643,682. Hydroplane vessels. BULL, Ltd., R., and GARDINER, N. W. Nov. 12, 1947, Nos. 30133 and 21896/48. [Class 113 (ii)] [Also in Group XXXV] In an automatic control system for a craft supported during its travel above a water surface by a hydrofoil, the water level ahead of the craft is detected electrically and the angle of incidence of the hydrofoil correspondingly adjusted to cause the craft to follow undulations of the water surface. A craft 1, Figs. 1 and 2, when at rest or moving slowly, floats on a water surface 66, but when propelled at high speed by an enginedriven airscrew 12 or a water propeller (not shown), it is supported in the elevated position shown, by a hydrofoil 5, having a fixed angle of attack, located between two rudders 14, 15 on a stern strut 3, and a pair of hydrofoils 4, pivoted on forward struts 2, the angles of attack of which are adjusted by rods 6 under the control of actuators 7 in turn electrically controlled by water level detectors 9 attached to arms 8 projecting forward of the craft. In an alternative arrangement the arms 8<SP>1</SP> are more widely separated to improve lateral stability. Retraction of arms 8 to permit landing the craft on wheels lowered prior to beaching, and a plurality of linked hydrofoils on each strut 2 are referred to. To compensate for the slower control of the aerofoil at high speeds an object may be towed behind the craft on a wire, change in strain of which operates contacts controlling an electric motor adjusting the distance between the craft and the arm carrying the water level detector. In one arrangement, Fig. 6, a water level detector 9c is arranged so that the water completes the circuit between a common replaceable contact 25 and a number of the contacts 23a-23h dependent on the depth of immersion so that a corresponding number of the relays 29a-29h are energized from a D.C. source 28 to energize, through relay 33 or 34, one or other pair of coils 42, 46 or 41, 47 controlling supply and exhaust ports of a reversible fluid-pressure servomotor 37 adjusting aerofoil rod 6 until by virtue of follow-up contact 51 the servomotor takes up a position corresponding to the water level. Automatic control may be disengaged by opening switch 35 and the servomotor controlled manually by push-buttons 54, 55. The following modified forms of water level detector suitable for the above and some of the other arrangements are described: (a) A series of pairs of contacts at different levels without the single common contact. (b) A series of switches at different levels, spring biassed to an open position and enclosed in a flexible waterproof cover, which are closed by the water pressure when immersed. The following modified forms of actuator for aerofoil rod 6, suitable for control by the electrical relay system of Fig. 6, are described: (a) A reversible D.C. shunt motor adjusting rod 6 through a worm drive, limit switches stopping the motor at extremes of travel. Manual control may be effected, after operation of a changeover switch, by selective energization of relays 29a-29h of the relay system. (b) A reversible fluid-pressure motor under the control of a valve operated by a reversible electric motor controlled as in the preceding arrangement. (c) A reversible electromagnetic slip coupling or an electromagnetic clutch in which a pair of rotating elements, continuously driven in opposite directions through gearing from an electric motor, are selectively energized to rotate a third element adjusting rod 6 through a screw and nut drive. In the case of the slip coupling the third element carries a shortcircuited winding, but it may alternatively carry a three-phase winding connected through slip-rings to an external resistance, the value of which may be varied manually or by relays at predetermined limits of water level to produce rapid correction under rough sea conditions. The third element may be fitted with a spring-urged mechanical brake which is released electromagnetically whenever one of the other elements is energized. Fixed energization windings for the elements, to avoid slip-rings, are referred to. (d) A mechanical friction clutch under the combined control of fluid - pressure servomotors, gravity and/or springs, or a hydraulic friction coupling under the control of electromagnetically-operated valves, involving rotating elements continuously driven in opposite directions co-operating with a third element coupled to rod 6. Alternatively the water level detector may be a pair of electrodes in the water, the resistance between which is dependent on the depth of immersion, arranged in series with the supply to coils 29a-29h, Fig. 6, which are then graded to operate at different values of current. In this arrangement supply to the coils is derived through a potentiometer which serves to adjust the aerofoil angle corresponding to a given water level, while an ammeter in series with the coils may be calibrated in terms of that angle. With the last-mentioned type of water level detector or with one of the multiple contact type associated with a tapped resistance, current variations resulting from variations in the depth of immersion may be amplified, e.g. by a carbon pile relay or electronically to energize a solenoid adjusting rod 6 against the action of a spring. A similar form of control is also obtained using a copper-mica condenser variable with water pressure on immersion but shielded to be independent of craft speed, associated with an A.C. supply source. There is a reference to a radar type of water level detector.