What happens to the water next to the sailboat when it achieves "hull speed".?

Question

Why is hull speed an important concept?

Answer 1

As a displacement (ie. non-planing) boat moves through the water it creates a bow wave. This wave flows down the sides of the boat as the boat moves forwards and creates a new bow wave. As boat speed increases the wavelength of the bow waves lengthens, in other words, the bow wave has moved further along the hull when the next bow wave is produced. At a certain speed the wavelength of the bow waves will equal the waterline hull length. The boat will be "floating" on the new bow wave at the bow and the old bow wave at the stern. If you try to go faster, the stern will fall into the tough between the two bow waves and you will then be motoring up the front of your bow wave. Non-planing boats (displacement boats) cannot do this, so "hull speed" is the speed at which the wavelength of the bow waves equals the waterline length of the boat.

A planing boat (most powerboats) employ huge amounts of power to drive up their bow wave and they "ride" this wave much like a surfer. This reduces their waterline length and their drag, which is one reason they go so fast (big engines is the other!).

Answer 2

In a displacement hull hull speed is the top speed at which a hull can move through the water. The water is displaced, as in pushed aside. If your hull planes (bow out of water and large bow wave) then you can achieve higher speeds.