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post #18 of Old 04-03-2019
Jeff_H
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Re: Generic Polars

Without well calibrated wind instruments and knotmeter it is hard to really use a polar plot to develop the best VMG or target speeds. My current boat is the first boat that I have owned with wind instruments and frankly my wind instruments are rarely calibrated even close to correctly and my knotmeter is only a little better. It just gets out of whack somehow. Integrated wind instruments and knotmeter systems will spit out a calculated VMG at any point in time. This can be useful if you optimize sail trim for one wind angle, note the average VMG, then change the wind angle a little, optimize sail trim and see which course is faster. Of course since wind speed, sea state, and wind angle play such a key role in selecting the correct target speed for any moment in time and all three change so frequently, its really hard to sail at the target speed shown on the chart.

Boats with sophisticated instruments do have displays that show target speed, VMG and boat speed, which provides input into the fine tuning of course and sail trim. The rest of us can only make course and speed decisions based on situational observations. Years ago I made the chart below. The way you use the chart is to sail a particular course and trim you sails until you get the maximum average speed on that course. That speed would be the initial speed that is shown in the left column on the chart. Then alter course to the course you think is faster and note the angular change in direction from your original course. Trim the sails for the highest speed on that course and look at the speed shown in the column for the change in course and the row for the initial speed, and if you are going faster than than number the new course has a better VMG. Lower than that number the original course was better.

VMG Table photo VMG Tables_001_zpsmzax55yr.jpg

The nice thing about situationally optimizing speed is that calibrated instruments are not required and you don't have to guess at sea state.

I would also note that boats like the Allmand 35 which have huge amounts of wetted surface, poorly shaped foils, and an inefficient sail plan, are not great candidates for using target speeds off a polar chart. Boats like these tend to make a lot of leeway when pushed hard and target speeds in themself do not consider leeway.

Respectfully,

Jeff


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Last edited by Jeff_H; 04-03-2019 at 03:02 PM.
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