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Old 07-01-2011, 07:57   #1
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Velocity Made Good Calculator

As we're planning a significant amount of down wind sailing soon (Mexico to the South Pacific) and I'm hoping someone can share with me a simple formula or calculator to determine Velocity Made Good. Obviously, sailing deep down wind is slower than a broad reach but one covers more distance sailing off the rhumb line.

My new Garmin chartplotter has a Wind Velocity Made Good field but there's no explanation for it. When I'm sailing straight to a waypoint, it shows a negative number, however, when I'm tacking or jibing to a destination, it shows a positive number.

Perhaps some will say that after sailing for 45 years, I should know the answer to this question and I won't argue. Regardless, I'd appreciate any feedback.

Fair winds and calm seas.

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Old 07-01-2011, 08:09   #2
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VMG is the straight line speed towards your waypoint or destination.

VMG can be calculated using a plotting sheet or using trig. Some GPS units and electronic charts have this feature.

The only time it should be negative is if you are getting further away from your destination as measured by a straight line between you and your waypoint or destination. Your VMG will always be less than or equal to your SOG.


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Old 07-01-2011, 13:05   #3
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VMG=speed x COSINE(course-bearing to mark)
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Old 07-01-2011, 13:32   #4
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Believe on the Garmin unit, it is showing you your VMG upwind... so positive for heading up and negative for downwind. If you are trying to make the best time downwind, you would want to sail with the largest negative number.
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Old 07-01-2011, 14:32   #5
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VMG to upwind = speed * cos( true wind angle )

I assume that you Garmin is hooked up to the wind instruments, in which case what jaerdko said. Positive VMG is upwind, negative VMG is downwind.

But some chartplotters display VMG as toward the waypoint, not the wind. So double check what you have displayed.
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Old 07-01-2011, 14:47   #6
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A lot of GPSs give VMG to the next waypoint. I've read that some will give VMG for upwind, which obviously requires a wind direction and I would assume a wind speed input so it can calculate true wind direction (also requires boat speed.) I don't have this function on my GPS, but I would assume that somewhere in the menu is a choice between waypoint and wind VMG.

Plotting sheet solution that David pointed out is simply find the straight line distance from where you started to where you are on the chart and divide by the time it took. Plotting sheets will have speed, time, distance nomogram so you don't even have to do the division.


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