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Hydrofoil Basics - Configurations

(Last Update 9 May 98)

Hydrofoil configurations can be divided into two general classifications, surface piercing and fully submerged, which describe how the lifting surfaces are arranged and operate (see Figure 1). In the surface piercing concept, portions of the foils are designed to extend through the air/sea interface when foilborne. Struts connect the foils to the hull of the ship with sufficient length to support the hull free of the water surface when operating at design speeds. As speed is increased, the lifting force generated by the water flow over the submerged portion of the foils increases causing the ship to rise and the submerged area of the foils to decrease. For a given speed the ship will rise until the lifting force equals the weight carried by the foils. As indicated by the terminology, the foils of the fully-submerged concept are designed...

Figure 1-- Surface-Piercing (Left) & Fully-Submerged (Right) Foil Configurations

to operate at all times under the water surface. The struts which connect the foils to hull and support it when the ship is foilborne generally do not contribute to the total hydrofoil system lifting force. In this configuration, the hydrofoil system is not self-stabilizing. Means must be provided to vary the effective angle of attack of the foils to change the lifting force in response to changing conditions of ship speed, weight and sea conditions. The principal and unique operational capability of hydrofoils with fully-submerged foils is the ability to uncouple the ship to a substantial degree from the effect of waves. This permits a relatively small hydrofoil ship to operate foilborne at high speed in sea conditions normally encountered while maintaining a comfortable motion environment for the crew and passengers and permitting effective employment of military equipment. It is this desirable characteristic which has caused the hydrofoil ship development in the United States to concentrate on the fully-submerged foil concept.

The basic choices in foil and strut arrangement are canard, conventional or tandem as shown in Figure 2. Generally ships are considered conventional or canard if 65% or more of the weight is supported on the front or the aft foil respectively. If the weight were distributed relatively evenly on the fore and aft foils, the configuration would be described as tandem.





0.00 < X/L < 0.35


0.65 < X/L < 1.00


0.35 < X/L < 0.65

Figure 2 - Foil/Strut Arrangements


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