Understanding polars

RobF

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I've managed to get hold of the polar data for my boat, which is as follows:

Wind velocity6kts8kts10kts12kts14kts16kts20kts
Beat angles43.9°42.1°41.2°40.4°38.8°38.1°38.2°
Beat VMG
3.11​
3.75​
4.26​
4.69​
4.97​
5.08​
5.11​
52°
4.78​
5.66​
6.35​
6.76​
6.99​
7.1​
7.16​
60°
5.06​
5.97​
6.59​
6.93​
7.14​
7.28​
7.38​
75°
5.27​
6.22​
6.79​
7.08​
7.29​
7.49​
7.77​
90°
5.46​
6.5​
7.05​
7.34​
7.49​
7.61​
8.02​
110°
5.45​
6.55​
7.09​
7.43​
7.77​
8.1​
8.49​
120°
5.26​
6.35​
6.99​
7.35​
7.69​
8.06​
8.77​
135°
4.73​
5.78​
6.63​
7.1​
7.43​
7.78​
8.55​
150°
4.01​
5.05​
5.94​
6.68​
7.1​
7.41​
8.09​
Run VMG
3.47​
4.37​
5.15​
5.87​
6.51​
6.98​
7.6​
Gybe angles146°150.6°152.6°157.7°180°180°180°

I've got two questions:
  1. Why are the Beat and Run labelled as VMG but the other angles aren't?
  2. Assuming I've got a clean bottom, reasonable sails and flat seas, how likely am I to be able to make these target speeds?
 

Motor_Sailor

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Because the VMG on all headings except beating and running, are pretty much along the heading. That is, if your course needed is 75° off the wind, then steering 75° off the wind is the best course and your VMG is simply your boat speed.

But if the course needed is either directly into the wind or directly away from it, then you have more options of the course to steer in order to make VMG in those particular directions. So for instance when trying to go at 180° from the wind, it pays to head up until 14 knots of wind, but from then on barrelling dead downwind will give you your best VMG.
 
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flaming

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1. No idea why they think VMG is helpful on its own there. I've never seen a polar that does that, doesn't seem very useful as what you really need to know is the angle and the boat speed you should be doing in that windspeed to know if you're up to speed. If this is all you've got, I'd do the maths to calculate what speed you should be doing at the given angle to give you a target.

2. Depends what the polars are based on. Are they based on having weight on the rail, or on doublehanded condition? If they're based on fully crewed then you might get pretty close up to about 10 knots, but over that you'll not get the speed if you don't have the bodies on the rail.

What boat is this for?
 

Birdseye

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A table like the one posted leaves you guessing what the data means - there should be some proper explanation attached to it. For example Velocity is a vector ie it has both speed and direction. So what direction? Presumably into the wind. In which case the numbers following are likely to be the same because they change as you might expect them to do when boat speed builds. Presumably the angles given must be real angles to the true wind but again it should say so. There might be conventions in sailing that answer these issues - Flaming will know - but looking at it from a maths / physics background there should be expalnations of what each number means.

For example, does the VMG take into account leaway.? Suggest you go back to your source and see if you can get this info.
 

lw395

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I suspect the reason for quoting VMG for the beat and run is that the boat will hold a very similar VMG over several degrees of pointing, the shape of the polar is a broad lobe not a spiky thing, unless you're talking about a 49er downwind.
Ideally you'd have data every couple of degrees around close hauled.
 

flaming

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I suspect the reason for quoting VMG for the beat and run is that the boat will hold a very similar VMG over several degrees of pointing, the shape of the polar is a broad lobe not a spiky thing, unless you're talking about a 49er downwind.
Ideally you'd have data every couple of degrees around close hauled.
That might be a good point if the beat angles weren't given to the nearest 0.1 of a degree....
 

RobF

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Thanks all, glad it's not just me who felt that the data was lacking a bit (although it was free!).
To answer Flaming's question, my boat is a Dehler CR 35. I appreciate I could spend the time on the water with a steady wind speed and try to create my own polars, but thought it would be helpful to have a starting point. I do most of my sailing single handed, so don't have movable ballast for the rail. I also need to upgrade myself and the sails if I am to achieve those speeds as they seem to be a minimum of 0.5kn more than what I currently achieve.

I'm not too bad at maths / vectors, but couldn't find any useful resources which could show me how to calculate my own polar :-(
 
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bbg

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I've managed to get hold of the polar data for my boat, which is as follows:

Wind velocity6kts8kts10kts12kts14kts16kts20kts
Beat angles43.9°42.1°41.2°40.4°38.8°38.1°38.2°
Beat VMG
3.11​
3.75​
4.26​
4.69​
4.97​
5.08​
5.11​
52°
4.78​
5.66​
6.35​
6.76​
6.99​
7.1​
7.16​
60°
5.06​
5.97​
6.59​
6.93​
7.14​
7.28​
7.38​
75°
5.27​
6.22​
6.79​
7.08​
7.29​
7.49​
7.77​
90°
5.46​
6.5​
7.05​
7.34​
7.49​
7.61​
8.02​
110°
5.45​
6.55​
7.09​
7.43​
7.77​
8.1​
8.49​
120°
5.26​
6.35​
6.99​
7.35​
7.69​
8.06​
8.77​
135°
4.73​
5.78​
6.63​
7.1​
7.43​
7.78​
8.55​
150°
4.01​
5.05​
5.94​
6.68​
7.1​
7.41​
8.09​
Run VMG
3.47​
4.37​
5.15​
5.87​
6.51​
6.98​
7.6​
Gybe angles146°150.6°152.6°157.7°180°180°180°
I've got two questions:

  1. Why are the Beat and Run labelled as VMG but the other angles aren't?
  2. Assuming I've got a clean bottom, reasonable sails and flat seas, how likely am I to be able to make these target speeds?
I can't answer Q2 (although it is related) but I assume the point is to say that if the speed drops below the VMG values upwind or downwind you are pointing to high or too low, respectively. Basically, don't let the speeds drop below those figures or else you are not making best VMG.
 

