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post #21 of 26 Old 04-03-2019
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Re: Generic Polars

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You don't need highly calibrated instruments to show optimum VMG in relative terms. Ours are always calculating VMG, and we don't use them to know the exact VMG - just to trim and sail the boat to the maximum VMG they show. Whether that absolute number is real or not is inconsequential. This works well, unless one thinks that the instrument calibrations are constantly changing in short terms. If they are, then they aren't very useful for anything, let alone VMG.

Our autopilot has an upwind setting to sail at maximum VMG, where it constantly monitors boat speed and wind and adjusts course so that VMG is optimum. Like us, it doesn't care about the absolute VMG number - only that it sails at the highest VMG calculated.

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Really does not work that way as the numbers need to be accurate on all the instruments to give you a VMG that is worth sailing . if the speeds are off on one instrument and not on the others then you will think you are sailing to best VMG. In reality you are sailing to what the instruments say is a best VMG but could be way off the mark. Garbage in Garbage out

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post #22 of 26 Old 04-03-2019
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Re: Generic Polars

A question colemj
I understand the autopilot adjusting course changes but it doesn't do anything regarding optimum sail angle etc...does it?
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post #23 of 26 Old 04-03-2019
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Re: Generic Polars

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Really does not work that way as the numbers need to be accurate on all the instruments to give you a VMG that is worth sailing . if the speeds are off on one instrument and not on the others then you will think you are sailing to best VMG. In reality you are sailing to what the instruments say is a best VMG but could be way off the mark. Garbage in Garbage out
I don't think it works that way. Even with mis-calibrated instruments, or one mis-calculated instrument and one good calibration one, the relative VMG will still be correctly optimized - just not accurate. In other words, your boat will still be sailing its best VMG, but the actual VMG value may not be correct.

For example, say the wind instrument is off by 5*. When you are sailing at optimum relative VMG, you might be making 5kts at 50* TWA by the instruments - where in reality you are making 5.5kts at 45* TWA.

In either instance (poorly calibrated, or precisely calibrated), if you fall off or head up from the optimum VMG, you will no longer be at optimum VMG. Whether you are actually making 5kt or 5.5kt VMG is inconsequential - it is still the optimum point of sail in those conditions and trim.

This becomes particularly true when using SOG, which is always calibrated.

Mark

Edit: I see you mentioned the need for accurate speed values for all instruments. I'm not sure why one would use multiple boat speed instrumentation to calculate VMG, but even if one did, the relative optimum VMG should still be the same.

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post #24 of 26 Old 04-03-2019
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Re: Generic Polars

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Originally Posted by paulinnanaimo View Post
A question colemj
I understand the autopilot adjusting course changes but it doesn't do anything regarding optimum sail angle etc...does it?
Our autopilot has a setting when on wind mode where it gradually moves around and optimizes the wind angle it is sailing at to be optimum VMG. This does mean that the sails need to be adjusted properly because all the AP can do is move the boat up and down the wind looking for the best VMG calculation. However, if close hauled, there really isn't much sail trim to be done after adjusting for conditions (I'm speaking from a cruising perspective here - racers will not be using the AP in this mode).

It really doesn't do anything different than a helmsman adjusting course while looking at a VMG value, except that it doesn't get bored doing it, and is more reliable at it than me.

Mark
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post #25 of 26 Old 04-04-2019
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Re: Generic Polars

the Alpha 3000 steers the course the skipper selects. As such I look at the data from the wind instruments (true angle and speed) and select what I know to be the best course for speed and VMG to my chosen mark (where's Mark Cole when you need him??:-)). More often than not I cannot fetch the mark.... and will have to tack or gybe. Then it's a good idea to have a decent VPP profile in you head or something you can refer to. But the instruments will tell you how your are doing... especially if you have entered the mark and have a VMG to destination to inform your course decision. The processor should tell you when you have the best course to get where you want in the shortest time... assuming no environment conditions change. But they do... so you have to correct the course. That's the joy of sailing!

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post #26 of 26 Old 04-09-2019
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Re: Generic Polars

Here is a thread over at SBO about generating polars for your boat.

https://forums.sailboatowners.com/in.../#post-1527187


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