Knut Grotzki

VPP for dinghies and skiffs

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hey dgmckim

Vpp stands for velocity prediction program. Its meant to show on a graph the speeds obtainable of a particular boat for different wind speeds and points of sail.

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59 minutes ago, edward mason said:

hey dgmckim

Vpp stands for velocity prediction program. Its meant to show on a graph the speeds obtainable of a particular boat for different wind speeds and points of sail.

The question remains -- of what use are they for dinghies?

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well, if you believe the vpp is accurate then you can get a speedpuck, put your dinghy on the correct heading in the specified windspeed and you can tell if you're sailing up to your dinghys potential. It's a target to shoot for. 

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1 hour ago, edward mason said:

well, if you believe the vpp is accurate then you can get a speedpuck, put your dinghy on the correct heading in the specified windspeed and you can tell if you're sailing up to your dinghys potential. It's a target to shoot for. 

.. and what's the correct heading?  How do you measure AWA (or TWA)?

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I think they are largely of academic interest in dinghies because a. They are likely to be wrong* and b. You don't know your true wind angle.  Generating them can be fun though as it allows you to virtually sail the boat and look at the sensitivity of sail settings and heading etc. 

* The 49er ones linked to, for example, suggest best VMG at 70 degrees TWA.  Presumably the units are m/s rather than knots too, else they're very slow!

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On ‎10‎/‎31‎/‎2017 at 11:48 PM, duncan (the other one) said:

.. and what's the correct heading?  How do you measure AWA (or TWA)?

GPS Action Replay gives an estimate of the TWA. Of course, its accuracy depends on having a big enough sample of data points & how steady the wind actually was.

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How many do you want, this is a 49er in Syd Harbour.   19th Oct

BOM, gives the wind at 10-12knts SW, but the wind beacon close to the sailing venue had it at 12-16knts, avg 14.5knts at 6m height.

Max boat speed was 20.6knts, long std gybe, lots of upwind, plenty of tacks, about 7nm sailed in little over a hour.

For those that know Syd harbour, most of the sailing happened around the Pigs.

 

image.png.7fba6e2a47cd048ee1236d4f68a1a88d.png

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You boys are getting awfully confused in your terminology.

A VPP is a theoretical calculation which predicts the performance of a boat.

A polar diagram is a circular graph, which in this case plots speed against direction.

A VPP may be used to generate a theoretical polar diagram, which is a graph showing predicted performance of the boat, and is usually displayed with true wind at 0 degrees.

A polar diagram may also be generated by real world data, typically GPS based tracking, or even just following a boat closely and carefully in another boat with calibrated instruments.

Julian is showing you polar diagrams of real world data, which look as if they have been constructed from GPS logs and by the looks of it some of them in quite shifty wind conditions. The big challenge with just having GPS traces is shifty wind, because the actual 0 degrees that you should be plotting from is moving all the time. If you have instruments that record apparent wind speed and angle and speed though the water its possible to reconstruct accurate polars you can use to predict performance.

 

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So the pragmatist asks, "How can I use this to learn to sail/race better?"  And how could I make a polar diagram/ VPP for a different dinghy?

Thanks!

US307

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The ultimate VPP is an empirical based polar.

Because the theory that creates the VPP better match the empirical data or it's not worth a crumpet (by definition).

(Way too many people try to get the data to match the theory and aint that an error! )

It quite easy to generate the ultimate VPP graph (because it's empirically based) from a polar, just takes some time. 

Watch this space.

         Jb

 

 

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21 hours ago, us307 said:

So the pragmatist asks, "How can I use this to learn to sail/race better?"  And how could I make a polar diagram/ VPP for a different dinghy?

Thanks!

US307

 

Just sail higher and then lower than the people around you in the fleet and see what happens. You'll find the sweet spot that way in real time.

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On 04/11/2017 at 12:11 AM, JulianB said:

The ultimate VPP is an empirical based polar.

