Polars and VPP

[Birdseye]

Have been looking at the Avalon VPP polars for my boat and gradually getting confused. Velocity means speed but in a specified direction - its a vector and thats its definition. But what is the direction specified in a Polar. It cant be into the wind or the downwind predictions would be negative. So it has to be speed through the water.

But if it is speed through the water not velocity that the VPP is predicting, then I dont recognise the numbers and I am not being silly here - my log is adjusted to match the GPS in the flat tideless water behind the barrage gates. The differences are more than a knot.

And whilst I am asking, do polars allow for leeway?

Anyone else compared their polar from avalon with reality?

 

[TLouth7]

I think velocity in this context is a misnomer (with the exception of VMG which is a proper vector).

They take TWA as an input which is defined relative to heading, but on the other hand finding VMG to windward clearly requires knowing speed in the direction of your course.

I am confident that the magnitude is STW as this is the only helpful thing for the helm when using the polars. Whether this is plotted against course through water (more internally consistent) or heading (perhaps more useful?) is less clear.

 

[anoccasionalyachtsman]

Your understanding of the terms is quite correct, I'm just not sure why you're not clear on reading the plot. If you sail at 45 degrees from the true wind it tells you the speed you should be getting - nothing more or less.

Are you using true wind on your gauges? Are the speeds you're seeing high or low compared to that which the plot offered?

As for leeway, that'll depend on how the inputs were derived. If boatspeed, heading, true wind direction and speed all came from real boats capable of measuring/calculating it all then it will be included. If not then it may all be guesswork and it probably isn't.

 

[Birdseye]

I am not clear because to me velocity is a vector and not a simple magnitude.

The speeds I remember are maybe half a knot higher. That can be memory - after all I havent sailed the boat since December. But I dont see how it can be calibration since one of the few benefits of birng cardiff based is that w have a large lagoon tide free behind the barage and can set the log to be accurate against GPS.

The inputs for the polar I am using ( the Avalon one) are a VPP, but I reckon going back to your original comment that it wont allow for leeway. It will simply be speed through the water for a certain angle to the true wind.

 

[anoccasionalyachtsman]

Assuming that we're talking about the usual heart shaped diagram. That shape represents the ends of an infinite number of lines drawn form the centre with a length proportional to the boat's speed. In other words the ends of an infinite number of vectors.

There could be a separate outline for different wind strengths and sail combinations.

To read it you take a chosen direction, say 45 degrees for upwind and follow that line until it meets the heart. Compare that distance from the centre to the concentric speed rings and it tells you how fast the data says you should be sailing at that True Wind Angle.

The last point raises a question - have you got your boat instruments switched to True Wind or Apparent?

 

[Little Dorrit]

If you constructed the polar diagram yourself you would note TWA and speed through the water. It makes sense to have various plots for different sail set ups and perhaps even tidal conditions. In theory you could have an infinite number of diagrams. If you have only one polar diagram it is obviously going to be a compromise.

 

[Laser310]

I work quite a lot with polars.

I do usually end up with several sets of polars for each of the boats on which I race.

Typically.., we will have a set provided by the designer - often these are somewhat optimistic...

next, we will have a set that comes from a rating - for example an ORC international certificate comes with a full polar set derived from the ORC VPP. This is obviously subject to all the biases of that rating rule.., and is more accurate for some boast than others. Still it is pretty useful as it always tells you whether you are sailing to your rating, or not.

Either the designer polars or the ORC polars will serve as the starting point, that can be edited based on observed performance, for another set of polars that are meant to represent the actual performance of the boat.

I usually end up with more than one version of the actual performance polars. one set - the inshore polars - is a bit aspirational.., and contains the target boatspeed numbers that are sent to the on-deck displays. we do not want to make it too easy for the crew to hit target.., better if they can never quite get there. Another set will be the best measure of the boat's actual performance, and is used in routing software. this set might reflect the effects of additional weight for distance races, as well as the effects of waves, and so on. The tacking angles may be a bit wider than the inshore polars.

Yet another set would be what are called "start polars" . this set contains no spinnakers, code sails., staysails.., and so on. These are needed when the boat wants to use polar boatspeed to calculate time to the starting line.., time to the pin end.., and so on. Th other difference is that even for upwind courses, they are slower than the inshore polars, because the boat is not in racing trim - weight is off the rail.., the wind near the starting line is very disturbed with boats reaching back and forth.., there are a lot of maneuvers.

only the top level race boats get to put much time into developing all these polars - for most of us they are simply educated guesses obtained by looking at the recorded logs from previous races

all of the polars are always a work in progress.

 

[Birdseye]

Assuming that we're talking about the usual heart shaped diagram. That shape represents the ends of an infinite number of lines drawn form the centre with a length proportional to the boat's speed. In other words the ends of an infinite number of vectors.

There could be a separate outline for different wind strengths and sail combinations.

To read it you take a chosen direction, say 45 degrees for upwind and follow that line until it meets the heart. Compare that distance from the centre to the concentric speed rings and it tells you how fast the data says you should be sailing at that True Wind Angle.

The last point raises a question - have you got your boat instruments switched to True Wind or Apparent?

