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Board » Technical Support » wrong VMG

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Posted by dunbur Posts 9 Registered at 2009-02-18 06:03:58	2009-04-20 23:30:37 - wrong VMG
	I think that has been mentioned before: the VMG is still showing wrong. At least for me (firefox 3.0.8 on linux). I think it is wrong since I rounded the mark at Fernando de Noronha. Before that I had the impression it was show the VMG directly to Boston (also not quite appropriate), but now the VMG is calculated as if the next mark would be directly upwind rather than towards Boston. See the attached screen shot. I am sailing with a TWA of 90° and the VMG is 0.00knts though I am pointing roughly to Boston (and the boat speed is not 0knts). Attachments vmg0.png (115.0 KB)
Posted by kalle Posts 413 Registered at 2007-06-01 09:58:04	2009-04-20 23:50:47 - Re: wrong VMG
	As said before, VMG is velocity made good against the wind. Waypoints has got nothing to do with VMG in ocean racing.
Posted by dunbur Posts 9 Registered at 2009-02-18 06:03:58	2009-04-20 23:56:57 - Re: wrong VMG
	Thanks for the quick answer. And sorry that I missed the answer earlier. May I turn that into a feature request, then. I see that VMG to way points is of limited usability, if you have to navigate around weather systems, but it would still be nice to have and very useful in shorter in-shore races.
Posted by Chappo Posts 53 Registered at 2009-04-17 12:00:35	2009-04-21 01:27:29 - Re: wrong VMG
	So VMG is only relevant when trying to find the best course when sailing upwind. Once reaching and running, SOL's VMG is not relevant and you need to calculate VMG with reference to the next mark or turn point. This is done by a calculation using BS and the cosine of the difference between your CC and direct course to the next mark.
Posted by jakob Posts 364 Registered at 2007-09-04 10:05:30	2009-04-21 05:08:31 - Re: wrong VMG
	Well, SOLs definition of Vmg is relevant both for upwind and downwind conditions e.g. whenever you want to maximize your speed component against or along the wind direction. A negative Vmg indicates you have a speed component downwind. ...one of the guys behind the game...
Posted by Chappo Posts 53 Registered at 2009-04-17 12:00:35	2009-04-21 05:53:47 - Re: wrong VMG
	Yes I agree a -ve VMG shows a downwind component and I can see that if your -ve VMG matches your BS, then you are dead square downwind. How would you use that to maximise speed once close or broad reaching and heading for a turn point or finish. It seems to me you have to incorporate the bearing to the buoy or finish in your calculation to get the best VMG. I have a simple Excel worksheet that does that for me.
Posted by jakob Posts 364 Registered at 2007-09-04 10:05:30	2009-04-21 06:19:58 - Re: wrong VMG
	You are right, the Vmg does not help you in all situations. We will most likely not implement any other form of Vmg/Vmc e.g. since it will be unclear what it means. Consider the example when you are approaching the finish-line. Shall Vmc be the speed to closest part of line, to mid point of line or maybe the part of the line that you can reach in minimum time? The latter would require route optimization... ...one of the guys behind the game...
Posted by Chappo Posts 53 Registered at 2009-04-17 12:00:35	2009-04-21 06:45:01 - Re: wrong VMG
	Yes I agree Jakob, you would need to input a bearing. SOL should leave some work for the sailors and not make it too mechanical like some aspects of Virtual Regatta when assisted by VR routing software. SOL weather is far more realistic as is the invisibility of the competitor's performance.
Posted by dunbur Posts 9 Registered at 2009-02-18 06:03:58	2009-04-21 06:56:32 - VMG to waypoint
	I don't quite agree. Of course one need a bearing. The bearing is quite easy to get. To the next way point. The argument with the finish line is rather weak. If you are far away, the exact point on the line does not matter. Just default to the midpoint as it is on the red line which connects all the way points. Then the VMG get useful. Currently, I fing the VMG not useful at all. When is the course really upwind? And then the VMG upwind is easily estimated. Just add horizontal lines to the polar and you have it.
Posted by kalle Posts 413 Registered at 2007-06-01 09:58:04	2009-04-21 08:05:54 - Re: wrong VMG
	It must be added that VMC, while being the instantaneously best speed against a waypoint, is almost never the best course. The best course is determined by intelligent routing w.r.t the weather forecast.

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