

Just an alternative....
In this presentation also seen what parameters need to be checked for it to be a good boat.

Please. Calculate, do not guess.

js

 

Thanks very much for the link. The design brief was very educational for me. I think that bottom would pound in a steep chop at 9 knots but that design does look fast and able.

 

It is not the deadrise which is the deciding factor for a boat to go smoothly. It is much more important that the bottom width is correctly calculated. A narrow bottom goes always softer than a wide bottom. This must be calculated using well proven methods as confirmed on some of my boats. This is the background of my boats with the trapeze bottom and the bottom with double chines.

Don't guess, "think", calculate.

js

 

"It is not the deadrise which is the deciding factor for a boat to go smoothly. It is much more important that the bottom width is correctly calculated. A narrow bottom goes always softer than a wide bottom. This must be calculated using well proven methods as confirmed on some of my boats. This is the background of my boats with the trapeze bottom and the bottom with double chines.

Don't guess, "think", calculate."

I appreciate your input and am intrigued to know why the trapeze would not pound at 9 knots in a short chop. My experience of a flat bottomed boat with 60cm bottom width, 60 degrees of flare amidships and very pointy bow and stern has been heavy pounding.

This boat, however, with a slightly longer waterline and more beam at the waterline, does not pound at all. I know nothing of calculations. Only 'thinking' and experimenting with models and self designed boats. I learn mostly by my mistakes and intuition. I'm on the forum to learn! Please enlighten me!

 

In my experience driving/riding small, outboard driven boats ... hulls that have a low dead-rise at the cut water pound badly.

Please note that's not a calculated conclusion it's based on observations of being on boats.

It's an interesting trade-off. Narrower hulls pound less, but for lateral stability may have to be flatter - which increases pounding.

 

"It's an interesting trade-off. Narrower hulls pound less, but for lateral stability may have to be flatter - which increases pounding."

That it exactly. A narrow boat does pound less, but it tips over much more easily too. You can always slow down if you get uncomfortable, but nosediving into a wave due to insufficient bouyancy is just dangerous.

 

Welcome to the convoluted concessions, in discontiguous compromise, that we know as yacht design. Making the "difficult design decisions" as Ted Brewer calls it causes all sorts of headaches. Finding the middle ground of the SOR will usually yield you a sort of narrow, sort of canoe body hull form and only some flow dynamics and/or testing will fine tune it.

There's a lot more to it than deadrise and beam/length ratios. For most, it's when does the dead rise flatten out (at what percentage of LWL), given it's pressure or wave train on anticipated operation speeds, entry half angle, optimized Cp, flow release (displacement speed forms) and reassembly, boat use, will lee bow surge waves need to be considered, etc., etc., etc. It's easy to go over the cliff, thinking a fine enry that picks up a fair bit of displacement as it's immersed, on a narrow canoe form is the way to, just to find you needed more volume in the forefoot or maybe a slightly smaller entry angle or a fuller set of WL's midship, etc. Prediction programs can get you close, but only testing can nail it down real fine.

This said, there's a number of tricks you can employ, such as a narrow water plane, yet dramatic flare to greatly increase volume as the hull becomes immersed (driven or weighted). Bolger was fond of this as was his penchant for box keels, which bore most of the hull's volume in a very narrow, streamlined box, with a shallow, skiff like hull attached above. Other tricks might include matching entry rocker with entry angles, hollow WL's in the forward sections, but convex aft (speed expectations dependant), multiple chines to reduce wetted area, and the list goes on and on.

 

Thanks for the eloquent reply.

 

Don't forget to factor in weight, when considering the prospect of pounding.....featherweight boats have a corresponding lack of inertia, and vertical acceleration/decelerations are greater, but a motorized canoe is a different proposition to a planning hull, too.

 

cat hulls for stability and smooth ride...

 

My concern with small cats is always the bridge deck clearance. It has to scale down to somee xtent with the rest of he boat and small cats tend to have a lot of slap and bang on the underside of the bridge deck both underway and at rest.

If you're designing a boat for fishing, that extra noise is most unhelpful.