Use
of VMG Display in Tacking to a Mark |
| To sail the quickest route to a mark, a boat
has to maximize velocity-made-good, or "VMG," in the direction of the mark. In
the example below, the boat to the left (east) of Alcatraz island is trying to tack upwind
towards mark "48" against an adverse current, which varies point to point. On
this scale one usually doesn't have a wind prediction file, but variations in wind will
also vary VMG (see second example below). Current affects boat motion directly, and also
changes the true-wind-angle (TWA) experienced by the boat (for discussion click here). The optimal route is one that most closely connects
points of favorable VMG. Force 4 is the
ONLY program to provide its unique VMG display, that shows at a glance points of favorable
and unfavorable VMG. There are four diameters of green circles on the graphic below; the
larger the circle, the greater VMG towards the mark. It is apparent that there is more
favorable VMG below the island (to the south) than above it, and hence one would expect
the southern approach to be faster. This is confirmed by the "best tack" (or
"what-if") solution, which compares port or starboard
tack to the corresponding layline and computes time to the mark. In this example, a
starboard tack to the lower current-corrected layline (magenta dots) is over 5 MINUTES
faster than the port tack solution - a huge difference in racing on this scale. A more
detailed routing solution (next graphic) results in a route that tracks the VMG circles.
As an alternative to setting up repeated routing
solutions, the VMG display updates automatically every few seconds and shows the helmsman
at a glance the favored part of the course. |
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| The Force 4 routing
solution (see Router) leads to an optimal (red) route that also
tracks the VMG circles. |
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Use
of VMG Display in long-distance (Ocean) Racing |
| For ocean races, which take a day or more,
the optimal route depends on the variation of wind, both point to point and over time. It
is difficult to visualize on a single screen how the wind will vary by the time the boat
reachs a given point. A routing solution integrates wind and current variations over the
period necessary to reach the mark, but you only see the final result, and need to repeat
the calculation as you go along. In the example below, the optimal red route is well
predicted by the VMG circles, which are computed at the time estimated for the boat to
reach that point. This display integrates wind and current (here the Gulf Stream), and
enables the navigator to see the situation at a glance |
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