FOIL EFFICIENCY: Hydrofoil Supported vs Fully Foiling
Hydrofoil-supported catamarans work on the principle that a significant portion of the boat's weight is carried on the foils, lifting the hull partially out of the water. The lift-to-drag ratio of a hydrofoil is typically I/D =20, while that of a typical planing hull is L/D = 5. Therefore, the load fraction on the foils is being carried far more efficiently than that on the hull. From a resistance point of view, it makes sense to carry as much load on the foils as possible and get the hull out of the water.
It is possible to lift the hull completely out of the water and carry 100% of the load on foils and indeed this is the most hydro-dynamically efficient application, but keeping the hull partially in the water offers two distinct advantages in stability and propulsion. First, the hulls being in contact with the water will provide the necessary stability in pitch and roll while the shallowly submerged hydrofoils are naturally stable under the surface. Second, with the hulls still in water contact, conventional propulsion such as outboard engines can all be used in their standard, proven configurations without difficulty.
It is possible to lift the hull completely out of the water and carry 100% of the load on foils and indeed this is the most hydro-dynamically efficient application, but keeping the hull partially in the water offers two distinct advantages in stability and propulsion. First, the hulls being in contact with the water will provide the necessary stability in pitch and roll while the shallowly submerged hydrofoils are naturally stable under the surface. Second, with the hulls still in water contact, conventional propulsion such as outboard engines can all be used in their standard, proven configurations without difficulty.