2007 © Francis Demange

2005 © Jean-Maris Liot

2007 © Francis DemangE

1995 © Philip Plisson

1990 © Olivier Chapis

## Hydroptère

From the Greek hydros (water) and ptère (wing), *Hydroptere* is a trimaran with foils. These underwater wings extract the boat’s hulls from the water when a certain speed is reached, which considerably reduces the ship’s hydrodynamic drag.

Thus, *Hydroptere* can attain very high speeds.

Principle

No longer floating on the water, but flying, the ultimate dream. To free oneself of the Archimedes’ principle, the boats’ hulls are lifted out of the water thanks to a vertical thrust given by the foils.

Then the foils are the only parts in contact with the water, which results in a considerably reduced drag and thus gives the opportunity to achieve far higher speeds than traditional boats.

The functioning of the foils is simple and based on the principle of dynamic lift, allowing the wings of a plane to make it take off.

Thus, when it manoeuvres in the water, the foil generates a difference in pressure between the lower and upper surfaces of the wing. This difference in pressure is translated physically by an upward force and applied to the foil, which is called lift. The greater the speed of projection of the foil, the more the lift increases. After a certain speed, the lift generated by the foils becomes superior to the weight of the boat and it causes the boat to be lifted out of the water. The geometry of the foils used on *Hydroptere* is conceived so as to limit the increase of this lift, so that the boat stops rising and is stabilized a few meters over the surface of the water. The foils used are V Autostable foils.

This very simple concept allows for an easy application, but the stress exerted is such that it was necessary to wait for the advent of new materials such as carbon and titanium in order to be able to make a large-sized boat fly over the waves.

## Archimedes’ principle

A body immersed wholly or partially in fluid (liquid or gas) is buoyed up by a force (buoyant force) equal to the weight of the displaced fluid. That force is called “the Archimedes’ Thrust”. In a field of uniform gravity, the thrust of the Archimedes’ PA is always given by the following formula:

where Mf is the mass of the fluid contained in the displaced volume V, and g is the value of gravity.

If the density ρ of the fluid is also uniform, we will have:

or if we consider only the greatness of the force:

The Archimedes’ thrust PA will be expressed in Newton (N) if the density ρ is in kg/m³, the displaced volume of fluid V in m3 and the value of gravity g in N/kg (or m/s²).