lets say you are sailing to a windward mark and following your VMG on your GPS. If you get to the point wherein VMG becomes zero then negative, aren't you at the layline for that mark and should therefore tack? (not counting leeward drift). It would mean that if you tacked 90 deg then you would be headed straight for the mark. Am I wrong?
If you are trying to use VMG for its best use, then yes, you are wrong. If you thought that simply because you are moving in a direction that is 90º to your target, that you are at the perfect place to tack, then yes, you are almost certainly wrong.
But using the VMG function, or using a bearing for determining a tack angle is not a good idea UNLESS: a) there is no current, and b) There is an exact 90º tacking angle on the boat upon which you sit, and c) there are no deviations in the direction from which the wind blows during your journey (wind shifts), and d) there are no seas (waves) which will affect your progress, and e) your boat does not do better on one tack than the other tack, and f) there are no wind velocity changes that will affect your boat's pointing ability.
If you are using this reading to determine when to tack, then you could just as well use the (proportionate) "bearing to mark" reading. When it hits the desired tacking angle, ... go for it.
As for the usefullness of VMG To Mark, it is very limited without the added datum "true wind angle." Additional data to include 'true wind direction,' 'true wind speed,' 'delta of distination:true wind,' and average boat speed (over the ground) on each tack,' would IMO be certainly be helpful.
GARY
