Hydrofoils
Early hydrofoils used U-shape foils. Hydrofoils
of this type are known as surface-piercing since portions of the U-shape
hydrofoils will rise above the water surface when foil borne. Modern
hydrofoils use T-shape foils which are fully submerged.
A hydrofoil is a boat with wing-like foils
mounted on struts below the hull. As the craft increases its speed the
hydrofoils develop enough lift for the boat to become foil borne - i.e.
to raise the hull up and out of the water. This results in a great reduction
in drag and a corresponding increase in speed.
Fully submerged hydrofoils are less subjected to the effects of wave
action, and are therefore more stable at sea and are more comfortable
for the crew and passengers. This type of configuration, however, is
not self-stabilizing. The angle of attack on the hydrofoils needs to
be adjusted continuously in accordance to the changing conditions, a
control process that is performed by computers.
Failure to make the proper
adjustments will result in the foil borne hull dropping violently back
into the sea.
The term "hydrofoil" is
also used to refer to the foil itself, especially when the airfoil profile
has been specifically designed for use in water (such as for a propeller
blade). Hydrofoils are now being applied in multiple marine applications.
Surfers have surfboards with hydrofoils, better suited for big waves
further out to sea.
Since air and water are
basically the same at the fluid level, albeit with different levels
of viscosity, the hydrofoil and aerofoil create lift in identical ways.
The water or air takes the same amount of time to travel different distances,
thus creating a difference in pressures, through application of Bernoulli's
Principle.
This difference in pressures
causes the plane to be sucked toward the area of lower pressure, usually
up, to provide lift. This upward force lifts the body of the hydrofoil,
decreasing drag and increasing speed.
The lifting force eventually
balances with the weight of the craft, reaching a point where the hydrofoil
no longer lifts out of the water, but remains in equilibrium. Since
the force of the waves acts over a smaller area of the hydrofoil, there
is a marked decrease in turbulence drag
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