A late contribution from me on the VMG question:
Thanks for posting
that article, whoever did so.
What I reckon we need to optimise sailing to windward are two measures:
1) Instantaneous Velocity made good through the water
in the direction of the instantaneous true wind
direction, also as measured relative to the water
('True' as distinct from Apparent, rather than vs Magnetic)
It could be termed "Upwind Vmg(w)"
This is pretty useful as a general aid to optimising sail trim, effectively giving you a point on the polar diagram for the boat
under present circumstances.
2) Same except over the ground.
It could be termed "Upwind Vmg(g)"
Seems to me this would be particularly useful when trying to continually re-evaluate the general strategy (not just picking the layline) to reach the top mark ASAP in a strongish current
. (eg tack up the shallows inshore, or head
up the middle of the harbour?)
Approach 1) has been available since the 1980s, requiring interlinked boatspeed and wind instruments
, but leeway is a confounding factor, as is damping of the wind direction
(a better approach to the latter problem, which I suggested at the time and has finally been implemented this last decade, is to factor in the rate of change of heel angle in order to correct for the swing of the wind indicator at the masthead, rather than applying damping, which delays and degrades the delivery
Approach 2) would require fairly sophisticated integration of the GPS
with the wind instruments
. I imagine it would be tricky to separate leeway from surface drift, tidal streams, ocean currents and other water-movement phenomena, but I guess a software-based 'lookup table' for leeway would do a reasonable job if the boat
was kept in good underwater nick, trim and helming were consistent .... It all starts to make my brain hurt a bit.
Those who decided to incorporate Vmg (towards the next waypoint) into GPS
without explaining the limitations in each and every manual must have been power boaters, I reckon, and/or marketers....
It would be a start if you could turn a dial to a particular True wind direction, (without fiddling around with waypoints) and get a real-time measure of Vmg in THAT direction.
Personally, when I'm cruising, I'd rather have a very simple GPS, one with a Lat/Long display only, and do everything else myself.
It's more interesting, there's usually plenty of time (there are always other things to do, but I'm not convinced they're more important) and it doesn't need to be done to three digit precision.
I'm horrified at the suggestion a cruising navigator should use a sharp pencil: i prefer to use a carpenter's pencil, and keep it nice and blunt, to remind me that the input data are generally inaccurate. I'd rather be approximately right than exactly wrong.