Unique Aids for the Tactican & Navigator: VMG Display
| Other programs may allow you to display
wind, or current, but often not both simultaneously. Even when both are displayed,
however, it is rare that the best route is obvious from inspection. Our unique VMG display
factors in boat polars, wind and current between your boat and the mark, and computes VMG
at representative points in between. It represents values by circles of varying diameter
(the bigger the circle, the better the VMG). The program computes VMG as velocity relative
to upwind or downwind direction for short legs; and velocity relative to rhumbline
direction for long legs, or when using a wind model. Our "best course" routing calculation applies the same data, and searches among many possible routes for the fastest. The navigator/tactician, however, may prefer to study the underlying factors, and the VMG display is a very useful tool to see the entire situation at a glance. Like the router, the VMG function corrects for transit time as well as location in computing currents and wind along the track, so you see VMG as it will be when you get there. This is a very rapid calculation which automatically
updates - in buoy races it is particularly useful in a windward/leeward leg, to pick the
best side of the course after a wind shift. |
| VMG Display assuming constant wind (15 knots from 45 T) and Gulf Stream currents as shown in right image. The larger the circle, the better VMG. The red line shows "best course" routing solution. | Gulf Stream currents (in blue), along with routing solution (in red). For this example, a beat with constant NE wind, the track pretty much follows the maximum current - and the big VMG circles. |
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VMG Display assuming wind pattern in right image and Gulf Stream currents. The stronger winds to upper right eventually dominate, and are reflected both in the VMG circles and routing solution. |
This example calculation used the Gulf Stream currents above and the wind pattern below. In this case the winds were held constant over the time of route, but in general the program varies them to suit. |
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