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The previous
discussion has assumed that the vehicle is completely surrounded by a
single fluid. This would be the case for an airplane wing or a hydrofoil submerged
in water. For a boat operating at the interface between two different
fluids an additional source of drag must also be considered. As the hull moves
through the water, bow waves are generated and the energy for this must
come from the propulsion system. A wave drag, CDw, must therefore be added to the
drag terms already considered. The diagram shows how this component depends
upon the hull speed of the boat, U, and the hull length, L. These quantities
are combined to form another non-dimensional number, Fr = U/(Lg)0.5,
known as the Froude number. At low hull speeds the wave drag coefficient,
CDw
= FDw/(rU2L2/2),
is small and it increases slowly until a critical condition is reached
with the bow wavelength equal to twice the hull length. The drag then takes
a sharp increase. The diagram also indicates that hull design can significantly
reduce bow wave amplitude and the associated wave drag. |
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