# Ideal W/L ratio in temrs of planing speed

Discussion in 'Boat Design' started by gordanm, Aug 26, 2009.

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### gordanmJunior Member

Is there ideal W/L ratio to achieve maximum planing speed in monohulls? In other words, is it better to have "fat" planing surface (more width than length) or "slim" one in order to achieve better speed? Of course assuming that wetted surface (while planning) is the same thus providing same hydrodynamic lift. Excuse me if I made some wrong statement because I have no boat design background whatsoever.

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### YellowjacketSenior Member

The short answer is "It depends"...

Technically the wider surface of equal area will be more efficient at creating lift. The problem is the amount of surface area required will vary with speed and location of the center of gravity, may or may not push the trim angle to be above or below the optimum for the speed that you want to go. Also involved are the location and angle of the thrust (prop) and things like hook in the hull, so it is not as simple as "wider is better". The faster you go, for a given center of gravity location, efficiency tends to improve once you get to the point were the center of gravity starts to make the boat run at very small trim angles.

So what happens is that, as you go faster, you need less planing area. If the CG is too far forward, your surface area won't decrease and the drag will stay higher, so narrowing the surface results in higher speed.

Suggest that you get a Savitisky calculator or spreadshet and try a number of different combinations. As you go very fast you don't need the planing area (width) and a narrow "pad" will be faster, as is now practice with padded v hulls that are very fast and efficient at high speeds.

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### tom28571Senior Member

Well said.

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### gordanmJunior Member

Thanks

Thank you Yellowjacket. It seems there is no simple answer to what is more efficient. Other factors have to be taken into account.

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### GilbertSenior Member

There is a simple rule of thumb about where the center of gravity should be, but of course it doesn't answer your original question about efficiency.
The center of gravity should be about 1.25 times the beam of the waterline ahead of the transom at the bottom.

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### gordanmJunior Member

Gilbert, you are right, this does not answer my original question but sure it's very interesting point. Thanks.

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### tom28571Senior Member

Actually your question was answered in more detail than you asked it. Specifically, width is more important than length in a planing hull. Unfortunately, the boat has to operate other than theoretically on a sheet of paper, in full planing mode or in other than ideal conditions such as flat water. First, the boat has to float and then it has to be powered, controlled, support the desired load and get from standstill up to planing speed in less than ideal conditions. These things introduce compromises to the ideal flat planing surface of infinite width. This "ideal" hull operates at an ideal trim angle of about 5 degrees and is only long enough to contain the wetted portion of the hull. In this sense, any hull length extending forward of the wetted waterline serves no purpose.

Thus the boat has to have buoyancy enough to support crew, cargo and powerplant. Then it needs length and an underwater shape like a V introduced to make the boat handle real water conditions. For rougher water, more V is introduced. To get more planing efficiency, keel pads, flats or strakes are then added to the bottom to compromise between the ideal flat bottom and the deep V. And so it goes, on and on. That's why designers make the big bucks

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