Although foils seem very recent, the first designs date back to the end of the 19th century when the Italian inventor Enrico Forlanini registered the first patents.
In 1906, Alexander Graham Bell began to be interested in hydrofoils, relying on the work of Forlanini.
In 1919, with the HD-4 hydrofoil shown in the image, he achieved a speed record (almost 115km/h) that stood for 20 years.
A foil is an appendage that is installed in the lower part of a boat and that, due to its hydrodynamic shape, causes the boat to leave the water at a certain speed, thus reducing its resistance to progress. This allows, depending on the objective sought, to increase the speed of the boat or reduce its consumption.
The design of a foil requires applying different technologies in the engineering process. Once the foil concept to be used has been defined, it is necessary to define the appropriate hydrodynamic profile for each of the areas and validate its operation through fluid dynamic simulation (CFD).
The next step of the process is the structural calculation, the definition of materials and the manufacturing process.
It is quite common to consider the use of potential foil usage at design time. What is not always studied is the possibility of refit an existing ship to improve her behavior. Foils for an existing ship could be useful in different configurations. As already mentioned, just partially carrying the weight of the ship can reduce fuel usage. Moreover, there are ample other opportunities for improvement. Adequately modifying the flow upstream of a propeller can help increase its efficiency. Good compromises can be found for existing boats too.
We’ve recently carried out internally such an exercise on an existing catamaran. As it is commonly the case with commercial ships, this vessel operates most of its time at a cruising speed. Such an operating envelope opens the door to substantial time savings through optimization. In this case, we took a shot at reducing power requirements for a given speed. The results show a great potential using a relatively small modification.
What we designed was a transverse foil adapted to the geometry of the existing hulls. Lift coming from this wing changes notably the trim of the vessel with a very small price in drag. The change in running trim translates to a substantial reduction in total resistance. Power requirements, therefore fuel consumption, are reduced by 12%. It is important to consider that no attempt has been made to optimize the foil for this specific case. It should be expected to reach at least 15% improvement with a shape optimized to include not only drag reduction, but designed to improve flow over the propellers.
Of course, a ship designed specifically for this configuration, or even to operate in an “skimming” condition, would be more efficient. However, we should not neglect the intrinsic value in existing vessels. Thousands of man hours have gone into designing and building those ships. They could very well get a new shot of life by implementing partial improvements that make them still competitive.