Because the theory that creates the VPP better match the empirical data or it's not worth a crumpet (by definition).

(Way too many people try to get the data to match the theory and aint that an error! )

It quite easy to generate the ultimate VPP graph (because it's empirically based) from a polar, just takes some time. 

Umm,

No arguments that at the current state of the art an empirically based polar diagram is the best means of evaluating performance for dinghies.

No arguments that a VPP that doesn't provide a reasonable match for empirical data is worthless.

No arguments that trying to get the data to match the theory is at even the very kindest description very seriously misguided. And its amazing how many people fall for it! Just because its a nice chart doesn't mean its right. A lesson government economists need to learn too I submit!

I agree that once you have sufficient good quality data its not that hard to produce an adequate velocity/wind speed/direction *chart* in the form of a polar diagram that resembles the charts typically generated from a VPP and can be used in exactly the same way - done it myself although not on the scale of the first class work done by your family.

What I have trouble with is calling  that a VPP. A VPP by every definition I know is a theoretically based program that *predicts* the performance of a boat based on measured dimensions.

The problem with getting sufficient good quality data is not so much that its difficult in itself, but that it requires serious sustained effort and hard work, plus ideally more instrumentation than the average dinghy possesses, which is why the work done by your father and the rest of your family was so exceptional. I'm not aware of many other folk who've put in this sort of sustained effort, although you'd think some of the Olympic teams would have done. Am I out of date? Do you know of anyone else doing serious work?

If one had a reasonably accurate VPP (which as far as I am aware does not exist for performance dinghies) then you could take all the details of a boat, even one that hasn't been built yet, and produce a chart that would, well, *predict* what its performance would be, and you'd only need a relatively small amount of empirical data to validate that the theoretical model was correct (or not), so it would save an awful lot of work generating a new empirical velocity chart for each new boat. Also save a lot of on line arguments!

 

 

 

 

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2 hours ago, JimC said:

I'm not aware of many other folk who've put in this sort of sustained effort, although you'd think some of the Olympic teams would have done. 

The RYA developed a datalogger with Caterham, so must have done some work. Probably not publicly available, though. 

http://www.yachtsandyachting.com/news/150612

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2 hours ago, sosoomii said:

A VPP for a Moth must be the simplest.  I'd imagine someone must have developed one for them?

Why? Because it is just L/D? No pesky residua\ry resistance wavemaking stuff?

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Exactly.  The resistance of a foiling Moth would be relatively simple, and the single sail is also the simplest thrust device. No worries about interaction between sails, or how to characterise planing etc.  

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VPP's for foiling boats are the most complex problem you can find. Take the Moth. Small changes i ride height have a noticeable effect on performance. Different size horizontal foils also make a big difference. You might use one foil for light winds and one for heavy winds, or you might use an all round foil. That's just the beginning of the equation. The AC boats needed new software to be developed in order to be able to predict performance. Over all, I doubt there are more than 4 or 5 people in the world who would have any chance of developing a VPP for a Moth. And as soon as they manage to produce one, a new development will make it obsolete.

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I think that is a very puritanical definition of a VPP.  A VPP just has to be able to predict a boat's velocity.  If it can do that using a limited number of parameters that represent the design, then great – but it is not a requirement.  A VPP is just a balance of forces calculator; if the drag force of the hull can't be calculated then it is a perfectly reasonable approach to use a lookup table based upon tank tests or towing trials or reading tea leaves.  And in this sense it is far easier to estimate the drag force of a fully foiling boat than a semi-planing boat, because foil drag as a function of speed, angle of attack, aspect ratio etc. is well characterised in the literature and more readily analysed using CFD techniques.

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4 hours ago, sosoomii said:

it is a perfectly reasonable approach to use a lookup table based upon tank tests or towing trials or reading tea leaves. 

I may be being puritanical, but if ones tea leaf or whatever based VPP produces results that have no useful correlation with the actual performance of the craft then , as Julian says' 'it aint worth a crumpet'.

