The present invention relates to a hybrid hydrofoil marine vehicle, and in particular to a marine vehicle having a hybrid hydrofoil with an extendable strut leading edge. 
Hybrid hydrofoil marine vehicles designs are known in the prior art.  Referring to FIG.  1, a marine vehicle 10 constructed according to the hybrid hydrofoil concept includes an upper hull 12 of the conventional monohull form with the addition of a long, slender, single strut 18 and a lower body or lower hull 14 added to its keel.  The lower body buoyant lift is augmented by the dynamic lift from the fully submerged foil system 16.  Foil dynamic lift comes into play at speeds of about 12-15 knots and above, at which time the upper hull is lifted from the water surface leaving only the small water plane of the single strut at the interface between air and water.  The foil automatic control system maintains predetermined flying height and provides a stable platform in waves.  The foil surfaces are sufficiently powerful to counter roll, pitch, and heave motions that would be imparted to a conventional monohull in high sea states.  Propulsion of the vehicle can be provided by one or more prime movers located in the upper hull driving through a Z-drive to a propeller on the stern of the lower hull, or by prime movers in the lower hull driving straight through to one or more propellers. 
Details of the hybrid hydrofoil concept and its applications can be found in "Hybrid Hydrofoil Technology Applications" by John R. Meyer, High Performance Marine Vehicles Conference and Exhibit, Washington, D.C., June 1992, incorporated herein by reference. 
One of the problems in designing the hybrid hydrofoil is to define the strut 18 dimensions and location (i.e. set-back A from the lower hull nose 20) in such a way that the marine vehicle has acceptable directional (lateral) dynamic stability in all foilborne modes.  These modes include the nominal shallow draft condition (where a relatively small part of the strut is immersed) and a deeper draft condition (where the water line is close to the bottom of the upper hull). 
Closely associated with the directional dynamic stability characteristic is the relative ability to execute coordinated turns.  If the marine vehicle is too stable, it will be difficult to turn; if it is close to being dynamically unstable, it can achieve relatively high turn rates. 
Recent analyses of the directional dynamic stability and turning characteristics of a hybrid hydrofoil marine vehicle, as shown in FIG.  1, indicate that the directional dynamic stability is sensitive to strut leading edge set-back (distance A from the nose 20 of the lower hull) and overall strut length.  Apparently, small decreases (one to two feet) in strut leading edge set-back can change the hybrid hydrofoil's directional dynamic stability characteristic from positive to negative.