Hullspeed and the
Speed/Length Ratio

The theoretical hull speed is the maximum speed that a non-planing boat can achieve in displacement mode, when the wavelength of its bow wave is equal to its waterline length. Beyond this speed, the boat will encounter increasing wave resistance and will need more power to overcome it.

The main factor that affects the theoretical hull speed of a boat is its waterline length, which determines the wavelength of its bow wave. The longer the waterline length, the higher the theoretical hull speed. Other factors that may influence the actual speed of a boat include its hull shape, displacement, draft, trim, sail area, wind and sea conditions, and propeller efficiency.

Planing boats are boats that can lift themselves partially or fully out of the water and ride on top of their own bow wave, reducing their wetted surface area and drag. Planing boats can exceed their theoretical hull speed and reach higher speeds with less power. Non-planing boats are boats that remain fully submerged in the water and cannot climb over their own bow wave. Non-planing boats are limited by their theoretical hull speed and require more power to increase their speed.

The half angle of entrance is the angle between the waterline and the centerline of a boat at its bow. The smaller the half angle of entrance, the finer the bow shape and the lower the wave resistance. A fine bow can slice through water with minimal disturbance, while a blunt bow can generate large waves and drag. The half angle of entrance is one of the key factors that determines the wave-making resistance of a boat.

There are several ways to increase the speed of your non-planing boat, such as:

• Increasing your sail area or using more efficient sails;

• Reducing your displacement or weight;

• Optimizing your trim or balance;

• Improving your propeller efficiency or reducing your propeller drag;

• Choosing a finer or longer hull shape;

• Sailing in favorable wind and sea conditions.

Some common misconceptions about hull speed are: - Hull speed is a fixed limit that cannot be exceeded by non-planing boats. In reality, hull speed is a theoretical estimate that can be surpassed by some boats with sufficient power or sail area, but at the cost of increased wave resistance and drag.

• Hull speed is the same for all boats with the same waterline length. In reality, hull speed can vary depending on the hull shape, displacement, draft, and trim of the boat, as well as the wind and sea conditions;

• Hull speed is the optimal speed for non-planing boats. In reality, hull speed is often too high for non-planing boats to maintain efficiently or comfortably, especially in adverse conditions. A lower speed that minimizes wave-making resistance and maximizes fuel or power efficiency may be more desirable.