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Board » Technical Support » wrong VMG

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Posted by
NZL_PaulR


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2008-07-22 12:21:30
2009-04-21 10:53:40 - Re: wrong VMG
I use the VMG all the time.

SOL is ocean racing, not harbour racing.

Certainly there are times when VMG is not critical, but on broad reaching it is critical as well as when close hauled beating.

If sailing dead downwind, then you are naturally very slow, and at TWA=180, BS will equal -VMG, but who would ever sail that slow?

At TWA=90, there VMG=0.0 as you are sailing exac6tly across the wind, neither upwind nor downwind.

Try setting your TWA=0.0 and see how fast you can go!


--- Last Edited by Paul Rosser at 2009-04-21 11:01:09 ---
If it breaks, it's not strong enough!
Posted by
Chappo


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2009-04-17 12:00:35
2009-04-21 12:46:32 - Re: wrong VMG
I am curious how you use - VMG when close and broad reaching, can you please explain. Doesn't the polar give you the best course to sail when reaching when the - VMG is just a -ve number that is neither a maximum or a minimum.
Posted by
dunbur


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2009-02-18 06:03:58
2009-04-21 16:22:41 - vmc
That's for sure. Looking at the weather is essential. And that's why SOL would not be mechanical with VMC as a previous poster claimed. But showing the VMC would help to judge losses and gains on detours due to weather.


--- Last Edited by dunbur at 2009-04-21 16:24:48 ---
Posted by
jakob


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2009-04-21 17:52:33 - Re: wrong VMG
Great discussion. SOL listens carefully and follows the debait but are not yet convinced to implement vmc. :-) We want to be unambiguous, fair and not too detailed in the info since we want sailor skills to be vital.
...one of the guys behind the game...
Posted by
NZL_PaulR


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2009-04-21 20:14:11 - Re: wrong VMG
Before developing any VMC, I would prefer development of the predicted course indicator to include any DELAYED COMMANDS. Currently the predicted course "bends" when a TWA is being steered, which is a great first step. However once you enter a delayed command, the course predictor indicator becomes useless because it does NOT show that planned change and its effect.

Showing planned changes is very important if tacking or gybing betwwen islands, rounding marks etc.

I would be happy to limit the course prediction indicator to Delayed Commands that exist for the next 6 hours, as per existing course predictor indicator limitation.

This I see as far more useful and important for "ALL" sailors.

VMC will not be used much, but if used, it will be used only by a few. Once you move off a planned course, the old course becomes irrelevant and is of historical and limited use.

Racing Navigation is always about where you are NOW and that "NEW" best course to the next mark. Who cares where you planned to be yesterday? That's gone forever. Your "here" now and what is your next plan? How are you going to get to the next mark faster is all that matters.

Last weeks meals are of little importance if you are hungry today!!!!

The wind is the most important factor, follows by planned course. Thus boat speed and VMG to the "wind" is MOST critical.

--- Last Edited by Paul Rosser at 2009-04-21 20:22:40 ---
If it breaks, it's not strong enough!
Posted by
jakob


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2009-04-21 20:39:44 - VMG
I hear you Paul and add the request to the list.
...one of the guys behind the game...
Posted by
NZL_PaulR


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2009-04-21 20:53:01 - Re: wrong VMG
Chappo said @ 2009-04-21 12:46:32:
I am curious how you use - VMG when close and broad reaching, can you please explain. Doesn't the polar give you the best course to sail when reaching when the - VMG is just a -ve number that is neither a maximum or a minimum.
REPLY:
The polar is a great diagram but it is hard to be accurate, given the small scale, when trying to read off resultant speed prediction.

Press the "wheel" icon and as you move the course, the polar diagram shows the Boat Speed and VMG to 2 decimal places, far more accurately.

I am quite happy to beat you by sailing x.01knots faster, as long as I'm faster!!LOL (My current leaderboard position is not necessarily my planned one!! LOL)

When reaching, Boat Speed and VMGs are a trade off, with VMG becomming progressively less important as TWA approaches 90 degrees. Therefore where VMG is maximised is generally at about 45 degrees upwind and 135 degrees downwind. BUT, and it is always a big "BUT", this does not apply in all wind strengths and IRL with different wave patterns. VMG to windward maximises between 30 to 50+ degrees dependant upon wind stength and boat design, weight and trim. In SOL waves, weight and trim are not factored in.

Planning a course needs to consider both bearing and VMG speed. Hence gybing down wind to maintain "hot" (faster) angles is very important e.g. Americas Cup or any match racing shows this up critically. 2nd is still "LAST". Fleet racing clouds the effect a lot as you may be trying to cover several boats at once. It's all down to trigometry and how accurate your wind forecast and hence boat speed forecast on the other leg will be.

When to tack or gybe or adjust course, especially with wind shifts, is what is the difference between 1st and ALL other positions.

--- Last Edited by Paul Rosser at 2009-04-21 20:54:49 ---
If it breaks, it's not strong enough!
Posted by
dunbur


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2009-04-21 21:43:50 - Re: wrong VMG
NZL_PaulR said @ 2009-04-21 20:14:11:
Before developing any VMC, I would prefer development of the predicted course indicator to include any DELAYED COMMANDS.
Oh yes, I second that. I also would wish for the course predictor to extend to e.g. 12 hrs rather than six. But I would give the inclusion of delayed commands higher priority. Then an extension of the course predictor and last but not least I would rather have VMC than VMG.
Posted by
Jeroen


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2009-04-22 06:49:46 - Re: wrong VMG
HI Paul, great discussion going on here.

I like he idea of including the delayed commands in the course predictor. But this might give people the impression that its fail safe. Then the course predictor plots out their exact course and they start thinking less about the course. (not the veterans of course).

I think this is the same by extending the course prediction to 12h, now its a clear indicator of the 6h blocks the weather comes in. Showing more might lead to the illusion that the course shown will always be true.

--- Last Edited by Jeroen Tiel at 2009-04-22 06:52:11 ---
I am a ex-admin of SOL who joins in from time to time.
Posted by
NZL_PaulR


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2009-04-22 10:24:24 - Re: wrong VMG
If the courses are all "COMPASS" courses, then the predictor is very accurate and the margin of error is indeed small.

If "any" of the commnads (current or delayed) are TWA based, then with wind shifts, increases and decreases the accuracy of prediction decreases.

In that case a wavy line (lke a current arrow on a chart) should be used to indicate a widening course of probability with the hourly circles getting larger in area. Perhaps a widening arrow but this could obscure some boats. So a wavy line that widens further out will show greater uncertainity of future course.

So with combinations of CC & TWA delayed commands, the course predictor could have a most interesting look. Would you like a diagram example?


However with most SOL Racing, it is very rare to have lots of different commands with many tacks and gybes within any 6 hour period. Certainly I have used many commands (say up to 10 if one is using Sail Planner) but these DC's are generally minor course alterations on the same basic course / tack. Just minor and slow increments in changing the TWA to suit a weather front or major wind shift as it passed by. Three or four DC's if sailing between islands.

--- Last Edited by Paul Rosser at 2009-04-22 10:27:07 ---
If it breaks, it's not strong enough!

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