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Posted by
AGage


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2008-12-27 10:31:57
2010-01-15 09:37:18 - Re: Game Feature Suggestions
HenryB said @ 2010-01-14 19:55:14:
As I see it VMG is 'Velocity Made Good' directly to windward; whereas, VMC is also a velocity made good, but to where? If it is to the destination, then it assumes that one is taking a direct great-circle path to the nearest point about which one has to turn. As well most sailors know, that isn't necessarily optimal.

In my youth I raced a Tornado and often found I could beat the competition, to a downwind mark, by tacking downwind - the boat was so fast on a broad reach that it got to a downwind mark much more quickly NOT heading straight for the mark.
=======================================
Well, VMC = Velocity Made to Course recognising that course legs arent always directly up or down wind. IRL we all use this concept downwind when there is a local shift just prior: Sail the gybe that takes us closest to the mark. Or either upwind or downwind to tack on the headers (lifts downwind): coz VMC is decreased and increased on other tack/gybe.
So VMC is the component of your current velocity (being a vector with magnitude and direction) in the direction of you next most immediate waypoint/mark on your course, VMG is with respect to the wind direction. So, no it doesnt assume one is on a direct course, great circle or otherwise, to the next waypoint, but rather uses that direct course instead of the wind direction.

In a windward/leeward course set exactly to the wind VMC = VMG. If a shift has occured and you are on the favoured tack/gybe then VMC > VMG, converely on the unfavoured tack/gybe VMC < VMG.
If you can, say, reach directly to the next waypoint/mark then VMC = BS. And in the case where that firection is 90 degrees to the wind, VMG = 0.

Say the TWD is 0, but the heading to the mark is at 15. You're heading at 40degrees doing 10kn. Then VMG = cos(40-0) * 10 = 7.66kn, and VMC = cos(40-15) * 10 = 9.06kn.
Now say we footed off 5 degrees and our BS increased to 10.6kn. our VMG is now cos(45-0) * 10.6 = 7.49kn, our VMC is cos(45-15) * 10.6 = 9.18kn. So our VMG has decreased, but VMC has increased by footing off for more speed.
And to illustrate what you said about sailing directly to the next mark not always being optimal, imagine the heading to next mark IS 40 degrees above.
Sailing directly toward it, VMC = BS = 10kn (cos(0)=1). By footing to 45@10.6kn our VMC is cos(45-40) * 10.6 = 10.55kn, so we will get there faster heading 45, even though over a 30nm leg we will sail an extra 1nm approximately (the cosine(angle) function automatically accounts for extra distance, but assumes no manouvering penalty). So given performance penalaties for for tacks/gybes, the longer the leg the more the benefit will be, and if the leg is short enough you WILL still get there quicker going straight for it.
This is my understanding of the term VMC, again maybe I am missing something, then pls inform me of it.

Of course, on really long legs a straight line on the mercator map will not be the shortest distance no, but you can still calculate a fastest heading, convert it to a great circle heading, and slowly follow the circle.
And of course you can always pick an arbitrary point to calculate VMC to, like if you need to round a headland before you get to a waypoint, then use that instead. C can stand for any chosen course, but we usually like to sail the course set by the RC, so it is usually that which we choose to use (or deviations due to obstructions).

Yes, High performance dingies, some cats, and performance yachts will now always reach the leeward mark faster by tacking downwind. That is simply the concept of VMG put to practical use downwind (if vmg > at an angle off 180) And that is such a good thing! I hated the 'processional runs' in the Junior classes I sailed. Downwind is now the leg generally least understood, which can provide good opportunities for gains, but equally for losses.
I dont think I have yet seen a boat on SOL that doesnt benefit from tacking downwind...
IRL main trouble is determining the best angle downwind... We have developed excellent methods to do this upwind, but not down... Its easy when you have a nice polar and that is the only thing that contributes to performance... Add in to that handling, trim, wave surfing etc etc and its quickly gets very complicated. Not to mention some boats like the ones I sail, benefit more if you dont sail ANY straight line, but a snake course, up first to increase the apparent windspeed and swing it more toward the beam (even the bow), then head down to decrease distance sailed... Headache time! :-)
High performance skiffs (18s, 49er etc) effectively never sail downwind, as the apparent wind is always coming from the bow, and this impacts handling and trim enormously...

Gettin too far off topic here, maybe we should start a 'Sailing Terms' Thread?
I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by
ernissss


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2008-12-15 12:42:03
2010-01-15 11:11:23 - Re: Game Feature Suggestions
Excellent explanation, think no new thread needed.

Ernestas
ernissss
LT
Posted by
Jawz


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2009-07-21 09:51:19
2010-01-16 07:01:56 - New Steering line etc.
Please guys do something about the zooming issue. Not being a routing sailor I rely on accuracy and shortness of course to keep my efficiency high. With this new system I cannot get this right at all. In my humble opinion this feature has taken us a step backwards and not improved the game engine at all. Lets take a step backwards and rebuild please!

Thanks
George
Posted by
jakob


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2007-09-04 10:05:30
2010-01-16 07:28:24 - Re: Game Feature Suggestions
Hi. What zooming issue?
...one of the guys behind the game...
Posted by
AGage


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2008-12-27 10:31:57
2010-01-16 07:33:28 - Re: Game Feature Suggestions
Jakob, there is an issue with the steering tool line, where it changes the length of the line shown on screen, depending on the zoom level. Some have commented that at maximum zoom it disapperas completely, but I have not personally seen this particular issue, only that it gets shorter with more zoom...
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
2010-01-16 12:57:50 - Re: Game Feature Suggestions
Can I get some screenshots of that please. I cannot reproduce... jakob@sailport.se
...one of the guys behind the game...
Posted by
jakob


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2007-09-04 10:05:30
2010-01-16 13:02:05 - Re: Game Feature Suggestions
Can I get some screenshots of that please. I cannot reproduce... jakob@sailport.se
...one of the guys behind the game...
Posted by
Schakel


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2008-12-19 09:19:00
2010-01-16 14:00:43 - Re: Game Feature Suggestions
jakob said @ 2010-01-16 13:02:05:
Can I get some screenshots of that please. I cannot reproduce... jakob@sailport.se
Note where the cursor is and where the line ends.

Schakel
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Posted by
jakob


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2007-09-04 10:05:30
2010-01-16 16:04:19 - Re: Game Feature Suggestions
Ahh, now I understand. Thanks!!
...one of the guys behind the game...
Posted by
LiquidMotion


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2010-02-14 14:08:05
2010-02-21 13:18:00 - Re: Game Feature Suggestions
Hi - Thanks for all the hard work.
I was wondering if it would be possible to overlay 'real terrain', something like Raymarine does on the chart plotter. Or incorporate Google maps. Just a thought.
Dan. aka LiquidMotion

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