Présentations

Hydrofoils
2. Presentation

a) Historical account

Contrary to what most people think, hydrofoils appeared as of 1869, ie. before the first flying vehicles.

. On September 11th 1869, the Frenchman Emmanuel Denis Farcot takes out an English patent on a removable foil system (in fact, it really is a serie of inclined planes and wedges) designed for a rowing boat. Given the propulsion mean, you need some run-up before reading the speed at which the foils could be installed and partially lift the hull out of the water.

The propulsion was one of the bigest drawbacks of hydrofoils and curbed their development.

. In 1894 in Chicago, the Meacham brothers demonstrate for the first time a 14 feet long speed boat equipped with foils.

. In 1911, the first landings and takeoffs of a seaplane equipped with a system of foils designed by Guidoni take place. These foils are in fact a combination between a multilayer foil system and the V-shapped form invented in 1907 by Croco for boats.
From 1910 to 1921, Guidoni will contribute to the upgrading of a system which, installed under floats, will enable seaplanes to take off at a speed lower than 50 knots and will make landings on rough seas easier.

Concerning more usual boats, the stress is put on speed.

. In 1919, the world record for speed on water is broken by a 60 feet speed boat equipped with foils (built by Alexander Graham Bell and Frederick W. Baldwin) which reaches a top speed of 61.5 knot/h.

Since 1869, foils have evolved quite a bit, from a simple flat from to a bulging ans streamlined form, but the principle remains that of a fully submerdged foil.

. In 1932, professor O. Tietjens takes out a patent on a V foil system (surface piercing foils). He tests it on a small and fast 500 pounds boat which reaches 25 mph with a low power engine.

. In 1950, J.G. Baker accomplishes the first "takeoff" of a sailing boat eqipped with foils.

Afterwards, the first electronic control systems are developped. They make it easier to take off and land at sea and they especially improve the stability of boats using fully submerdged foils. The evolution of hydrofoils then mostly concerns improving existing techniques in order to go faster, to be more stable and to be able to support more weight. In the mean time, the use of foils appears on human or solar propulsed vehicles or in water skiing and windsurfing.

b) Presentation of our model

The aim of our work was to visualize the elevation of a hydrofoil model above water. We would then be able to realize different experiments and mesure lift and drag. We had to our disposal the canal of the Fluid Mecanics Department of the INPT-ENSEEIHT. It is a rectangular glass-made canal which is 10 inches wide. The flow can be adjusted between 0 and 23 liters per second and the water height between 0 and 1 feet (the maximal speed is therefore close to 0.8 m/s).
The hydrofoil model we decided to build has a T foil, is made ou of polystyrene with an aluminium bar on each side to fix the foil to the hull. This sytem enables the foil to rotate around a horizontal axe and so we could adjust its incidence.

We used the "hot wire" technique within the IMFT (Institut de Mécanique des Fluides de Toulouse) to make the main parts of our model. The principle of the method is quite simple : an electric current is used to heat up a wire which in contact with polystyrene makes the latter melt. In this way, we could cut up the hull and two different foils (one symetrical and the other asymetrical).

Vidéo 2.60 Mo

In order to increase the foils’ resistance to water and to make them smoother, we added a synthetic fibre on their surface, which we then covered with resin to make the different layers hold together.

Vidéo 1.45 Mo

Our model being built, it was time we moved on to the experimental part. we first tested the symmetrical and assymmetrical foils. As we could not directly fix the speed of our model (it has no propulsion mean), we had to adjust the flow rate in the canal. Indeed, when the flow rate increases, either the water height increases, or the flow speeds up (the model being still, it is equivalent to accelerating the model in a steady flow). In our case, the water height can more or less be fixed, so that an increase of the flow rate results in an increase of the flow speed (at the beginning though, the water height increases a little bit). Once the water height has reached its peak, the flow speed increases and the drag created be the foils increases until the hydrofoil reaches an equilibrium: at this point, drag compensates weight, and the hull is out of the water.