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how to read polar diagram (chart, plot)?


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#1 seacow101

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Posted 29 December 2005 - 02:50 AM

please recommend learning website(s), thank you!

#2 BIAM

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Posted 29 December 2005 - 02:52 AM

North U. has a book titled something like Performance Sail Trim......

has a good explanation of how to use a polar......

#3 B.J. Porter

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Posted 29 December 2005 - 04:26 AM

You have three sorts of lines that are relevant.

1) Straight lines radiating out from the center. These are lines representing the True Wind Angle (TWA)
2) Circles, radiating from the center, like a Radar display. These represent Boat Speed. They are dashed in the example diagram.
3) Curves, not radiating from anywhere. These are plots for different True Wind Speed (TWS). There are often two sets, one for upwind sails, one for spinnakers.

The interesting lines are #3, because these show data points that predict the boat speed, at certain Wind Speeds, for certain Wind Angles.

How you read it is you look at one of the curves for TWS. Say the 8 knot line below. You can then follow that curve along for various TWA's, and see the predicted boat speed. For the 8 not curve, 50 degrees TWA shows you about 6.5 kts boat speed, and your optimal TWA is somewhere around 43 degrees with a target boat speed around 5.9 knots.

Some helpful diagrams like this one also plot the maximum Velocity Made Good (VMG), where you make the best boat speed/angle of sail combination to get max VMG for going dead upwind or down. These are shown as little squares on this diagram.

Make sense? To the peanut gallery - did I get it right?

Posted Image

#4 DA-WOODY

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Posted 29 December 2005 - 05:33 AM

You have three sorts of lines that are relevant.

1) Straight lines radiating out from the center. These are lines representing the True Wind Angle (TWA)
2) Circles, radiating from the center, like a Radar display. These represent Boat Speed. They are dashed in the example diagram.
3) Curves, not radiating from anywhere. These are plots for different True Wind Speed (TWS). There are often two sets, one for upwind sails, one for spinnakers.

The interesting lines are #3, because these show data points that predict the boat speed, at certain Wind Speeds, for certain Wind Angles.

How you read it is you look at one of the curves for TWS. Say the 8 knot line below. You can then follow that curve along for various TWA's, and see the predicted boat speed. For the 8 not curve, 50 degrees TWA shows you about 6.5 kts boat speed, and your optimal TWA is somewhere around 43 degrees with a target boat speed around 5.9 knots.

Some helpful diagrams like this one also plot the maximum Velocity Made Good (VMG), where you make the best boat speed/angle of sail combination to get max VMG for going dead upwind or down. These are shown as little squares on this diagram.

Make sense? To the peanut gallery - did I get it right?

Posted Image


I wasn't even wondering about that

But it's sooo clear now :o :lol: :lol: :unsure: :blink:

#5 B.J. Porter

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Posted 29 December 2005 - 05:40 AM

That's a lovely diagram BJ. Seacow, BJ got pretty much all you need to know. Whatever the true wind speed is, you just find the #3 curve for that wind speed, and find where it intersects with the radial line for the angle between your course/heading and true wind. Then the distance to that point from the center of the diagram is your boat speed. (That's what the circles tell you.) The little squares are just the tops and bottoms of the curves, like if you lay a ruler horizontally so it just kisses each curve. The vertical axis is VMG: top for upwind, bottom for downwind, since the wind comes straight down from the top of the page.

Notice that max upwind VMG occurs near 40-45 degrees depending on wind speed. 90 degrees is a beam reach, and 180 is dead downwind (DDW). Note also that max downwind VMG is not quite DDW, so if you have room, that boat can benefit from gybing downwind rather than heading straight down.

P.S. Are the radials based on true course or apparent heading? It seems like you could make the chart either way.


I'm not sure if they are True or Apparent - I always thought they were true.

Those LOOK like True numbers to me, that's the Polar for a Beneteau 40.7, and those numbers are close to the TWA's that I remember not AWA. Though that is a vanilla configuration, our downwinds our different.

One thing that has always puzzled me on the polars is sail choice. Meaning - are those numbers for ALL one size sail, or is it assumed that you have an optimally sized sail for the wind conditions on at all times.

