Optimizing VMG is far more important than just going fast. In an upwind/downwind race VMG is the “bottom
line”. Plain speed is only important in the context of how it affects your VMG. For example, if you sail on a
beam reach, perpendicular to the wind, you will go really fast but you won’t get any closer to the windward mark
and your VMG will be zero. On the other hand, if you point straight upwind you won’t go anywhere, so your VMG
will again be zero. Somewhere in between head-to-wind and a beam reach lies a “magic angle” that will optimize
your upwind speed and make you a champ. Unfortunately, this elusive angle depends heavily on your gear, the
prevailing conditions, your size and your sailing style.
How Does The S5 Calculate VMG?
The S5 calculates the heading from the downwind reference point to the upwind reference point. This heading is
assumed to be the wind direction. Since the only information derived from the reference points is this heading, the
position of the reference points in absolute terms does not matter. The only thing that matters is the position of the
upwind point relative to the position of the downwind point.
The S5 takes GPS readings every second to get your actual speed as well as the heading of your course. The device
then multiplies your speed with the cosine of the angle between your heading and the wind direction to extract the
component of your speed that is aligned with the wind direction. This process is repeated every second. Some
averaging is then performed to make the output smoother and easier to interpret. The results are displayed on the
LCD screen.
How accurate is the S5?
In speedometer mode the accuracy of the S5 is better than +/- 0.2 knots (0.36 km/h). In VMG mode accuracy of the
S5 depends on what angle to the wind you are sailing but is generally better than +/- 1 knot (normally closer to +/-
.3 knots).
How long do the batteries last?
Rechargeable Nickel Metal Hydride AAA batteries will last longer than 10 hours. Disposable batteries will last
just under 10 hours.
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