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Old 23-02-2013, 10:12   #1
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Hull Shape Compromises

I'm just beginning the process of considering a powered boat over sail and have been reading some old posts to gain some context. I have a couple of questions for which I haven't yet found answers. In fact, I don't understand everything I know.

In my reading I continue to come up against hull shape compromises that everyone seems to understand - displacement, semi-displacement, deep-vee, had and soft chines. I suspect I'll be able to locate the definitions of these, but I have been having trouble with their implications.

Is it fair to say that a rounded, full displacement trawler hull will run more economically at cruising speed that a more aggressive hull? Is it also fair to say that the more aggressive hull will exhibit less roll?

Thanks.
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Old 23-02-2013, 10:51   #2
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Re: Hull Shape Compromises

It depends on the cruising speed. Full displacement hulls have rounder surfaces designed to help preserve the laminar flow across the hull. Semi-displacement hulls have more deadrise and are designed to ride better in chop off a plane and yet still be able to get up on a plane. A planing hull has less deadrise or no deadrise aft and is designed to burn less fuel while on a plane than would a semi-displacement hull.

There are also catamaran powerboats to take into consideration.

Which hull form you want depends on how fast you want to go, how comfortable you want to be and how much money you want to spend in fuel going from point A to point B.

Roll has more to do with the the amount of hard chine, keel shape and length, stabilizers, windage and metacentric height and less so with the above hull types.
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Old 23-02-2013, 19:29   #3
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Re: Hull Shape Compromises

Thank you, David. So a semi-displacement hull allows you to plane more economically than a full displacement hull and also tends to be kinder in chop.

I expect we'll want all the kindliness we can find and will be willing to sacrifice speed (and a little economy) for it.
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Old 24-02-2013, 08:47   #4
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Re: Hull Shape Compromises

Hi Jaywalker,

Wanted to make sure you don't have the wrong impression. A displacement hull will not ever plane, no matter how much power you throw at it. It's limited to about a speed in knots of about 1.34 x the square root of its waterline length in feet. Thus a full displacement boat with a 25-foot waterline can go no faster than about 1.34 X 5, or 6.7 knots. A 50 foot waterline could get you up to only about 12 knots.

A semi-displacement boat like many "trawler" type cruisers in the 30-40-foot range may be able to go 12-16 knots, but at a major cost in fuel. They will also go 7 knots well, but not quite as efficiently as the full displacement hull.

Our 26-footer is a planing hull, with a deep-v shape, that is to say a lot of deadrise at the transom. The aft sections of the hull are largely flat fore and aft, but the hull cross section is quite v-shaped if you look at it from the rear. Deadrise is the angle from keel to side between the horizontal and the hull bottom. Ours is 19 degrees, a moderately deep-v. So it can plane, and travel at 25 knots or more at full power. The v-shape helps it crush through the chop comfortably at planing speeds.

We most often go slow however, at 6-6.5 knots, and save a ton of money on diesel - it uses roughly 3x as much fuel per mile at planing speeds than it does at 6.5 knots.

Chines are the point where the hull bottom turns up to become the sides. Our planing hull has hard chines, or a fairly sharp angle between bottom and side. Soft chines are more rounded. Soft chined boats can be a little more efficient at displacement (slow) speeds, all other things being equal, than hard chined ones. But chines are a minor factor compared to the basic hull types we've been discussing.

One of your key decisions will be how fast do you want to be able to go, and how much are you willing to spend to be able to do that. Another key one is how much distance will you realistically cover. We do thousands of miles every summer, so fuel cost is quite important to us.

We're happy to travel slow - it's more relaxing, and we see and appreciate more of the beauty around us on the PNW coast. If we were not retired, and had only weekends or a week or two for an outing, we might be willing to go fast more often.
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Old 24-02-2013, 08:55   #5
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Re: Hull Shape Compromises

Quote:
Originally Posted by Jaywalker View Post
Thank you, David. So a semi-displacement hull allows you to plane more economically than a full displacement hull and also tends to be kinder in chop.

I expect we'll want all the kindliness we can find and will be willing to sacrifice speed (and a little economy) for it.
Full displacement hulls are not intended to plane at all. They can be made to plane if you had enough horsepower but a lot of energy would be wasted compared to getting a planing boat of the same displacement up on a plane. The engine horsepower in full displacement hulls will not exceed what it takes to bring the boat up to hull speed and perhaps a little over hull speed, provided the boat was engined correctly. Some full displacement boats do not have the horsepower to reach hull speed.

Catamarans are slightly different animals in that they can exceed hull speed with proportionately less energy wasted in trying to get the boat to rise out of the water onto a plane.

Generally, full displacement hulls will give you the best ride in chop.

Hull speed is about the 1.4 times the square root of the length of the waterline. The units are knots and feet.

A semi-displacement hull will ride better in chop both on and off a plane than will a full planing hull.
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Old 24-02-2013, 10:25   #6
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Re: Hull Shape Compromises

Thanks both - in general, I'm not looking at multihulls, so the hull discussion is helpful.

FWIW, I don't expect to want to travel fast, but it might be useful to have the capability, especially if there are benefits in rougher seas. Cost is definitely an issue.

Again, thanks.
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Old 25-02-2013, 09:41   #7
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Re: Hull Shape Compromises

For cruising you probably want a deep V with a sharp bow and a good flare for reserve buoyancy. The sharpness will let the boat slice small waves rather than hit them, and will save fuel. You'll want a relatively flat run aft, transitioning from the v form to the flat gradually over the course of the boat's length. The flat stern will make her more stable and efficient.

If you're going to be running in rough stuff or might ground and want to keep your running gear, consider a boat that has a keel deep enough to protect said gear. It'll cost you some efficiency, but will give you more directional stability in a stern sea, and the protection it gives is valuable.


Depending on how heavy she is, and how deep that v is at the bow, she might be a spendy boat to get up on a plane, sucking lots of fuel to do it. That just is what it is.

For what it's worth, I'm describing a Hatteras Sportfisher hull. Coincidentally, I just happen to own one.
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