The evolution of an all-round 6-metre hybrid displacement/flying day-sailing catamaran is described. A 4.9m prototype was a relatively conventional, but wide, beach-cat type platform with a single fractional rig. Fitted with mechanically incidence-controlled horizontal lifting surfaces beneath the twin daggerboards and a single inverted T rudder, the boat could be sailed in one of three modes: fully displacement with the lifting foils locked in neutral; one hull displacement and the other flying under automatic height control; two hull flying. The small size and structural fragility of the 4.9m boat led to the construction of a 2-man 6m version which, despite being overweight, performs satisfactorily over a wider range of operating conditions than her predecessor.
A simple VPP suggested that a second pair of lifting foils with greater area would enlarge the fully oilborne performance envelope upwind, although reducing top speeds on other courses. In practice these foils performed poorly other than on flat water. Other errors contributed to the ultimate failure of one foil-strut assembly, the mode of which is described in detail. Prior to this it was found that operating the foils as stabilisers, i.e. in their active mode but with the hulls in the water, provided a remarkably comfortable, safe and fast ride, particularly upwind in gusty weather. Subsequently, "flying displacement" became the preferred upwind mode, with the original, smaller lifters.
Because the author's working hours have limited the opportunity to sail against other similar sized beach cats, instruments to record the boat's performance have been developed. Based around readily available low cost microcontroller technology, the data gathered is processed to identify short, steady periods of sailing. The resulting polar diagrams have compared favourably with predicted performance. Supporting the weight of a boat, as well as resisting roll and pitch moments, through a more subtle combination of dynamic foil lift and hull displacement than presented here provides a continuing opportunity for further developments.