Kinetic energy = 1/2 * mv^2
Potential energy = mgh
assume g=9.81, you can convert all the kinetic energy into potential energy when 1/2 * mv^2 = mgh
Mass cancels out, v^2 = 2gh
v = 30 knots = 15.4m/s, so v^2 = 237.16
so h = 237.16/(2 * 9.8) = 12.1metres.
This ignores friction and other losses.