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post #11 of 26 Old 04-02-2019
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Re: Generic Polars

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Originally Posted by Teamstone View Post
...the one thing I don't have is wind instrumentation.
I'm guessing you're not racing then.

If they make you happy, have at them. Why do you feel you need them?


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post #12 of 26 Old 04-02-2019
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Re: Generic Polars

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Originally Posted by Minnewaska View Post
I'm guessing you're not racing then.

If they make you happy, have at them. Why do you feel you need them?
They would be useful in order to see if you are sailing as well as could be expected. If Polars show that you should be going 6 knots on a beam reach in 15 knots of wind, and you're only doing three, you know something isn't right.
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post #13 of 26 Old 04-02-2019 Thread Starter
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Re: Generic Polars

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Originally Posted by paulk View Post
They would be useful in order to see if you are sailing as well as could be expected. If Polars show that you should be going 6 knots on a beam reach in 15 knots of wind, and you're only doing three, you know something isn't right.
Exactly!
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post #14 of 26 Old 04-02-2019
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Re: Generic Polars

It seems to me that it would be very difficult to account for tidal currents; when our boat seems sluggish, or extra quick, that is usually the reason.
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Re: Generic Polars

Teamstone-
Generic polars could be useless. Your boat needs the first reef at 18 knots, mine needs it in 12. Your boat runs fasted dead downwind, wing and wing, mine is faster gybing off. No real way to "average" those variables into a generic.
I'd also found a great Excel sheet to guesstimate polars for boats that didn't have them. It accommodates questions about sail plan and area (is your normal headsail a 130? 150? 110?) and crew aboard, i.e. can you put 600# on the rail or not?
If you can't find an online tool to generate something useful, then you are left to either looking for polars from "similar" boats (perhaps the molds or the design were borrowed or shared?) or taking lots of sails, with pen and paper or the GPS dropping lots of breadcrumbs, to note how it really goes.
Since polars were originally (AFAIK) a competitive racing tool, and racing is a money game...Yeah, they often have to be bought, or run off for your boat, at a price.
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post #16 of 26 Old 04-02-2019
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Re: Generic Polars

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Originally Posted by Minnewaska View Post
Unless you sail in flat water, with a perfectly clean bottom, brand new sails and have perfect trim, they are totally useless. If you have all these things, they are modestly useless. Besides, getting to know how a new boat sails in various conditions is part of the mating process. Embrace it, don't overthink it.
Ah, well, not really. In everyday sailing one doesn't always have a "clean bottom", few actually have "brand new sails"; and, most can't achieve "perfect trim". One can, however, generate a set of useful "polars" for one's own boat sailed under "normal conditions" as to bottom, sails and trim so long as one can get a day with relatively steady winds in a body of water with relatively little tidal current, or, one can determine the set and drift of the current which isn't so hard if one simply lays ahull, absent sails, for awhile and keeps track of one's COG and speed as one "drifts" with the currents. With that, one simply sails a set of courses at reasonable intervals on each tack/gybe from DDW to as close to the wind as one can point and records one's boat speed, apparent wind, true wind as computed by ones instruments or manually if necessary (assuming one has a reasonably will calibrated Speedo). One would, of course, want to do this several or more times in various true wind speeds but it wouldn't take much effort to generate a reasonably decent set of polars for say, 10 to 25 knots, for each of one's head sails, assuming that one has more than a single sail and might switch sails to account for anticipated conditions on or during a given passage, tho' most likely do not but adjust a given head sail for the conditions as they evolve.

In our case, we have very elaborate polars for our boat as it was developed for racing but I found the foregoing approach quite helpful and fairly easily done with a pad, pencil and an Excel spreadsheet. Making up one's own polars as described is also more effective in some respects as they are specific to one's own boat which may have performance characteristics quite apart from that of a "test boat" for a given model/design.

FWIW...
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post #17 of 26 Old 04-03-2019
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Re: Generic Polars

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Originally Posted by paulk View Post
They would be useful in order to see if you are sailing as well as could be expected. If Polars show that you should be going 6 knots on a beam reach in 15 knots of wind, and you're only doing three, you know something isn't right.
If that works for you, enjoy the process.

I still donít get it. A beam reach, with 15 kts of breeze, should be damn near hull speed on just about any boat. A good focus on sail trim is going to get any boat within a narrow margin of a polar prediction. I donít see the polar being the right tool, if one canít get close enough.

The real advantage of a polar, as I see it, is to determine optimal VMG to a waypoint. This will be less than optimal, without wind instruments.

You can try to make your own polar, but theyíll be specific to ones skill to trim and condition of the sails. Generic polars, as others have stated, are fairly unrealistic. I know the sails I just retired were doing just fine deep downwind, but not pointing worth a damn.

Iím really good with everyone sailing their boat any way that makes them happy. My encouragement is to look at the windex and sails and not in the book. After a while youíll know whether youíre sailing efficiently, without having to check, but you do you. Enjoy your boat. Cheers.


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post #18 of 26 Old 04-03-2019
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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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post #19 of 26 Old 04-03-2019
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Re: Generic Polars

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.

Mark

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post #20 of 26 Old 04-03-2019
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Re: Generic Polars

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Originally Posted by colemj View Post
You don't need highly calibrated instruments to show optimum VMG in relative terms.
Mark
I agree. That is why I said, "Without well calibrated wind instruments and knotmeter it is hard to really use a polar plot to develop the best VMG or target speeds."

Jeff


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