Calculation of Sail Performance
The determination of the theoretical speed applies the following princi-
ple:
When the hull moves through the water, the driving power created by the
sail is compensated by a resistance force of the same magnitude acting
in the opposite direction.
The driving power is calculated from wind velocity, angle and the drive
polar values stored in the instrument.
The water resistance acting on the hull increases with the speed of the
ship; it increases with the speed. This ratio mainly depends on the hull
length or the submerged part of the hull.
As the driving force equals the resistance force, the theoretical speed
can be calculated if the waterline length is entered.
The rarely stable wind conditions must be taken into account. The ship
cannot accelerate enough when the wind velocity suddenly increases in
a gust. The time needed until both forces are equal again depends on
the mass of the ship - a light ship will have quicker reactions than a hea-
vy one. This explains the need of entering the relative ship weight to ob-
tain reasonable values in such cases.
The ratio of the actual speed to the theoretical speed ( V(th) ) is display-
ed in percent ( P(%) ) in addition to the theoretically maximum possible
speed. The advantage is that, independent of wind conditions, a quick
trim evaluation is possible without reading the ship speed.
The correct sail performance calculation depends on several factors,
such as ship waterline length, ratio of sail surface to ship size, ratio of
ship weight to ship size. These ship-specific values must be set in the
CLOSE HAULED (see Basic Settings).
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CALCULATION OF SAIL PERFORMANCE
Contents
Summary of Contents for Logic Close Hauled
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