Hydrofoils commonly are in the form of thin generally planar structures which are suspended beneath the vessel and extend in directions substantially perpendicular to the vessel's direction of movement. The hydrofoils are usually maintained in rigid relationship to the hull of the vessel and are contoured such that at high speeds the vessel rises at least partially out of the water and rides or "planes" on the foils.
Submerged foils produce more lift at lower speed than a surface piercing foil of the same size because of its larger wetted area and the more vertical orientation of the lift vector. However, the lift vector of the surface piercing foil is generally inclined, as viewed from the front, toward the vehicle center of gravity, making the surface piercing foil the more stable configuration. Stability is a factor of some importance, since all hydrofoils are top heavy when foil borne.
Surface piercing foils are the simplest in design because they are generally self-stabilizing in roll, and in height above the 'water. However, because a portion of the surface piercing foil is always in contact with the water surface, and therefore the waves, this type of foil is more susceptible to adverse effects of wave action that results in a rough ride.
In contrast, fully submerged foils have no contact with surface waves and therefore a smoother ride can be attained in rough water. Boat designers have found, however, that this type of design, where the hull raises out of the water and thus becomes airborne, is generally not passively stable, i.e., it is not self-stabilizing. Consequently, to maintain a specified height above water and a straight and level course, a boat having totally submerged foils usually requires an independent control system to adjust the angle of attack of the foil surface. This control is much like that of an aircraft requiring multiple control surfaces.
In a smaller boat, while it is desirable to become foil borne at as low a speed as possible and to operate the foils when hull borne in a shallow draft or beaching mode, stability must also be considered. Crossing the wakes of other boats, manuevering, and operating at angles to significant wave action are a few examples of operating regimes where stability is desired. Submerged foils suffer interference drag losses at the intersection of the foil and the supporting strut. At higher speeds, the surface piercing mode of operation is far more efficient, enabling operation at higher speeds and with greater economy than at the submerged mode, since this form of drag can be eliminated.
At constant speed, hydrofoil boats control attitude, height above the surface and trim by control tabs on the foils or by changing foil angle of attack. At constant speed, changing from submerged to surface piercing operation and back, as taught by this invention, raises and lowers the boat in the water and, by operating the foils independently, attitude and trim of the craft can be controlled.
A few hydrofoil boats have utilized differential angles of inclination of hydrofoils to bank the boat. When the foils on one side of the boat are inclined away from the horizontal less vertical lift is produced and the boat lists toward the more vertical foil. However, this maneuver and maneuvers executed by the more conventional (no bank control) hydrofoils, produces a momentary side force opposite to the direction of the turn, the horizontal component of the lift vector of the more vertically inclined foil. Side ventilation of the struts in the submerged mode or foils in the surface-piercing mode can occur during these uncoordinated turns causing increased drag and stress.
Pairs of foils may be independently inclined from approximately horizontal or parallel to the plane of the lateral and longitudinal axes of the boat to surface-piercing positions at an angle to this plane many factors may be controlled by the operator including draft of the boat at low speed, height above the surface at any given foil borne speed, bank angle for turns, trim and attitude about both lateral and longitudinal axes and stability. Changing the sweep back angle differentially between pairs of foils also permits banking without creating a momentary sideforce opposite to the direction of banking and turning insofar as the lift vectors of the foils are concerned. The less highly swept foil may, depending upon the banked, wetted area of each foil, have a higher drag which will cause yaw without a counteracting yawing moment.
Hydrofoil boats have used conventional rudders to effect turns, incurring a drag penalty during use. This penalty can be significant during a long run with a strong cross wind or current. Others include a rear, symmetrical wing foil of positive dihedral which may be rotated to produce a yawing moment with no loss of total foil lift and no increase in drag.
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