Originally Posted by Rucksta
Is reliance on boat speed for true wind calculation a hangover from and era where GPS did not exist?
No, not at all...
In MOST cases the difference is small, but not in all. The difference is that "TRUE WIND" as calculated with Speed Through the Water is relative to the water surface, which is where (hopefully!) your boat lives. If you use GPS to do the calculation you actually end up with GROUND WIND, i.e., wind relative to the unmoving ground. If there is no current they are the same... but...
Let's put together a simplistic hypothetical designed to make the math easy and avoid trigonometery...
A boat with a heading of 000º, in a current with a set of 000º, and a drift of 5 knots. The wind is from 180º at 8 knots. Speed through the water is 0 kts.
Apparent wind is 3 kts with an AWA of 180º.
A calculation of TWS and TWA using STW would tell you your TWA is 180º and your TWS is 3 kts. Which is the right answer, because that
is what you really have to sail with.
A calculation of TWS and TWA using SOG gives very different answers. It would give a TWA of 180º and a TWS of 8kts. While this is great data if you are sitting still on the ground, you don't actually
have 8 knots of wind to sail with.
Just as an example, my boat sails
fine downwind (slow--but fine) with 8 knots TWS. With a TWS of 3 kts, our sails would just hang there. So this is a real difference, in the real world.
You might decide this is not relevant to you if you sail in a place with no currents, or that you don't really care. That's fine. But the difference can matter to your boat even if you decide it doesn't matter to you.
It is not required information, but it does help sail a boat efficiently.