What does "hull speed" refer to in sailing?

• A. The speed at which a hull cannot go faster due to its shape
• B. The theoretical maximum speed of a displacement hull determined by its waterline length
• C. The speed that can be achieved only in calm waters
• D. The speed at which a boat begins to plane over the water

Answer: (B)

Hull speed is a concept that relates to the maximum theoretical speed that a displacement-hulled sailboat can achieve based on its waterline length. Essentially, the waterline length is the length of the boat that is in contact with the water, and it plays a significant role in determining how fast the boat can move through the water.

The formula used to calculate hull speed is often expressed in knots as:

Hull Speed (in knots) = 1.34 x √(Waterline Length in feet).

According to this principle, as the boat tries to exceed this hull speed, it begins to encounter increased resistance known as wave-making resistance. This resistance occurs because as the boat accelerates, it creates a wave pattern as it displaces water. The deeper and more pronounced these waves are, the more friction and resistance the boat experiences, ultimately making it more difficult to exceed this speed.

Understanding hull speed is crucial for sailors, as it helps them set realistic expectations for their vessel's performance under varying conditions, particularly in terms of efficiency and energy expenditure while sailing.