# Barge efficiency-displacement speed

Discussion in 'Hydrodynamics and Aerodynamics' started by jbo_c, Mar 30, 2020.

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### jbo_cJunior Member

Whether I go displacement or planing, it will be a flat bottomed boat, so no deadrise.

The program for this boat is protected to downright small water, possibly the odd lake crossing, but more typically if in the lake proper would be shoreline hugging. Most of the time will be in protected sloughs or in small river channels.

We have no interest in running fast or in crowded water. I currently run a center console fishing boat and spend maybe 5% of the time on plane.

Jbo

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### jbo_cJunior Member

Do like those bluejacket boats. Nice classic look.

Jbo

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### Mr EfficiencySenior Member

What does "inefficiency" matter with a 10-15 hp motor, anyway, you will still be able to motor along economically at slow speed, and if at some point you get sick of going slow, with a planing hull form, you can then change to a larger engine, and get the speed that was lacking.

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### jbo_cJunior Member

Thank you, Mr. Efficiency. You nailed the original purpose of my question.

So, how close, then, to full hull speed would the planing hull begin to behave poorly?

20 foot boat so let’s say for easy math 16 feet of waterline. That’s 4x1.34 or 5.4 knots hull speed. We “know” that at 5.4(ish) it will be struggling to get out of the hole and likely will be misbehaving in the steering department. So then the question is ‘at what portion of hull speed is the boat still well behaved?’ 80%? That would be 4.3 knots or about 5mph. I could live with that as a ‘cruising speed.’

Thank you for continuing the discussion.

Jbo

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### DCockeySenior Member

A "planning hull" designed to operate at semi-planning speeds will not behave poorly approaching "hull speed".

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Define ... poorly

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### Mr EfficiencySenior Member

4 or 5 knots certainly is heading in the direction of useless, going upstream in a flowing river, or against a strong tidal run, but on a lake or sheltered estuary, less a problem. And there is no necessity for there to be handling difficulties pushing along off-plane with a planing hull, especially one with minimal deadrise.

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### fallguySenior Member

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### jbo_cJunior Member

I like escargot. Spent lots of time looking at it. Should probably at least spend the buck on the study plans.

Jbo

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### TANSLSenior Member

Define.... define

11. Joined: Jul 2017
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### jbo_cJunior Member

Hmm. I can feel it more than define it.

Maybe bow up attitude and grudgingly responsive to steering.

I was just perusing more(while I’m supposed to be working). Looking at Devlin’s Millie Hill, that hull is straight aft as if for planing, but clearly intended for displacement cruising.

Jbo

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### Mr EfficiencySenior Member

If the planing hull is bow up at a given speed, the displacement hull will also be, at that speed, in a boat of similar proportions.

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