design boat speed

[sundance]

On this forum and elsewhere, I have seen wide variations in the equation for arriving at a displacement hull's maximum design speed. These variations have ranged from the square root of the waterline length multiplied by anything between 1.20 and 1.45!
Can anyone confirm, once and for all, what exactly is the correct figure to multiply the square root by - and why so?

James

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[beachbum]

In short, no - because there is no "exactly correct figure".

As a displacement hull approaches speeds of this order, the power requirement for further increases in speed becomes rapidly greater. It's a progressive thing, though, a classic law of diminishing returns, there is no definite "cut-off". You can go on piling on more power 'till the engine weighs so much it sinks the boat, I s'pose that would be some kind of "absolute" value. Otherwise, you make your own choice as to how much power you want to provide - e.g. if 10hp will get you to 4 knots, say, 20hp to 5, 40hp to 5.5 etc... - where do you draw the line?

1.20 is a very conservative figure, and 1.45 rather optimistic, but that doesn't mean either is wrong. The most commonly quoted figure seems to be 1.40

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[rwoofer]

The differences in the factor are more down to the wave generating properties of the hull. If things like weight are equal an efficient race boat will have a higher factor than a tubby cruising boat. This reflects the real difference in wave making properties.

Some hulls can break displacement speed by a considerable margin and still not be planing eg. catamarans.

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[extravert]

> Some hulls can break displacement speed by a considerable margin and still
> not be planing eg. catamarans.

That's because the formula applies to typical mono-hull designs with a LWL to BWL ratio of about 3:1. Often multi-hulls have a completely different ratio. My main hull LWL to BWL ratio is 10:1, floats are 18:1. Hence the formula does not apply.

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[cameronke]

Hello Extravert

Are we looking at getting into wave piercing rather than displacement here?

How did you get on with the wet & dry vac?

Regards
Cameron

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[extravert]

> How did you get on with the wet & dry vac?

Sucks like a tart on speed. Thought I'd try it pointing at the sky next time it starts dribbling.


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[snowleopard]

the limiting hull speed occurs when the wavelength of the bow wave equals the waterline length and the boat sits in the hole it creates.

the relation of the wavelength to the hull depends on the shape. at one end of the scale a boat with long overhangs like a j class or 12 metre effectively increases its waterline as the overhangs become immersed. at the other end a blunt ended beamy boat creates a short steep wave.

the wavelength of the bow wave relates to its speed so a boat which creates a long wave will travel faster than one with a short wavelength. this means that two different shaped hulls of the same waterline length will travel at slightly different speeds. this is reflected in the different constants.

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[boatmike]

For a fairly average monohull with modern beam to length ratios and fin keel a reasonable average figure would be 1.3. Other correspondents are correct though in that this is the point where hull resistance becomes significant not a maximum. It will require disproportionate power to go faster in other words. It is also correct to say catamarans or other hulls with a narrow beam will have a much higher ratio and are limited mainly by the combination pressure wave between the hulls not the hull shape itself. also of course fat little tubs with wide ransoms and blunt bows wont go as fast so use 1.3 as a general rule and then apply common sense.......

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