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Old 06-09-2020, 07:50   #1
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Am I using polar diagram right?

I was out sailing yesterday in winds of 10 to 15 kts. Nice day!


While sailing upwind I was paying more attention than usual to my instruments. That is mainly due to recently closely inspected the polar diagram for my boat. What I was observing and what was on the polar didn't agree so well. I know part of the problem is I didn't have the sails trimmed as well as they should be.



But then I looked closer at the polar and now I have some questions about whether I am reading it correctly. From what I understand, it shows TWA and boat speed. The curves are drawn for different TWS.



Here is an example of my confusion: For a 10 kt TWS, with a heading with TWA at 50º the boat should moving at 6.5 kts. Great. Except that means the AWA is 32º and I don't believe the boat can sail at that angle.


What am I missing?
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Old 06-09-2020, 08:19   #2
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Re: Am I using polar diagram right?

It may help if you let us know what type of boat you have.

It sounds like you are interpreting your polars correctly, but keep in mind most cruising boats will have a hard time achieving 100% polar boatspeed.

Default boat polars are usually made by a VPP software program which assumes a lightship boat, perfectly clean bottom, and optimal sails.

Past this, driving and trimming make a huge difference. You will likely need to “work your way up and in” to your optimal upwind speed and angles. Use your foils. Speed = lift. Lift = point.

Some cruising boats just will not have the sail cut and sheeting locations to allow for best speed and angle. While cruising we shoot for 90% polar boatspeed average.
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Old 06-09-2020, 19:01   #3
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Re: Am I using polar diagram right?

If you're only moving at 6.5 knots in 10 knots of wind with a true wind angle of 50º, how does your apparent wind angle change by 20+ degrees? You're not going fast enough for that to happen. Polars are supposed to tell you how fast you should be going with the true wind at a specific angle and speed. They have nothing to do with apparent wind.
This article may be helpful:What are Polars - Ockam Sailing Instruments
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Old 06-09-2020, 19:32   #4
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Re: Am I using polar diagram right?

Quote:
Originally Posted by psk125 View Post
If you're only moving at 6.5 knots in 10 knots of wind with a true wind angle of 50º, how does your apparent wind angle change by 20+ degrees? You're not going fast enough for that to happen. Polars are supposed to tell you how fast you should be going with the true wind at a specific angle and speed. They have nothing to do with apparent wind.
This article may be helpful:What are Polars - Ockam Sailing Instruments
By my calculations I get 30 degrees apparent wind.
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Old 06-09-2020, 21:34   #5
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Re: Am I using polar diagram right?

Quote:
Originally Posted by Dr. D View Post
I was out sailing yesterday in winds of 10 to 15 kts. Nice day!


While sailing upwind I was paying more attention than usual to my instruments. That is mainly due to recently closely inspected the polar diagram for my boat. What I was observing and what was on the polar didn't agree so well. I know part of the problem is I didn't have the sails trimmed as well as they should be.



But then I looked closer at the polar and now I have some questions about whether I am reading it correctly. From what I understand, it shows TWA and boat speed. The curves are drawn for different TWS.



Here is an example of my confusion: For a 10 kt TWS, with a heading with TWA at 50º the boat should moving at 6.5 kts. Great. Except that means the AWA is 32º and I don't believe the boat can sail at that angle.


What am I missing?
Can you post the polar table?
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Old 06-09-2020, 22:46   #6
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Re: Am I using polar diagram right?

As suggested above, there are so many real world influences not reflected in the polar table (age of sails, current, etc.) the polars seem like a theoretical target to strive for, but as they say, “your mileage may vary.”
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Old 07-09-2020, 04:23   #7
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Re: Am I using polar diagram right?

when was the last time you calibrated your instruments ? AWA / BS ?

this needs to be done regularly and with care...the old saying applies...'garbage in, garbage out'

anyway, an AWA of 32 is not that great. race boats are usually 26-27

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Old 07-09-2020, 06:58   #8
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Re: Am I using polar diagram right?

Quote:
Originally Posted by psk125 View Post
If you're only moving at 6.5 knots in 10 knots of wind with a true wind angle of 50º, how does your apparent wind angle change by 20+ degrees? You're not going fast enough for that to happen. Polars are supposed to tell you how fast you should be going with the true wind at a specific angle and speed. They have nothing to do with apparent wind.
This article may be helpful:What are Polars - Ockam Sailing Instruments

I did the trig and it comes out to that value. You can do the trig, or use an online calculator like this: https://l-36.com/polar_calculate.php...10&ws=T&bs=6.5
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Old 07-09-2020, 06:59   #9
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Re: Am I using polar diagram right?

Quote:
Originally Posted by chrisr View Post
when was the last time you calibrated your instruments ? AWA / BS ?

this needs to be done regularly and with care...the old saying applies...'garbage in, garbage out'

anyway, an AWA of 32 is not that great. race boats are usually 26-27

cheers,

Good point. I will try that next time I'm out, after I determine how to do that...
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Old 07-09-2020, 07:07   #10
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Re: Am I using polar diagram right?

Polar diagram attached.
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Old 07-09-2020, 08:08   #11
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Re: Am I using polar diagram right?

The squares on the graph should indicate the best VMG upwind and down. I would read it as at 10 knots of wind, 6.1 knots of boatspeed, 42 degrees true, 26 degrees apparent.

As others have said this is everything perfect, new sails, trimmed correctly, etc, and theoretical.
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Old 07-09-2020, 08:27   #12
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Re: Am I using polar diagram right?

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...I would read it as at 10 knots of wind, 6.1 knots of boatspeed, 42 degrees true, 26 degrees apparent. As others have said this is everything perfect, new sails, trimmed correctly, etc, and theoretical.
Mostly, entirely, absolutely theoretical. Is anyone ever able to make 6.1 SOG pinching 26 degrees off a 10 knot wind? I don’t think so. I’d guess the average boat owned by folks on this forum would be lucky to make half that boat speed.

Just my $0.02
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Old 07-09-2020, 09:00   #13
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Re: Am I using polar diagram right?

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Mostly, entirely, absolutely theoretical. Is anyone ever able to make 6.1 SOG pinching 26 degrees off a 10 knot wind? I don’t think so. I’d guess the average boat owned by folks on this forum would be lucky to make half that boat speed.

Just my $0.02
It appears to be a fractional rig with a non overlapping jib. It should be able to point high without pinching.
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Old 07-09-2020, 10:18   #14
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Re: Am I using polar diagram right?

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Originally Posted by cal40john View Post
It appears to be a fractional rig with a non overlapping jib. It should be able to point high without pinching.

The genoa is said to be a 102%, so just a bit of overlap.



Thanks for all the good feedback. Thinking through the day I realize I had several trim errors:


-- The leech line on both the main and genoa need to be tightened


-- I should have eased out the vang to let the top of the main dump air (the heel was more than I like and the boat sails better on her lines)


-- The genoa cars were too far back


Probably others, too....
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Old 07-09-2020, 10:35   #15
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Re: Am I using polar diagram right?

A polar diagram made by your builder or sailmaker would be a good starting point for making your own. One made by the instrument maker will help only to learn about them in general. A working polar diagram without adjustments for seaway is still a work in process.
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