Low Reynolds Number Hydrofoils

Most hydrofoil sections, like the Eppler E817, are designed for small ships traveling at high speeds, typically 40 knots or more. Sailing hydrofoil boats have different requirements. Their hydrofoils are physically smaller, and they operate at slower speeds, say, 20 to 30 knots, so their Reynolds number range is much lower. This calls for a different design philosophy. The H105 hydrofoil section was designed to avoid laminar separation and ventilation when operating at low speeds and moderate angles of attack, while still having low velocities at small angles of attack to avoid caviatation at high speeds.

Eppler used a rooftop philosophy in the design of the E817, which has a constant maximum velocity at the design angle of attack. This results in a pressure recovery zone aft of the rooftop which is too short for low Reynolds numbers, and the flat rooftop makes for a tendency toward leading edge stall.

The H105 section has convex velocity profiles, which use the entire surface to control the position of the laminar separation bubble. The location of the maximum velocity also changes with angle of attack. The H105 is restrained in its use of aft loading to avoid possible trailing edge separation problems. The result is a hydrofoil which sacrifices some of the upper range of the cavitation envelope for a significantly higher maximum lift, while still maintaining low drag and a high incipient cavitation speed.

The H002 section shown for comparison is an earlier design. This hydrofoil was designed for a higher range of angles of attack than the H005. It had a similar upper surface design approch to the H005, but he lower surface used a rooftop distribution similar to the Eppler hydrofoils. This section had a higher minimum drag, and slightly lower incipient cavitation speed. Its biggest drawback was its excessive aft loading, which may not have worked well in practice. Its chief advantage, higher maximum lift, was more than required since hdyrofoil craft tend to operate within a narrow range of pitch attitudes.

Last Updated on 1/16/99
By Thomas E. Speer
Email: tspeer@gte.net