# Calculating True Wind – a sailboat’s fuel tank

Simple Geometric Calculation
Y = 90 – D
a = AW * ( cos Y )
bb = AW * ( sin Y )
b = bb – BS
True-Wind Speed = (( a * a ) + ( b * b )) 1/2
True-Wind Angle = 90-arctangent ( b / a )
True-Wind Calculator

{ Apparent-Wind (AW) and Apparent-Wind Degrees off the bow (D) are known. }

http://www.sailingusa.info/true_wind_calculator.htm

This simple calculation doesn’t take into account leeway. Leeway is the drift of the boat to the side due to the push of the wind against the sails.

Leeway depends on:

• Angle to the Wind – is greatest on a close reach and zero on a run.
• Velocity of the Wind
• It will be different for every boat.

This simple calculation doesn’t take into account current. Current creates a ‘water frame of reference’ which is different from the ‘earth frame of reference’

Earth Frame:

• GPS
Water Frame:
• Wind Vane
• Anemometer

Current Deviation
C : Is the angle of the Current to Boat’s Bow

sine C = c/ac = sine C * a
cosine C = b/a
b = cosine C * a
d = power – b
CD = arctangent ( c/d )