RE: [Paddlewise] Hull speed isn't wave climbing

Niels Blaauw wrote;
>
>  You are partly right: The bow wave will have the same speed as the boat.
>  However, there is a relation between wave speed and wave length. I don't
>  have the exact formula handy,
>
A wave's speed (in knots) equals 2.5 times the square root of the wave
length in metres.

A boat's waves are the result of pressure differences in the water created
by its forward movement. Above very low speeds, one wave crest is created at
the bow, and one at the stern. Although the wash travels outwards, this wave
profile travels along with the boat. The length of the wave is thus
approximately equal to the waterline length of the boat, and because of the
relationship between wave length and speed, the boat cannot theoretically
exceed a speed of 2.5 times the square root of waterline length without
something drastic happening to the wave. This speed is considered as the
boat's maximum displacement speed (all other things being equal, <G>).

If the boat attempts to exceed this speed, the wave travelling with the boat
must increase its length, which is achieved by the second crest at the stern
falling back further behind the boat, with the boat's stern settling into
the wave trough while the bow remains on the crest of the bow wave. A
further increase in power only makes the bow wave larger and the wave length
longer. This puts the stern deeper into the trough giving an even larger
hill to climb.

A planing hull is designed so that it will lift and overtake its bow wave
(given sufficient power), and then has only a small proportion of its hull
in contact with the water.

So hull speed isn't wave climbing, but attempting to exceed it with a
displacement hull is.


Allan Singleton


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