# how would more or less gravity effect hull speed?

Discussion in 'Boat Design' started by Squidly-Diddly, Sep 1, 2009.

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### Squidly-DiddlySenior Member

I'm guessing more gravity would increase hull speed.

How about more or less dense "water"? I haven't heard of any difference between fresh and salt water hull speeds.

How about more or less atmospheric pressure (discounting changes in "air" density)? I'm guessing no change.

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### masalaimasalai

I have never been anywhere where I could try it out? - There are stories of boats being caught in an up-welling of methane and the reduced density is thought to have caused them to sink... then there is an "ice boat", in which the Europeans seem to get up to incredible speeds, but there the water is less dense but frozen and how do you compare the performance between fresh and saline sailing venues? no capability for "side-by-side" comparison there... and your last question is available but again no opportunity for side-by-side racing.... Just thimk...

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### daiquiriEngineering and Design

Talking about the long waves, the ones that are used for the calculation of the hull speed, the wave speed in a deep water is given by the equation:

V = sqrt ( gL / 2pi )

Where "L" is the wave length, "g" is the gravitational acceleration, "pi" is 3.141592...
So, you can see that increasing "g" leads to an increase of "V" and vice-versa.

P.S. (just saw that you didn't ask about gravity only)
The density and the athmospheric pressure do not compare in the equation because, in the range of speeds and wavelengths that we are concerned about, their influence is small enough (when compared to the gravity) to allow us to neglect both of them. They would be important should you consider the sound waves, which are dominated by the compressibility effects.
Both pressure and density become important again when you have to deal with waves having very large (read: global) length scales - the ones meteorologists have to deal with, for example.
At the other end you have very small waves and ripples, which are very much influenced by the water surface tension but it's a whole different story.

Mas, the story about methane up-welling is more about buoyancy than about the wave propagation speed.

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### masalaimasalai

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### baeckmoHydrodynamics

If there are two fluids with different density, in stationary layers, the formula for the wave propagation as daiquiri expressed it [V=sqrt( gL/(2pi))], will have a correction and becomes V=sqrt( gL/(2pi)*((d1-d2)/(d1+d2))). Here d1 and d2 are the respective densities. With an object moving on the intermediate surface, there is an additional factor to be included as well.

It is quite instructive to use this phenomenon; pour water and an oil into a long bottle, let it lay horizontally for some time to let the fluids separate. Then you can study the gravity waves in the median surface. In some waters, the effect of salinity plus temperature will generate strictly separated strata deep in the water. For instance, a submarine can be very difficult to spot if hiding in the lower layer and moving with a speed that corresponds to a dominant wavelength in the intermediate layer. The "hidden wave surface" will act as a partial reflector to sound waves from a sonar or an echo-sounder.

6. ### mark775Guest

The term for this is "internal wave".

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### baeckmoHydrodynamics

Yup, you are correct there. My note above referred to reflexion of incoming sound waves against the higher density medium. Here, the reflector is the surface, not the wave, thus my citation marks.

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### daiquiriEngineering and Design

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