Deep and shallow water waves

cb625

1. The problem statement, all variables and given/known data

Large ships in general cannot move faster than the phasr speed of surface waves with a wavelength equal to twice the ship's length. This is because most of the propulsive force goes into making big waves under these conditions rather than accelerating the ship.

How fast can a 200m long ship move in very deep water?
As ths ship moves into shallow water, does its maximum speed increase or decrease?

2. Relevant equations

For deep water waves, water depth h>lambda/4
and v(ph)=square roots of (g/k)

3. The attempt at a solution

The wavelength of surface waves = 2*200m=400m ...
But then I don't know what to do ?