Post by viv on Nov 29, 2017 9:27:06 GMT -8
What's VMG?
This acronyms stands for Velocity Made Good, i.e. the speed vector pointing to your destination as you tack back and forth against the wind (or gybe downwind)
Because when you need to go East and the wind is blowing from the East, you have to tack back and forth, and find the right angle which will take you fastest upwind.
Basically, VMG expresses the pros and cons of pointing higher : lower boat speed but better angle i.e. less distance travelled.
A more direct course at a lower speed may or may not be faster than a longer course at higher speed : VMG provides a quick answer to that discussion, summarizing the trade-off in one simple figure.
Tools exist to help the sailor find the theoretical best VMG, starting with the boat "polars", i.e. the boat theoretical speed projection across points of sails, expressed visually on a polar chart (see illustration)
It is about applying long forgotten maths concepts: geometry, vectors, trigonometry.
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The chart above (courtesy of Liveskipper) presents an Imoca polars for a 15-knot wind. The different colors represent the various sails available to the skipper to cover all points of sail (here: foresail, genoa, code 0 and spinnaker)
The tool let you check what the boat speed will be at any degree to the wind (0º = head to wind, 180º = full downwind)
You can see the speed dropping dramatically as one points higher than 40º to the wind, boat stopping totally when facing the wind (speed=0 when angle =0)
The boat speed is read along the circle, here 12.5 knots at 49º from the wind.
The VMG upwind is calculated taking the vertical vector component of the speed, i.e. projecting the speed point on the vertical axis, here 8.2 knots, with same 49º angle.
What it means is that the best angle to go into the wind and maximize VMG is to tack on a 49º angle to the wind (or more likely a range between 45 and 50º).
At a 60º angle, the speed would be about the same, but the VMG would drop to about 6 knots.
NB: All angles to the wind are TWA - True Wind Angle (not AWA - Apparent Wind Angle)
NB2: For instance, at about 50º TWA, a Trudeau 12m sailing at 12knots is on a 30º AWA or so.
The same tool can be used downwind, with a range of possible angles to the wind to maximize VMG.
With the same 15-knot wind, the best set-up downwind is to raise the spinnaker and point between 140-150 TWA, for a speed of 14-17knots, and VMG around 12.5 knots.
We see clearly here that going straight downwind is detrimental to performance as both speed and VMG drop when one points further down.
At 180º, i.e. straight downwind, speed=VMG at about 9.5 knots.
Of course, every boat behaves differently, with optimum angles to the wind varying greatly, depending on the shape of the polar curve.
The wind strength too has a significant impact on the shape of the polar curves, usually pushing the optimum angle away from the wind as wind drops (or becomes very strong).
This is not always reflected in SL as boat makers have different approaches to modelling the performance of their boats (different "wind engines"), some being more realistic than others.
This acronyms stands for Velocity Made Good, i.e. the speed vector pointing to your destination as you tack back and forth against the wind (or gybe downwind)
Because when you need to go East and the wind is blowing from the East, you have to tack back and forth, and find the right angle which will take you fastest upwind.
Basically, VMG expresses the pros and cons of pointing higher : lower boat speed but better angle i.e. less distance travelled.
A more direct course at a lower speed may or may not be faster than a longer course at higher speed : VMG provides a quick answer to that discussion, summarizing the trade-off in one simple figure.
Tools exist to help the sailor find the theoretical best VMG, starting with the boat "polars", i.e. the boat theoretical speed projection across points of sails, expressed visually on a polar chart (see illustration)
It is about applying long forgotten maths concepts: geometry, vectors, trigonometry.
The chart above (courtesy of Liveskipper) presents an Imoca polars for a 15-knot wind. The different colors represent the various sails available to the skipper to cover all points of sail (here: foresail, genoa, code 0 and spinnaker)
The tool let you check what the boat speed will be at any degree to the wind (0º = head to wind, 180º = full downwind)
You can see the speed dropping dramatically as one points higher than 40º to the wind, boat stopping totally when facing the wind (speed=0 when angle =0)
The boat speed is read along the circle, here 12.5 knots at 49º from the wind.
The VMG upwind is calculated taking the vertical vector component of the speed, i.e. projecting the speed point on the vertical axis, here 8.2 knots, with same 49º angle.
What it means is that the best angle to go into the wind and maximize VMG is to tack on a 49º angle to the wind (or more likely a range between 45 and 50º).
At a 60º angle, the speed would be about the same, but the VMG would drop to about 6 knots.
NB: All angles to the wind are TWA - True Wind Angle (not AWA - Apparent Wind Angle)
NB2: For instance, at about 50º TWA, a Trudeau 12m sailing at 12knots is on a 30º AWA or so.
The same tool can be used downwind, with a range of possible angles to the wind to maximize VMG.
With the same 15-knot wind, the best set-up downwind is to raise the spinnaker and point between 140-150 TWA, for a speed of 14-17knots, and VMG around 12.5 knots.
We see clearly here that going straight downwind is detrimental to performance as both speed and VMG drop when one points further down.
At 180º, i.e. straight downwind, speed=VMG at about 9.5 knots.
Of course, every boat behaves differently, with optimum angles to the wind varying greatly, depending on the shape of the polar curve.
The wind strength too has a significant impact on the shape of the polar curves, usually pushing the optimum angle away from the wind as wind drops (or becomes very strong).
This is not always reflected in SL as boat makers have different approaches to modelling the performance of their boats (different "wind engines"), some being more realistic than others.