Is an angle really a direction? I can see what you are saying so best drop that point before I end up arguing semantics.

As for your last question the answer is - it depends! . When I say that speeds from the VPP ( and avalon provide two covering our typical sail set of roller genoa or spinny) are different to those we achieve I am relying on memory and thinking either hard on the wind or beam reaching.

Sounds like a project for this year. If ever we get released I will put some effort into data collection now that I have the polars to compare with. Not the level that Laser 310 talks about , mind you.

 

[anoccasionalyachtsman]

A. An angle is a direction when you use a compass! Agreed on semantics - otherwise we'll end up on ground wind, water wind and club bar wind.

B. You need 'em switched to true, that's what the diagram is based on.

c. Laser 310's level, me neither, but I would if the boat had any instruments - keelboat, class rules etc..

 

[Laser310]

anyoccasion - if you have a common boat.., check for an ORC certificate.

if someone has a boat like yours, and it has an ORC international certificate, you can get the polars from the certificate, You can get the certificate for free on the ORC website, but you have to register. Or just post the type of boat and I will look.

I don't have any idea how well the avalon polars work.., but often the ORC polars are not that bad, and you can compare them with the avalon polars.

Depending on what you want to do with the polars, the ORC numbers are not necessarily in the best form, and you may need to do some simple trigonometry to use them.., or post them here and someone will do it.., it takes a minute.

 

[anoccasionalyachtsman]

anyoccasion - if you have a common boat.., check for an ORC certificate.

if someone has a boat like yours, and it has an ORC international certificate, you can get the polars from the certificate, You can get the certificate for free on the ORC website, but you have to register. Or just post the type of boat and I will look.

I don't have any idea how well the avalon polars work.., but often the ORC polars are not that bad, and you can compare them with the avalon polars.

Depending on what you want to do with the polars, the ORC numbers are not necessarily in the best form, and you may need to do some simple trigonometry to use them.., or post them here and someone will do it.., it takes a minute.

It's a common enough old one design, but much to my surprise half a dozen have ORC certs (none have current IRC and I was going to get one for this year just to upset the apple cart...). On the one hand not having, or being allowed, any instruments is A Good Thing because it saves a heap of cash when the bows and sterns of surrounding boats would tell you all you need to know about your trim. On the other, it's a shame because we can't go out and get any meaningful comparisons with polars.

 

[Birdseye]

Pity. My boat has an old CHS certificate but nothing else. I'll try the RORC to see if they have polars since they obviously use some sort of VPP to give the boat its TCF

 

[Motor_Sailor]

I'll try the RORC to see if they have polars since they obviously use some sort of VPP to give the boat its TCF

You can try, but I don't think you'll get anything. The iRC is a measurement rule and the formula is a secret.

However the ORC Handicap is based on a VPP generated polars and they are far more amenable to selling their material.

 

[Laser310]

You can try, but I don't think you'll get anything. The iRC is a measurement rule and the formula is a secret.

However the ORC Handicap is based on a VPP generated polars and they are far more amenable to selling their material.

The ORC VPP is public and available for free
any existing ORC certificate is public and available for free - the "international" certificates have the polar information

almost nothing from IRC is public or free

 

[anoccasionalyachtsman]

The ORC VPP is public and available for free
any existing ORC certificate is public and available for free - the "international" certificates have the polar information

almost nothing from IRC is public or free

I wish I'd asked you first, but having registered on the ORC site and found the boat type I was led to believe that I had to pay 50 credits to see the 'Speed Guide' or 20 for the 'Target Speed'. I stumped for the latter (a credit is nearly a whole Ingrish pound), but that turned out to be some speeds for AWAs 26-30 and TWAs 145-180 in a few windspeeds. Without knowing what the Speed Guide is the £50 is staying in my pocket!

 

[Laser310]

I wish I'd asked you first, but having registered on the ORC site and found the boat type I was led to believe that I had to pay 50 credits to see the 'Speed Guide' or 20 for the 'Target Speed'. I stumped for the latter (a credit is nearly a whole Ingrish pound), but that turned out to be some speeds for AWAs 26-30 and TWAs 145-180 in a few windspeeds. Without knowing what the Speed Guide is the £50 is staying in my pocket!

the speed guide is not what you want - that is more of a custom product for as specific boat
what you want is a rating certificate- they are free

 

[oceanice]

Have been looking at the Avalon VPP polars for my boat and gradually getting confused. Velocity means speed but in a specified direction - its a vector and thats its definition. But what is the direction specified in a Polar. It cant be into the wind or the downwind predictions would be negative. So it has to be speed through the water.

But if it is speed through the water not velocity that the VPP is predicting, then I dont recognise the numbers and I am not being silly here - my log is adjusted to match the GPS in the flat tideless water behind the barrage gates. The differences are more than a knot.

And whilst I am asking, do polars allow for leeway?

Anyone else compared their polar from avalon with reality?

Just got the latest news from Avalon. They have added a polar capture module to their app. It collects real STW data at sea and then you can tune the data provided by Avalon VPP. Can be useful for boats that Avalon VPP cannot handle such as multis and foilers