HYDRODYNAMICS

• Foils: Which part in our speed machine ?
  In the boat definition, at high speed, the main part of the boat lift will be coming from the aerodynamics. But before "flying", the boat must take off !! In order to accomplish this phase, we choose to use foils to creat lift at mid-speed.
  
  
• The different kind of foils
  Many people are familiar with airfoils. Hydrofoil is simply another word for a wing (such as the wing of an airplane). A hydrofoil is a wing that "flies" in water.
  
  
  • T-foil
    Schematical representation of a T-foil.
    [At the same time, you can learn some french words !!:
    "bateau" means "boat", "surface de l'eau" means "water surface",
    "Portance" means "Lift" and "Foil en T" means "T-foil"].
    The foil (in red) under the water generates a lift which moves the boat out of the water. A T-foil has a very good efficienty but is naturally instable.
    
    
  • V-foil
    Schematical representation of a V-foil.
    [At the same time, you can learn some french words !!:
    "bateau" means "boat", "surface de l'eau" means "water surface",
    "Portance" means "Lift" and "Foil en V" means "V-foil"].
    The foil (in red) under the water generates a lift which moves the boat out of the water. This foils is naturally stable.
    
    
• Ventilation :
  Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the water; So the foil generates much less lift.
  
  
  Schematical presentation of the ventilation (+ means high pressure, - means low pressure)
  (Sorry still in french...).
  Ventilation is a complex phenomenon:
  • The lift decreases.
  • It can occurs at low and high speeds.
  • To avoid ventilation, you can put fences on the foil. Fences are small fins placed on foils to prevent ventilation air from migrating down along a foil. The fences are attached to the foil so as to be parallel to the direction of the fluid flow.
  • Very few experimental datas available.
  • Ventilation is very hard to model.
  
  
• Cavitation :
  Caviation occurs when the pressure is lowered to the point where the water starts to boil. When the local speed is high enough, the foil generates so much lift (i.e the pressure on the lifting surface of the foils goes down) that the water flowing over the blade begins to boil. When cavitation occurs, the foil no longer generates engough lift (something you like to avoid !!).
  
  So the cavitation consequences are the following :
  
  • The lift decreases (again !!).
  • Chemical or mechanical erosion.
  • Noises.
  • Vibrations.