Planing boat at displacement speed

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jbo_c
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Planing boat at displacement speed

Post by jbo_c »

I’m about to start a shanty boat build. Plan is to only ever use at displacement speed. However, have the option to build true displacement(both transoms out of the water) or planing hull.

I notice Jacques has at least on design(GT23) that is a planing hull, but he recommends displacement speed usage. Why would that be?

That causes me to wonder, should I build a planing hull even though I plan to use at only displacement speeds? I suppose at least then I would theoretically always have the option of a bigger motor and planning speed where the inverse would never be true. But what else would be an advantage?

Thanks.

Jbo

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Re: Planing boat at displacement speed

Post by jacquesmm »

A displacement hull will work better at displacement speeds but will not get on plane properly or at all even with a very large engine.
In some cases, the program of the boat must included displacement and planing speeds. In that case, the hull shape is either semi-displacement or planing.
A planing hull will work at displacement speeds but will not be efficient and not as seaworthy.
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TomW1
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Re: Planing boat at displacement speed

Post by TomW1 »

jbo that was Jacques response to some one who wanted both displacement and semi- planing spedd in the mid-upper teens.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

jbo_c
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Re: Planing boat at displacement speed

Post by jbo_c »

I think this comment from the GT27 study plan is what brought up this question. I had thought the comment was a little more direct, but maybe not.

“ However, this is not a speed boat and she will be much happier at a tranquil pace of 8 to 10 mph with a 25 HP.”

So my question became, why recommend displacement speed for a planing hull.

Since I’m building a boat for displacement speed, my plan is a displacement hull. But never having owned a displacement cruiser(outside a canoe), I have some worry that I will find the slow speed cumbersome in practice. In that case, would be handy to build for planing in the event the slow life is really just too slow. :).

Jbo

Fuzz
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Re: Planing boat at displacement speed

Post by Fuzz »

I feel you are wise for thinking about it now. You may or may not ever use plaining speed but it would be nice to have the option.

jbo_c
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Re: Planing boat at displacement speed

Post by jbo_c »

Exactly my point, Fuzz. As it is, I have a fishing boat that I use 95%(or more) of the time below the hump. My wife and I just aren’t go-fast people. . . . But it’s there if I want it. :)

Program for the boat is 90-10 lake and river use transitioning to probably 70-30 when I retire. So overcoming current could occasionally be needed. But that can be planned for - like planning a takeout downstream, going upstream on the first leg of the trip rather than down, etc.

Probably won’t build to this scale again, so don’t want to set myself up with a boat that doesn’t work for me.

All that said, I am 98% confident in my choice for displacement.

Jbo

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