wind you feel onboard a moving boat is a
combination of true wind
and boatspeed (the apparent wind).
solving the wind triangle,
speed and angle is obtained from the measured apparent wind, heel and
boatspeed. Wind direction
is then the sum of true wind angle and
Correctly calculated wind direction and speed does not change
with the boat's sailing condition.
True wind is a tricky thing to get right. Any error in the inputs
throws off the output, causing the true wind to change with the sailing
There is a good way to test for a valid solution: by seeing how much
the wind direction changes when you tack or gybe (the 'wiggle').
wiggle = (wind direction before tack) -
(wind direction after tack)
There are two frames of
reference for wind, each with its own advantages.
The Ockam system
unless COG/SOG is replacing boatspeed/heading. OckamSoft has a BIF for
relative. This is the 'normal' way of thinking when on
land. "Wind's from the nor'east at 60". If you strapped an
instrument on a car and drove around, the wind direction would always
report as 45° (ignoring magnetic variation of course).
- All meterology is earth-based and true north oriented,
so earth-true is
the number you have to use when route planning.
- Using earth-based boatspeed & heading prevents
the calculation of current.
- When converting to water-true, say for next-leg
apparent, the local current and magnetic variation must be considered.
- Water relative.
On the water, the true wind that the boat feels is a combination of the
earth-true and current. If the current is
significant, water-true wind will be different
- The boat responds to water-true
because the keel is immersed in the water. Polar plots always use water-relative reference.
- True wind angles
are the same on both tacks (except for wind shear).
If wind direction
were calculated using COG/SOG, the true wind angles would be different
on the opposite tacks (more on the difference here).
- When being used for performance, because of
this symmetry, water-true is the number to use.
- Current is calculated by comparing water-based
boatspeed & heading+leeway with COG/SOG.