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What's with these pointy polars downwind?

ISomething isn't right about that. I have a hard time believing that the speed through the water drops off like that between running and beam reaching. That just does not make pnyiscala sense.

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6 hours ago, fastyacht said:

That just does not make pnyiscala sense.

It does, it's all about apparent wind. Your sails power basically comes from the amount of air your sails slope down/change direction. If the apparent wind is forward of the beam every increase in speed is more fresh air coming faster across the deck, and you're getting more and more power. More and more drag too of course.

But if the apparent is behind the beam then every extra increment of speed means less wind across the deck, means less air processed means less power. 

So for really high performance boats you get this really extreme polar diagram, and TBH there ain't much to picking angles, sail that boat as fast as she'll go and you'll be right.

All boats are really slow on dead runs though, no matter what a few degrees off one way or the other, is faster, but following don't realise. They do largely sail like that naturally though. Next time you think You're on a dead run gybe the boat. If not just as comfortable then weren't on dead run.

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Where it gets interesting is a boat without such an extreme spike as say a 49er. I may have a go at drawing something up tomorrow.

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That 20kt curve is absolue bullshit. Under windspeed at beam reach but tgen even sliwer rapidly as yo sail freer? Not in thecreal workd! Thec29 knot beam reachvisvoverpowered but at 120 you have full powet and rhe boat will be steadily accelerating as you turn deeoer.

 

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I'm afraid I am less than convinced by any of those posted curves. They all look as if they are based on rather simplistic VPPs Julian's crumpets would be safe from them I think. I don't think they have had an awful lot of checking against real world data. But it is interesting getting GPS action replay and the like to do the sort of plots Julian has posted for us. I've found my perception of speed on different points of sailing doesn't match actual speed nearly as well as I would have thought. 

-----------------------------------

Going back to plots for less extreme boats, what you tend to see is an apple shape. The greatest speed is not that far below a beam reach, and reduces steadily but slightly as you go lower. A *really* simplistic VPP that utterly ignores drag looks like below. You get more and more speed as you get to maximum apparent wind, which i this case is 20-25 degrees off a beam reach, and then the speed drops away drastically. 

Now the major point of this is that although the max vmg is at the lowest point on the graph, There's actually quite a wide range where the speeds are fairly similar - say 155-135 on this wholly theoretical and quite useless-in-real-life plot, and across that range speed is diminishing drastically as you go deeper, but the shorter distance sailed makes up for it. In UK parlance this is called soaking, and its a question of working out the best speed and direction at any instant, quite possibly with the crew or helm stuck in the middle of the boat. 

This theoretical plot is for a single or two sail boat, but the same principles apply for a slowish asymettric or a spinnaker boat, although there would be a speed lobe for the spinnaker, which at the shy end is flat because the kite is oversheeted and  won't deliver much net power. 

Where it gets really irritating is on the very slow asymettric boats, where the speed lobe from the asymettric is actually off to one side, and you get better vmg on deep legs by taking the kite down and sailing two sail, although even then it usually pays not to sail an exact 180 degree run. Of course given a light enough wind it pays to take the kite down on runs on just about any boat, but outside Brit ponds I suspect few actually go out in conditions so light a kite will only fill on a beam reach...

A pole kite theoretical, and practically will have a much wider speed lobe at the deep end. But even then it will usually be better to sail an angle of some sort, the individual class and weather will depend weather that 170 degrees or 135...

 new-1.png.9be45a986dedb24319e2021db69865e0.png

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This polar shows the difference between an RS200 and a 59er (chosen because they are opposite ends of the hiking asymmetric spectrum and the 59er drag curve is available in HPS2).  The 200s VMG downwind is pretty consistent across the angles, giving a choice as to whether to soak of go for speed, depending on tide, fleet position shifts etc.  The 59er is much spikier, as the kite can't be carried too high and speed drops off quickly if pointed too low.  Get it right though and you can go downwind at windspeed, and stay in a gust for the whole leg.

