As a boat’s hull passes through the water, it creates waves emanating from both the bow and the stern. In the water behind the boat, the waves cause a wake. On the boat itself, the waves are felt as drag resistance, tending to slow the boat down.
The distance between wave crests, or wavelength, of both the bow and stern waves is dependent on speed. As the boat travels faster, the wavelength becomes larger. At some speed, when the wavelength is equal to the length of the boat, the bow waves and stern waves add together to dramatically amplify the drag resistance felt by the boat. The speed at which this occurs is known as the hull speed of the boat, which is given by the following formula:
Here, LH is the length in feet of the hull at the waterline, and sH gives the hull speed in knots.
While it is certainly possible for a boat to sail faster than this speed, it takes much more force to overcome the drag resistance and increase speed. So hull speed is a useful estimate of the peak speed under normal sailing conditions and without the spinnaker.
