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post #1 of 13 Old 10-10-2013 Thread Starter
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Understanding the Polar Diagram

http://www.cc27association.com/pdf/iii_polar.pdf

Per the above link, if the wind is at 20 knots on a beam reach, my boat should theoretically be traveling at a speed of 7.25 knots? Is this diagram based on a 100 jib and main?

Thanks.

1977 C&C 27 MKIII
Sea Mist
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post #2 of 13 Old 10-10-2013
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Re: Understanding the Polar Diagram

Yes to the first part of your question. Since the configuration isn't specified in the diagram, I would assume what other manufacturers call "race conditions" which means that the polars are generated in the best possible configuration - often with the lightest fixed boat weight, lots of crew on the rails, and as much sail as the boat can carry. If the C&C can carry a 125% or more then that is what I would assume.

More important than the actual (and only hypothetically reachable speeds) are the curves themselves. So in this example, if you were to change your course by 30 degrees more off the wind you would speed up to almost 8 knots. With a bit of trigonemetry (or a race computer) you could figure out which wind angle will get you the fastest speed over ground- which is not necessarily going to approach that shown in the polars.

I've got official polars for my boat that I find useful despite not even coming close to their posted speeds on any point of sail (except for when heading closer to the wind than the angle in my polor, when my actual speed of 0 matches the theoretical speed of 0 )


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Last edited by Zanshin; 10-10-2013 at 09:35 AM.
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post #3 of 13 Old 10-10-2013
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Re: Understanding the Polar Diagram

There are two plots for each speed. One is downwind with a spinnaker, the other is upwind with the jib. You can see where they cross.
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post #4 of 13 Old 10-10-2013
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Re: Understanding the Polar Diagram

Probably safe to assume the plot includes a genoa. This is something you enter into the data before you plot.

I really don't use polar plots. I use the tabular form where there is far more data presented including VMG's upwind and down, heel angle and an indication of when to reef. You don't get any of this on the polar plot. It is very helpful to be able to see the difference in boat speed to VMG.

I tried to upload on of the tabular VPP's here but it doesn't want to take it.
I'll see what I can do.

OK, let's ee if you can read this sample from one of my boats.
It works! It's a bit small to read but I think you get the idea.
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Last edited by bobperry; 10-10-2013 at 01:08 PM.
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post #5 of 13 Old 10-10-2013
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Re: Understanding the Polar Diagram

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Originally Posted by bobperry View Post
Probably safe to assume the plot includes a genoa. This is something you enter into the data before you plot.
You can see there are two plots. One is for a head sail, and the other a spinnaker.
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post #6 of 13 Old 11-10-2013
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Re: Understanding the Polar Diagram

Is there a place I can obtain a polar diagram for several different boats? Or am I going to have to search for every boat specifically?
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post #7 of 13 Old 11-10-2013
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Re: Understanding the Polar Diagram

Aaron:
You can go to U.S.Sailing and purchase polars for a wide variety of boats. Maybe you can find some nice yacht designer willing to spend some time digging through his files and providing you with some examples. I don't know any nice yacht designers, try Bruce Farr, so I'd go to U.S. Sailing.
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post #8 of 13 Old 11-10-2013
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Re: Understanding the Polar Diagram

Trying to post that plot of Bob's a bit larger...


Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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post #9 of 13 Old 11-10-2013
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Re: Understanding the Polar Diagram

Thanks Faster.

I think Aaron will need much more info on each boat than just a polar. It would be helpful for him to have an overview of the entire design. Try an assemble a complete picture is what I have in mind.

Here is a sail plan for the polar you posted of my boat. This rig design is now obsolete but I think it was the one used for that polar. Maybe you can fix the sail plan so it's bigger please. Thanks.

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Last edited by Faster; 11-10-2013 at 11:24 AM.
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Re: Understanding the Polar Diagram

Quote:
Originally Posted by bobperry View Post
.... Maybe you can fix the sail plan so it's bigger please. Thanks.
Done

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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