Did some more scratching around on wind based VMG. I hope I'm correct on all
this. :)
First, let's define some terms and abbreviations
* VMG - Velocity Made Good. Velocity directly up or down wind
* WCV - Waypoint Closure Velocity. Velocity towards a waypoint.
(Shows on GPS as VMG)
* TWA - True Wind Angle. The angle of the boat to the true wind.
* TWS - True Wind Speed. The speed of the wind across the water
* AWA - Apparent Wind Angle. The angle of the wind across the boat**
* AWS - Apparent Wind Speed. The speed of the wind across the boat**
* BSP - Boat Speed. The speed of the boat through the water**
Note: The only one of the above that requires a GPS is WCV. The remainder
require either a wind sensor, a knot transducer or both. The ones marked **
are observed, the rest are calculated.
Also note that TWA should not be confused with true wind direction, a compass
bearing. None of the above require a compass.
The calculation of VMG is a bit obtuse.
In order for your instrument system to calculate VMG it must first calculate
TWA. It does this from BSP, AWA. and AWS (apparent wind speed). It's vector
addition or trigonometry.
Here is a decent explanation and calculator with diagrams.
http://www.sailingcourse.com/keelboat/true_wind_calculator.htm
Next, it calculates VMG. Simply put,
VMG = BSP x cosine (TWA)
Here is another site which may help visualize it.
http://lagoon-inside.com/2012/01/faster-thanks-to-the-vmg-concept/
Now, let's point out that VMG does not consider current. It does, however,
contain leeway because of the apparent wind component.
Now let's look at WCV. Many GPS units display VMG. This is really WCV. It is
the velocity at which the boat is approaching the active waypoint set in the
GPS. A stand alone GPS has no idea what the wind is and therefore can not
calculate wind based VMG. WCV will work at the start of a race or cruising
leg, but will become less useful as the boat sails up the course. This becomes
apparent as the boat approaches the windward or leeward laylines and the racing
mark has been set as the active waypoint. As you approach the laylines, WCV
will tend to zero as you are sailing past the waypoint at a 90 degree angle and
are no longer closing on the waypoint. In fact, many GPS units will display
negative WCV once you have overstood the layline. A useful tool if you also
have instruments that calculate VMG.
So how do you use your GPS to approximate VMG for a windward/leeward race?
Simple. Before the race begins, turn head to wind and observe the true wind
direction. Next, enter a waypoint in your GPS that is 20+ miles beyond, and
directly upwind of the windward turning mark.
Now sail your race maximizing WCV. In some ways, this may be better than
maximizing wind based VMG because it accounts for current.
Darn, my brain is tired. I need to go chat with my friend Johnnie Walker.
Dennis C.
Touche' 35-1 #83
Mandeville, LA
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