It seems like the latter, but it is hard to tell. Someone must know this one.

#6 seacow101

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Posted 29 December 2005 - 05:48 AM

BJ and Gilligan, thank you very much! :lol:

#7 B.J. Porter

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Posted 29 December 2005 - 05:59 AM

Yes, it's definately true wind rather than apparent. What I'm wondering about is leeway. If you assume the radial stands for true course through the water (relative to TW), you're being more optimistic. If it really means apparent heading of the bow (still wrt to TW), your course won't be quite as high once you factor in leeway. :( That's also a good point about the sails.

That's why I figure it has to be TWA & TWS, because AWA and AWS can be affected by other things.

Apparently Farr designed the 40.7 for a 144% genoa...we use 151% around here, as it's the max without penalty - I wonder what the impact on the < 15 kt ranges is.. It's hard to tell without your instruments being mega-calibrated to see if you are spot on the numbers or not anyway - but they tend to be in the ballpark.

#8 West to East to West

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Posted 29 December 2005 - 06:52 AM

Polars can be EXTREMELY detailed- you can get into sail sizes sure, but you can also have weight factored in via crew, where on the boat they sit, reefs, etc. The programs have the ability to be amazingly detailed- want to know the effect of your case of beer in the cooler in the cockpit where you can reach them versus next to the keel? In theory, it can be predicted.

#9 RandomSailor

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Posted 29 December 2005 - 07:19 AM

I agree that the diagram supplied looks like true wind angles. You may encounter either true or apparent polars in your lifetime. Both are useful. Mostly depends on how much you trust your wind instruments to provide accurate true wind direction and speed.

What you really ought to be wondering at this point is what those cute little square boxes on each curve mean and how you know where to place them.

For pure windward/leeward racing, each represents your VMG to the mark for a particular wind strength. You read from it both the expected speed through the water and the optimal angle. Got that? You instantly know your tacking and gybing angles. Also notice that for the example polar supplied, dead downwind is truly DEAD. VMG for downwind at 10kts requires you steer 150, not 180!

The way you place these squares is to find the horizontal line which just intersects the curve at a single point (tangent). The line must be perfectly horizontal in order to find exactly the right spot. The intersection point is that square box on the curve. :)

You can make your very own, personalized polars with a simple bit of diligence. Go out for practice sails in a handful of different wind speeds (aim for reasonably steady conditions on each sweep). Record your speed for a variety of wind angles at a given wind speed. Plot them on a blank polar diagram and connect the dots. You can always refine your data over time if you have the time and inclination. :P

#10 ockam-nokam

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Posted 29 December 2005 - 02:14 PM

The Ockam web site has a good article on polars - http://www.ockam.com/ocku1.htm

Jon

#11 SemiSalt

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Posted 29 December 2005 - 03:15 PM

Looking at the polars for reaching, you can see that the VMG at 90 deg true is obtained by bearing off further, to say 120 true. Of course, to get up to your mark, you also have to spend some time closer to the wind, at say 75 deg true. As practical matter, this may mean it could pay to run a little low with a chute, then close reach up to the mark with a genoa. Has anyone ever seen codified advice for this situation? In the real world, does it ever pay?

#12 Rail Meat

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Posted 29 December 2005 - 04:34 PM

One thing that has always puzzled me on the polars is sail choice. Meaning - are those numbers for ALL one size sail, or is it assumed that you have an optimally sized sail for the wind conditions on at all times.

It seems like the latter, but it is hard to tell. Someone must know this one.



They had to have been run assuming a specific sail size. At different sail sizes, you would get a different plot.

If you were to request polars from US Sailing, you can get them run them for the different sail sizes, and for the down wind sails you can specify Asymetrics on the pole and on the center line. Each iteration is going to cost you.

#13 B.J. Porter

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Posted 29 December 2005 - 05:06 PM


One thing that has always puzzled me on the polars is sail choice. Meaning - are those numbers for ALL one size sail, or is it assumed that you have an optimally sized sail for the wind conditions on at all times.

It seems like the latter, but it is hard to tell. Someone must know this one.



They had to have been run assuming a specific sail size. At different sail sizes, you would get a different plot.