 

IMG_0951.PNG

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Where on earth did the RS200 plot come from? I find it hard to believe. 

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4 hours ago, JimC said:

Where on earth did the RS200 plot come from? I find it hard to believe. 

Well, I made it up... I took the 59er drag curve and amended by nothing more than judgement - less wetted but shorter length etc so less draggy at low speeds and more (much more) draggy as speed builds. Aero model is essentially the same between both boats, but with the right sail areas and some allowance for the 59er sails being v flat in comparison. 

I know the 200 looks too fast, maybe I need to add even more drag. Or it may be that the 200 rig is much draggier/less adjustable than I allowed for.  But if you take the ratio of the VMG at 45 degrees, you get pretty near the ratio of their PYs, so maybe it isn't that far off after all.  

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Its a good piece of work, Sosoomi, although I reckon empirically in 10kts or more SOG at VMG angles would be ~ 4.5kts for RS200 - reckon if you fudge to fit to that the rest of the polar would be very valid.

Computed polars are the output of VPPs and are used for 2 things - some yacht handicap rules, and for getting target boatspeeds. On a yacht with (properly calibrated!) instruments with SOG and TWS and TWA then you look at your polar especially upwind and also VMG running (when not in tactical situations where you are not on VMG angle) to work out whether you are sailing too high and slow, or too fast and free, i.e. to make sure that the optimum balance between induced drag (reduces with speed at same RM) and frictional drag (increases with speed) is met. Think many VPPs have optimum heel angles as well - important as with heel comes RM but you lose some efficiency and CLR moves.

I'll be honest I don't see the value in the measured stuff. Even if you were clever and only sampled it during "steady state" sailing. You never know the TWS or TWA accurately enough on a dinghy for it to have value, and of course both change pretty dynamically, as do most things on a dinghy.

Dan

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26 minutes ago, Daniel Holman said:

I'll be honest I don't see the value in the measured stuff. Even if you were clever and only sampled it during "steady state" sailing. You never know the TWS or TWA accurately enough on a dinghy for it to have value, and of course both change pretty dynamically, as do most things on a dinghy.

I've used it to evaluate downwind options in my IC. Given a short track and reasonably frequent gybing the immediate true wind was fairly obvious, which also gave the angle, and speed changes give an idea of gust or no. I felt I had enough data to establish I was getting the best vmg for a downwind track  with the jib poled out to windward and reverse flow on the jib. Both sails setting cleanly the same side was clearly sailing too far, and a conventional stalled out goosewinged jib was too slow.

Obviously you could learn much more with a much more structured session than I was doing. A boat on boat two boat tuning session is better if you have no instrumentation I think, but for those who can't organise that I felt I had enough to be reasonable confident about the best downwind configuration.

 

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I don't think VPP's or polars are very much use to dinghy sailors for reasons already stated. Using a GPS is the way to go. It takes a bit of time to get used to converting pure speed into VMG - easier 2 boat tuning than on your own - but you can still analyse tracks after and gain some good insight into what is going on. After a few sessions with a decent GPS, you will get a pretty reasonable idea of your best VMG. I would suggest spending your time with GPS will yield better results than thinking about VPPs and polars.

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On 11/10/2017 at 12:09 AM, Team_GBR said:

I don't think VPP's or polars are very much use to dinghy sailors for reasons already stated. Using a GPS is the way to go. It takes a bit of time to get used to converting pure speed into VMG - easier 2 boat tuning than on your own - but you can still analyse tracks after and gain some good insight into what is going on. After a few sessions with a decent GPS, you will get a pretty reasonable idea of your best VMG. I would suggest spending your time with GPS will yield better results than thinking about VPPs and polars.

I could see that being the case. Only used that technique once, don't own a GPS, but it was interesting and esp if more than one boat in a race compares tracks it could be pretty interesting.

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