Hydrofoils 1. Principle

a) Introduction and principle of the hydrofoil

When you want to increase the speed of a boat without having to boost its power, you face problems due to the drag on the hull and the resistance of waves. To overcome these difficulties, there are 3 solutions:
- reduce the surface in contact with water and give it an aerodynamic form (this leads to multihull boats)
- reduce the submerdged volume (this is done on hovercrafts)
- take the hull of the boat out of the water and maintain it there

The later can be implemented thanks to lifting elements similar to wings, called foils, which create a vertical force capable of compensating the effect of gravity. This system is very interesting because it posesses good performances both in terms of efficiency (with an important decrease of the global drag compared to classical boats) and of navigation since such boats are stable and easy to manoeuvre (which has enable such ships to set many records). However, the existence of a minimal speed under which the hull stays (at least partially) in the water means the hull also has to present the lowest possible friction.

Foils can in fact equip any kind of vehicle going on water, even occasionally (as for example seaplanes or amphibious vehicles for which foils are retractable).

Vehicles equipped with foils can be classified according to their mean of propulsion:

• a motorised propulsion (internal combustion or solar engine) linked to a propeller or using jet propulsion. This is used for:

…military hydrofoils	…civilian transport hydrofoils such as the one between Hong-Kong and Macao	…the "Golden Eagle V" which is a solar hydrofoil

• a propulsion using sails, essentially used on multihull boats (which are more stable) and for boats aiming to beat speed records on short distances such as:

…the "Hydroptère"	…the "Yellowpage Endevour"	…and some windsurf such as the "Foilboard"

• a human propulsion (with oars or pedals). Such boats are usually based on the use of a crankgear linked to a propeller either over or under the water.

• a external propulsion. This is principally the case of water skis pulled by speedboats as the "AirChair".

Whatever the category, the size of hydrofoils can vary from:

the model which weights a few hundreds grammes	to the ferry or the warship of several tons

However, in spite of the numerous vehicles capable of using foils, the principle and basic rules remain the same whatever the scale and power may be.

The foils used on these vehicles can be divided into two categories based on functioning and perfomances:
- on the one hand, the surface piercing foils (also called first generation foils)
- on the other hand, the fully submerdged foils (also called second generation foils)
In practice, hydrofoils are equipped with at least 2 foils (one in front on the other in the back) installed accordingly to the weight repartition of the boat (cf. layout of the foils on a ship).

b) Principle of the foil

The forces exerted by the fluid on the foil can be divided into two categories according to the phenomenon which creates them.

• The first category includes the pressure forces due to the movement of the foil versus the fluid. These forces, perpendicular to the profil in each point, can be represented by a unique resultant, perpendicular to the main direction of the flow and oriented upwards. This force is called lift.
• The second category includes the viscosity forces due to the viscosity of the fluid. These forces are tangential to the profil and can be represented by a unique resultant, parallel to the main direction of the flow and oriented opposite to the foil's movement. This force is called drag.

In order to compare the ability of different foils, coefficients without dimension, called aerodynamic coefficients, are calculated (see also the part dealing with Aerodynamics).

• Cx, the drag coefficient, is the intensity of drag on a wing similar to the wing studied but 1 sqm large and placed in a tube filled with air at a dynamic pressure of 1 kg/sqm.
  
• Cz, the lift coefficient, is the intensity of lift on a wing similar to the wing studied but 1 sqm large and placed in a tube filled with air at a dynamic pressure of 1 kg/sqm.