Birdseye

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Thanks all, glad it's not just me who felt that the data was lacking a bit (although it was free!).
To answer Flaming's question, my boat is a Dehler CR 35. I appreciate I could spend the time on the water with a steady wind speed and try to create my own polars, but thought it would be helpful to have a starting point. I do most of my sailing single handed, so don't have movable ballast for the rail. I also need to upgrade myself and the sails if I am to achieve those speeds as they seem to be a minimum of 0.5kn more than what I currently achieve.

I'm not too bad at maths / vectors, but couldn't find any useful resources which could show me how to calculate my own polar :-(
In the end you can just use the data as something to compete with, record how you do against it and what tweeks get you nearer the numbers.

The calculations of a polar are usually the designer using a VPP program. It would be difficult to make one of your own because of external effecs from the state of the hull to leeway and wave action. Plus winds arent that steady.

My boat is a 35 ft fin and I dont have polars for it so I will use your numbers and see where I get.

P.S. Pottering about on the net after writing the above I came across Avalon VPP system which gave me a polar for my boat and might well do for yours. Just google them
 
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Laser310

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I've got two questions:
  1. Why are the Beat and Run labelled as VMG but the other angles aren't?
  2. Assuming I've got a clean bottom, reasonable sails and flat seas, how likely am I to be able to make these target speeds?

Polars like those posted come from a VPP - Velocity Prediction Program. Many rating systems employ a VPP to predict the speed of boats across a range of wind speeds and TWA's, and thus to rate them.
In particular, those may have come from an ORC rating certificate, and from the ORC VPP. Certificates for all rated boats are available on the ORC website. If you have a common boat, chances are you can find polars there that will work for you. just check the configuration - asym vs sym.., and so on. even if your exact boat isn't there, you may find one that's close, and you can modify them based on experience. For example, for a delivery on an Oyster 625, i started with polars from a Hallberg Rassy 64 - worked well enough for basic routing.

there are a few reasons the polars are presented as they are:
1) knowing the optimal beat and run angles is important - often you will see these posted in the cockpit as a table.
2) pretty much all boats can sail at TWA = 52, 60, and so on.., but closer to the wind than 52 is going to be their upwind target angle, and these vary between boats.., so if you want to compare polars between boats.., it is better to compare the upwind targe VMG at say 8kts TWS, than the upwind target BSP - if the angle is 5deg different, the BSP comparison will be meaningless. But comparing upwind VMG at 8ktsbetween boats is fine.

in any case, it is trivial to find the target BSP from the VMG at those upwind target angles - indeed you will need to do so if you want to take that table, and make polars formatted for routing programs.., as they want BSP at all angles, not VMG.

Also, if you are going to post a table of targets, typically you want up/down target TWA and BSP for all TWS

as to why they don't just make it easy for you.., and save you the trouble of doing the trignometry, by including both the VMG and the BSP ath the target angles.., i don't know....
 

Daydream believer

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I've managed to get hold of the polar data for my boat, which is as follows:

Wind velocity6kts8kts10kts12kts14kts16kts20kts
Beat angles43.9°42.1°41.2°40.4°38.8°38.1°38.2°
Beat VMG
3.11​
3.75​
4.26​
4.69​
4.97​
5.08​
5.11​
52°
4.78​
5.66​
6.35​
6.76​
6.99​
7.1​
7.16​
60°
5.06​
5.97​
6.59​
6.93​
7.14​
7.28​
7.38​
75°
5.27​
6.22​
6.79​
7.08​
7.29​
7.49​
7.77​
90°
5.46​
6.5​
7.05​
7.34​
7.49​
7.61​
8.02​
110°
5.45​
6.55​
7.09​
7.43​
7.77​
8.1​
8.49​
120°
5.26​
6.35​
6.99​
7.35​
7.69​
8.06​
8.77​
135°
4.73​
5.78​
6.63​
7.1​
7.43​
7.78​
8.55​
150°
4.01​
5.05​
5.94​
6.68​
7.1​
7.41​
8.09​
Run VMG
3.47​
4.37​
5.15​
5.87​
6.51​
6.98​
7.6​
Gybe angles146°150.6°152.6°157.7°180°180°180°
I've got two questions:






  1. Why are the Beat and Run labelled as VMG but the other angles aren't?
  2. Assuming I've got a clean bottom, reasonable sails and flat seas, how likely am I to be able to make these target speeds?
You may have found the data here but if not
Try this site for a few more. Just type in the yacht class at the top & a selection will come up. It is updated annually
Sail boat data

Do not like to tell you this----- but looking at data for Dehler 35 SV "Hinder"---- you will have to go a bit faster :unsure:
I have compared mine with a few & can happily say that on most levels I can get there OK - but only if my son helms (y)
 
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RobF

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... Do not like to tell you this----- but looking at data for Dehler 35 SV "Hinder"---- you will have to go a bit faster :unsure: ...

I did find the data there :) Fortunately, Hinder is new Dehler 35 (designed by Simonis en Voogd) which is a much more sporty boat than mine. I've got the old Van De Stadt designed Dehler 35 which has a much shorter LWL

Sail boat data - Gast Vrij
 
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