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How to use-vmg? |
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Flemse 
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Joined: 30 December 2007 Location: Denmark Status: Offline Points: 84 |
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 Topic: How to use-vmg?Posted: 01 January 2009 at 16:58 |
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Happy new year friends!
i hope some of you "racers" can teach me how to use the vmg indicator in the IS20 wind , .i understand the basic term" velocity -made -good" So going upwind ,are you just trimming/sterring for the highest vmg number? And how do you use it down wind ? I only race Hobiecats ... we dont have any instruments,but i really want to learn to use it. Kind regard Flemse FreeSpirit 370 |
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flemze@mac.com
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Johan Hackman
Admiral of the Fleet 
Joined: 24 August 2005 Location: Sweden Status: Offline Points: 4361 |
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Posted: 01 January 2009 at 20:39 |
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I got so excited when I found out how to use VMG on my (then) IS15 instruments that I wrote a lenghty post about how to use it. The post can be found here together with a number of fine contributions to the thread from other members. It's well worth reading and it will probably give you a few answers to your questions.
In essence, the number you get on your wind instrument is the speed (in knots) by which your boat moves towards the eye of the wind. You want the number to be a high as possible. The only difference between sailing upwind and downwind is that you get a minus sign in front of the number when you go downwind. Johan |
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panos
Admiral 
Joined: 02 March 2008 Status: Offline Points: 1939 |
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Posted: 02 January 2009 at 16:27 |
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Hi,
I read the very interesting posts about the VMG and I would like to emphasise one more point : The VMG should be only used as a long term instrument. By this I mean that the one who should read it is the tactician and not the driver. The tactitian will use the information to select the compass cource that the driver must follow as well as the exact moment the boat must tack. If the driver is allowed to steer the boat (short term) by VMG he will constantly tend to drive the boat more upwind than he should or deeper than optimum when running. The reason is simple to understand: Since the boat has inertia and will keep its velocity for a while, even if the sail force is reduced, the VMG increases if the driver turns upwind, since the speed remains the same (for a while) while the angle between course and the direction to the wind is reduced. (VMG is the product of boat speed and the cosine of the above angle). The same happens driving down wind where the VMG will guide the driver deeper than optimum. Of course this is a mistake since soon the speed will drop and the instrument will register the correct VMG, the driver will overcorrect, since the speed doesn't increase as fast and so on. |
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Panos
Hanse 630e - selling her - |
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Johan Hackman
Admiral of the Fleet
Joined: 24 August 2005 Location: Sweden Status: Offline Points: 4361 |
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Posted: 02 January 2009 at 17:12 |
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That's a very good point, Panos!
Johan |
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Gregor
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Joined: 11 February 2006 Location: Netherlands Status: Offline Points: 1256 |
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Posted: 02 January 2009 at 17:30 |
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A lot has been written about vmg.
From what I understood it is not the compass course but the COG that is the course to steer. Gregor |
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Uisge Beatha
Currently sailing Dehler 36 JV (2002) Previous boat: Hanse 311 #80 http://www.uisge-beatha.eu |
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Johan Hackman
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Joined: 24 August 2005 Location: Sweden Status: Offline Points: 4361 |
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Posted: 02 January 2009 at 17:42 |
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Gregor, I think you are confusing VMG towards a waypoint and VMG towards the wind. VMG on your IS15 is the speed by which the boat is moving towards the wind (sideways) whereas VMG on your GPS is the speed towards a waypoint.
It's a shame that they use the same term for two different things. Johan |
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Gregor
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Joined: 11 February 2006 Location: Netherlands Status: Offline Points: 1256 |
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Posted: 02 January 2009 at 19:35 |
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I know that there is some nice (...) confusing in the name VMG. I understand what you are saying, Johan. But the steer correction is IMHO based on the COG and not on the compass course. Or do I overlook something?
Gregor |
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Uisge Beatha
Currently sailing Dehler 36 JV (2002) Previous boat: Hanse 311 #80 http://www.uisge-beatha.eu |
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panos
Admiral
Joined: 02 March 2008 Status: Offline Points: 1939 |
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Posted: 02 January 2009 at 21:04 |
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Hi,
VMG (Velosity Made Good) is the usefull part of the boatspeed. In around the cans races where the racecource is oriented along the wind, VMG maximum is the longterm cource to steer as long as the race cource is set correctly. In other races (and fast cruising without engine) the direction that maximizes the VMG to waypoint is the (longterm) cource to steer. I don't see any confusion besides the name. If while racing around the cans, the wind changes permanently (and the race course is not aligned to the wind anymore) ,the VMG to waypoint is more usefull than the upwind VMG. (In these cases the race commitee should think of canceling the race IMHO) I propose to the forum to adopt the name convention of our leader : a)VMG to wind or simply VMG and b)VMG to waypoint. The first is calculated by the wind instrument and the speedometer while the second by the GPS receiver and the waypoint selected. Edited by panos - 02 January 2009 at 23:13 |
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Panos
Hanse 630e - selling her - |
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Johan Hackman
Admiral of the Fleet
Joined: 24 August 2005 Location: Sweden Status: Offline Points: 4361 |
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Posted: 03 January 2009 at 15:15 |
I can't think of a better way to spend a Saturday afternoon than to make a cup of coffee and do the maths to see what the expected VMG would be for different wind angles. I used the designer's polar diagram for the 342 to extract the speeds in a 10 knot wind. 
So here's the result. I don't know if I have proved anything with it but I find it interesting to see for myself how the numbers work out. Johan 
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Johan Hackman
Admiral of the Fleet
Joined: 24 August 2005 Location: Sweden Status: Offline Points: 4361 |
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Posted: 03 January 2009 at 15:43 |
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By doing vector summation on paper I found out that the corresponding apparent wind angle for a true wind angle of 40 degrees is 25, and for 45 degrees it's 32. That confirms what I have already experienced when sailing - depending on the sea I try to sail the boat 25 to 30 degrees to the apparent wind when tacking in that kind of wind.
I haven't taken any leeway into consideration but as far as I know the Simrad instruments don't either. I hope I haven't ruined anyone else's Saturday afternoon by being this theoretical. Johan 
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panos wrote: