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Board » General Discussion » Wrong Speed VMG

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Posted by
kalle


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2007-06-01 09:58:04
2009-02-04 17:55:13 - Re: Wrong Speed VMG
There is no such thing as 105% performance :)
Posted by
AGage


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2008-12-27 10:31:57
2009-02-09 04:44:56 - Re: Wrong Speed VMG
I have read Jakob ask if anyone has ideas on how to implement the performance loss on manouvers. How do you currently implement it? If you could provide something like a pseudo algorithm would be good. I do have some ideas on this, but it could simply be a variation on what you already have in place.
I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by
jakob


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2007-09-04 10:05:30
2009-02-09 07:10:33 - Re: Wrong Speed VMG
Well, the current model determines if the maneouver is a tack or a gybe or just a minor change in twa (dTWA is the twa change).If (tack or gybe) {e = 1.0-self.bs/200.0} else {e = 1-abs(dTWA)/25.0}. perf = perf*e. However if perf<0.93 no further maneouver loss is done. something like that.
...one of the guys behind the game...
Posted by
AGage


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2009-02-12 03:32:36 - Re: Wrong Speed VMG
OK firstly, I do not sail large boats so I am unsure if any of this is realistic but...
From what you wrote Jakob, the faster your bs, the greater perf loss, whereas I would have thought the opposite would be the case, that perf loss is inverse to bs?
And I asssume you also use abs(e), in cases where dTWA > 25? This part seems a bit off, as a dTWA of 50 would result in 0 loss and 25 in maximum performance loss. Also a bear away takes time reach full bs, whereas pointing up doesn't...

What determines the rate at which performance is re-establised, as this seems to be the issue that concerns people.
In my sailing experience gybes usually return to full bs quicker than tacks?
Perhaps making it a function of TWS would work? Or a function of bs and TWS as that would simulate acceleration?

Maybe I will play with some code and try some different algorithms for all this.
How simple do you wish to keep it? ie, should a manouver that changes from headsail to spinnaker (and back) be penalised more than simple course change or tack/gybe? Obviously you need a quick algorithm for this...

As I said above... I have had little experience on large yachts, so all of this could be completely unrealistic!

also, to StuArt: As far as I know, racing rules prohibit techniques where you gain performance advantage from tack/gybe manouvers... In dinghies for example, it is sometimes possible to gain speed through a tack (but very hard to do) and this is prohibited [although very hard to prove in protest ;-)]...
I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by
jakob


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2007-09-04 10:05:30
2009-02-12 06:20:56 - Re: Wrong Speed VMG
Hi. You are very welcome to try some new algorithm for speed recovery. The current models are designed by myself and Magnus Woxén, a very experienced yachtsman with 3 laps around the globe and a place on the Ericson 3 from Qindao. So some thought is put into this. :-)
...one of the guys behind the game...
Posted by
Eddie C. Dost


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2008-08-23 11:40:29
2009-02-12 09:51:39 - Re: Wrong Speed VMG
Maybe a different view on the performance loss effect helps to be less scared. What you are interested in is not the % performance or the duration of recovery, but the overall loss against the other boats.

In the example above I will use worse case numbers I have seen sailing here.

Assume we tack at a BS of 10kn and experience a performance drop to 90% with a recovery time of 20minutes. In almost all cases I have seen the loss is less and the recovery faster.

I use the term e for performance below, so with 90% performance e = 0.9 and the term t_r for recovery time in hours so with 20 minute recovery time t_r ~ 0.333

The distance you travel with the boat is s = v * t. The distance you loose when assuming linear recovery is s_l = v * (1 - e) * t_r / 2
which is s_l = 10 * (1 - 0.1) * 0.333 / 2 = 0.167

The overall loss in this extreme example is 0.167nm or with t_l = s_l / v this is 60 seconds.

These figures sound very reasonable, as we are not sailing in calm waters and stearing up to the new wave pattern and optimizing trim on the new tack takes its time.

Also the loss of 60 seconds or 0.167 nm does not introduce a penalty as high as "felt" by the 90% and 20 minutes recovery. It will only hurt in very tight racing situations where your opponent does not have to tack anymore. But then again you made some mistake earlier by not covering your opponent. ;)

All in all I think the algorithm is reasonable and I hope I could take away some of the fear caused by looking at the performance figure or the recovery time alone. I have attached a quick drawing showing the amount of loss.
_/)_/)_/)
The sea is lovely, dark, and deep, But I have promises to keep, And miles to sail before I sleep, And miles to sail before I sleep.
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Posted by
Schakel


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2008-12-19 09:19:00
2009-02-12 09:58:41 - Re: Wrong Speed VMG
Eddie C. Dost said @ 2009-02-12 09:51:39:
I use the term e for performance below, so with 90% performance e = 0.9 and the term t_r for recovery time in hours so with 20 minute recovery time t_r ~ 0.333

The distance you travel with the boat is s = v * t. The distance you loose when assuming linear recovery is s_l = v * (1 - e) * t_r / 2
which is s_l = 10 * (1 - 0.1) * 0.333 / 2 = 0.167

The overall loss in this extreme example is 0.167nm or with t_l = s_l / v this is 60 seconds.

And here I was thinking I could just wing it. ;-)

Philip
Posted by
K3 (Bernd)


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2008-08-25 12:34:07
2009-02-12 09:59:31 - Re: Wrong Speed VMG
I made some experiments with performance loss during the last practice session. I edited 10 gybes with a delta-twa of 178 deg in steps 0,05 h. The crew didn't liked me afterwards.
The minimum performance after the gybes was about 92 %. The effect on boatspeed was only about 9 %. So nothing to worry about, my own mistakes have always bigger effects.
Posted by
jakob


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2007-09-04 10:05:30
2009-02-12 10:36:39 - Re: Wrong Speed VMG
We introduced this performance loss not to punish normal sailing maneouvers but to prevent boats to tack e.g. once every minute during hours just to ride on a tws that optimizes vmg. The prformance loss implementation succeeded in preventing that behavior and is as stated above not a problem for normal maneouvers. Thanks all for caring!! :-)))
...one of the guys behind the game...
Posted by
AGage


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2008-12-27 10:31:57
2009-02-12 13:12:18 - Re: Wrong Speed VMG
Schakel said @ 2009-02-12 09:58:41:
And here I was thinking I could just wing it.

Philip
...........................
Just a bit of physics: s = 1/2*a*t^2 (s = distance, a = acceleration, t = time) and differential calculus... Yeah... brainaid has an appropriate name :-)

...........................
Jakob: I wasnt actually complaining myself about the way things are with the loss, just responding to others posts. And I certainly didnt mean to imply that thought had not gone into how it is implemented, you have obviously been developing this simulation for quite a while, just pointing out things that seem to *me* not quite right, based on the algorithm you posted. As I said, I dont sail ocean racing boats, so have no real idea how they perform... I was simply extrapolating from my own sailing experience. I was also unaware that it was implemented such as to discourage abuse as you say...
Having said that:
a) any performance loss should prevent abuse, yes?
b) do Ocean Racing Yachts really loose upt0 .15nm on a single tack in 10knots? I am asking seriously... I dont sail them... I suppose you dont do alot of leebow manouvuers! :-)
I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__

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