Hydrofoils and Hysucat
A hybrid hull consisting of an asymmetrical, planing catamaran hull and special hydrofoil system was developed by Prof. Gunter Hoppe at the University of Stellenbosch, South Africa, in the early 1980’s. The resultant hull form is known as a HYSUCAT (HYdrofoil SUpported CATamaran). The advantages of this unique, hybrid system is that it further amplifies many of the existing benefits of the catamaran hull-form such as:
- Reducing the running resistance of the catamaran hull for a given speed by approximately 35%. This advantage can be exploited by lowering the installed power (reduced capital cost and reduced fuel consumption) or increasing speed by approximately 35% when using existing engines.
- Improved ride quality in rough seas due to the motion damping effect of the hydrofoils.
- Reduced wake size as the catamaran hull is partially lifted out the water by the hydrofoils causing less disturbance to the body of water the hull is operating in.
The main hydrofoil fitted between the catamaran hulls acts like any classic foil operating in a fluid medium - the top surface generates a low pressure region, the lower surface a high pressure region and the mass of water flowing past the foil is deflected downwards. The resultant is lift generated far more efficiently than the hull planing surfaces – typically the Lift/Drag ratio of a planning hull is 4:1 while that of the hydrofoil is 20:1. Smaller trim hydrofoils are also fitted on the inside of the catamaran tunnel near the transom. These smaller foils play an important role in the automatic longitudinal trim stabilisation of the hull in that as they approach the water surface in the catamaran tunnel, the bow trims down, their lift decreases (due to free-surface effects) and then, due to lift reduction, the transom trims back down again.
In general, the hydrofoils are protectively fitted between the catamaran hulls as they are fitted to the hull keels as deep as possible so as to keep them fully submerged but no lower than the keels – as they do not protrude below the hull nor are they wider than the tunnel beam, there are no operational or aesthetic constraints as a result of fitting HYSUCAT hydrofoils.