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Board » General Discussion » VMC

Posted by
JimGodders


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2011-11-06 17:56:14
2018-09-11 20:30:24 - VMC
Hi all,

I've been reading about a little as I'm looking to improve, and have zeroed in on VMC as an interesting concept. But, for the life of me, I can't seem to get my head round it properly and I have a few questions that I'm sure you fine folks will be able to answer for me.

As I understand it, sailing to VMC maximises the boat's speed over the ground in the defined direction. Considering 2 boats, starting at the same point at the same time with both wanting to get as far north as they can, if 1 is sails to VMC, they will be further north than the other sailing due north, though the boat sailing to VMC will also be off to the left or right.

What I'm struggling to understand is how this is useful in racing. I know there's little point in sailing VMC direct to a mark if the wind ins't expected to shift as you'll lose whatever time you've gained on the leg trying to get back to the mark.

I've seen some suggest that in shifting wind, a reasonable VMC target is to sail to half the shift. I don't understand what this means though. I've tried google but to no avail - this doesn't seem to be a topic that is easily accessable. Could someone please explain what the VMC target would be if trying to get a mark due north, with a wind veering from NW to W over the course of the leg please?
Posted by
Bimmer


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2015-02-23 15:38:27
2018-09-12 07:05:40 - Re: VMC
Posted by
JimGodders


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3
Registered at
2011-11-06 17:56:14
2018-09-12 18:50:34 - Re: VMC
Thanks very much Bimmer. I had seen that article already, and others on the same blog and this is what is leading to my confusion!

Let's take this article. Outlaw has chosen to VMC to a bearing of 280*. For the given wind conditions, sailing a course over the ground of 307* gets Outlaw furthest down a line drawn from his/her current position on a bearing of 280*, though they will be to starboard of a point on a direct 280* bearing from their current position. Nevertheless, sailing a course of 307* gets them farthest up the 280* bearing.

Now, I understand that 280* was chosen as this is the TWD that Outlaw will tack at. But this is what I can't quite get my head around. A TWD of 280* seems incorrect - that could happen anywhere along the leg, and not necessarily in the best tacking position. Does he mean he'll tack when the bearing to the mark is 280*? I often see mentioned that a tack/gybe should be done when VMC = VMG. Why is this not a consideration here?

I feel like this is a missing piece of the puzzle for me, so I'd love to try and understand it better!
Posted by
outlaw


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2012-06-01 13:37:55
2018-09-13 21:48:43 - Re: VMC
1. I prefer 'his'. Not that I've ever claimed to be human.
2. The only claim is that if I sail a couple hours at a set VMC, and arrive at point A, it is fundamentally impossible to beat me to point A. Bimmer could sail to point B, then have the faster path to the finish. Happens.
3. The causality for picking the target IS confusing. In an ocean race I could decide to go 'west', wade thru bird entrails until I come up with VMC280, and tack at TWD280 because that's how the VMC function works.

OTOH in a round the buoys type race I could just do an SOTP estimate where/when I'm going to tack, determine the TWD @tack, and use that as the target. Post tack the boat would just point in the general direction of the mark, bearing ~240, probably.

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