If you were to request polars from US Sailing, you can get them run them for the different sail sizes, and for the down wind sails you can specify Asymetrics on the pole and on the center line. Each iteration is going to cost you.


I got those...they asked for max sail size if I recall, but not the full inventory. One price.

#14 Rail Meat

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Posted 29 December 2005 - 07:24 PM

Yeah, then they ran it for that max size. You could have asked for additional runs at the different head sail sizes, but they charge you for each. Whom ever buys my boat is going to get all the C+C 353 III data they ever would have wanted...

#15 Turtle

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Posted 29 December 2005 - 09:24 PM

Yeah, then they ran it for that max size. You could have asked for additional runs at the different head sail sizes, but they charge you for each. Whom ever buys my boat is going to get all the C+C 353 III data they ever would have wanted...



Rail Meat,
Is Indigo for sale? If so, I assume the deal is done for your new rocketship. I'd like to see the polars on that one.

Turtle

#16 Rail Meat

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Posted 29 December 2005 - 10:17 PM

Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.

#17 West to East to West

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Posted 30 December 2005 - 01:03 AM

Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.


Um, okay, so we saw the slow polars at the beginning of this thread, how about showing those "sweet" polars?

#18 B.J. Porter

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Posted 30 December 2005 - 04:40 AM


Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.


Um, okay, so we saw the slow polars at the beginning of this thread, how about showing those "sweet" polars?

Those polars aren't too slow for a 15,000 lb. 40 footer.

#19 Rail Meat

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Posted 30 December 2005 - 05:53 AM


Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.


Um, okay, so we saw the slow polars at the beginning of this thread, how about showing those "sweet" polars?



I don't want to jinx myself.

#20 DA-WOODY

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Posted 30 December 2005 - 06:19 AM


Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.


Um, okay, so we saw the slow polars at the beginning of this thread, how about showing those "sweet" polars?

Those polars aren't too slow for a 15,000 lb. 40 footer.


Posted Image


That's not the polars

of a slow Sail Boat



That's the polars

of a slow Sale Boat :o :lol: :lol: :lol: :lol:


I just couldn't help my self

#21 West to East to West

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Posted 30 December 2005 - 08:00 AM



Not a done deal yet, but good indications. If the commitment is there come March, then I will list Indigo.

And I have seen the polars for the new boat. Very sweet.


Um, okay, so we saw the slow polars at the beginning of this thread, how about showing those "sweet" polars?



I don't want to jinx myself.


Ahhh, fair enough. Knowing very well what you mean- it gets crazy messing around with the polars (non-overlapping versus 155% is nothing- what about 100% versus 103%, etc.) and hey- ultimately you never really know until it floats, but thank God (or is it Dog?) that the VPP programs are as good as they are now. Seriously easier than building multiple boats with varying appendages and cutting different sails and moving keels and tank testing and starting over again. Damn, what did we do before technology? Saw a nice wooden double-ender go by today- no fast, but sure pretty. Hmmm. Nah- like Ti and Carbon too much.

#22 scooby_simon

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Posted 30 December 2005 - 08:32 PM

Guys,

Re the question of heading low with the kite up and then coming back up again having dropped on an asymetric boat.

Yes it is fast, but you need to work it out carefully.

Sailing a Cat with a kite as I do, it is usually worth keeping the kite up and going low (ignore tactical situations for now) to start with, or sailing high and then popping the kite to make best VMG to the next martk that is too high to carry the kite all the way.

One thing to think about is who is around you as well. Do you have boats below you that can carry their kites higher (thus you need to make space first), do you have boats below you that cannot carry their kites as high (so you don't need to make space and can react if/wen they drop their kite), are you sailing on a lift now (so you mightcarry the kiet all the way at the moment, are you sailing on a header (so you might not carry it at all). Are you sailing in wind that is higher than average at the moment (so you will be forced lower) or are you sailing in lighter wind than average(so you can carry the kite higher at the moment than usuallly).

Does anyone know of any Polar plotting software ?

#23 Guest Anarchist jolab_*

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Posted 11 February 2006 - 10:16 PM

For an easy way to use polars look at

http://www.tacsail.be/




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