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Board » Sailonline Yacht Club » VMC or "by the numbers" in oscillating winds

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Posted by
WhyNot


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2011-04-09 15:12:58 - VMC or "by the numbers" in oscillating winds
I'm finally beginning to understand the concept of VMC, but I gave a question.

I'm familiar with "sailing by the numbers" when tacking/gybing in oscillating winds i.e. not tack-/gybing on the header, but on the "mean". When I sail by the numbers, my TWA is always max VMG.

Will this work if my TWA is VMC?
Posted by
NOR_Jawik


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2011-04-09 16:13:50 - Re: VMC or "by the numbers" in oscillating winds
76 Trombone's site

have a nice explanation and by the way VMG = Sped x cos(TWA)

--- Last Edited by RainbowChaser at 2012-02-18 09:50:10 ---
Posted by
Rod


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2011-04-09 17:34:12 - Re: VMC or "by the numbers" in oscillating winds
I must admit to being puzzled by the "choice of target destination", when the obvious target will not be reached before the new wx. That is, in a short race the obvious destination is the next turn "Mark". But in a multi-day ocean race, how to I pick an "intermediate destination" unless I use a router?
And if I were to use a router, would not all these decisions be made for me?

--- Last Edited by Rod at 2011-04-09 20:23:08 ---
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Posted by
WhyNot


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2011-04-09 17:48:15 - Re: VMC or "by the numbers" in oscillating winds
Thanks Jawik, but 76's explanation does not help me with oscillating wind. Or maybe it does, but I don't understand it:)
Posted by
WhyNot


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2011-04-09 18:00:07 - Re: VMC or "by the numbers" in oscillating winds
Rod: I don't say this is the answer, but I have the same problem and the following is what I experiment with:

Disregarding the usual persistent shift! I aim at the point on the predictor that is half way through the shift. Again - not saying this is the right answer! If anybody knows that I'm way out in left field - pls let me know.
Posted by
Rod


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2011-04-09 21:32:36 - Re: VMC or "by the numbers" in oscillating winds
to Tom Nordal:- Could you diagram what you mean and attach it to your next input? I am much more comfortable with diagrammatic explanations of these situations.
I have been using 76T 's ideas with regards to offsetting my course from the polar to account for the direction to the next clear 'turn point' such as a Mark or a Headland, but I don't understand these interim destination points at all.

--- Last Edited by Rod at 2011-04-09 21:38:53 ---
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Posted by
76Trombones


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2011-04-10 00:20:26 - Re: VMC or "by the numbers" in oscillating winds
Nope, I haven't covered choosing the target at all yet, and it is almost *never* the mark/headland... The whole idea of using a fixed point for VMC calcs is a result of GPS/computerised nav, ETA math and engines. For sail routing purposes we'll use an angle that we'll adjust over a leg to take advantage of TWS changes as well as TWD.

For sail routing, a good starting point for target angle is based on the average TWD. There are rules of thumb such as "half the magnitude of the shift" that give you a handle on how far to stray from VMG sailing SOTP. Exmeromotu is an expert on such - maybe he'll chime in? ;-)

Important points to remember are that a shift is required, and you usually need a tack/gybe as well to take full advantage. Sailing low and fast only works if you can sail low and fast the opposite direction as well. Try it in steady winds and the VMG crowd will be laughing.

If you work out a VMG track to the next mark, chances are that a VMC breakdown tacks at similar times. You *always* tack when your VMC=VMG so this is a decent way to start your figuring...

Also note (from a pure learning pov mind, we'd never blindly follow) that you can very easily pick the VMC target that the superpowers are sailing by scoping their angles relative to the polar... The steering line and a piece of right-angle card are a pretty sneaky tool, and no math required.
Posted by
Rod


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2011-04-10 01:03:11 - Re: VMC or "by the numbers" in oscillating winds
I sincerely hope, 76T, that you have a book on this subject in the 'works'!
I have a Norie,1840, Jeans 1853, W.A.Smith 1973, Walker 1976, Melges and Mason 1983, Turner and Jobson 1979,and Schlereth 1975, in my library, as well as several more books on dinghy racing, and not one of them mentions the ideas you are presenting here. Have you thought of presenting this in a mini-course on SOL as a preliminary to writing your book?
Speed the day!!!
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Posted by
ShamrockPirate


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2011-04-10 01:15:30 - Re: VMC or "by the numbers" in oscillating winds
How about doing a SOL Webcast?

Get the BlogMeisters (76, AGage, SWE54 and others)to do a Web-based "primer" and advanced seminar? This is a terrific tool to reach out over multiple timezones.
Posted by
WhyNot


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2011-04-10 11:22:18 - Re: VMC or "by the numbers" in oscillating winds
Shamrock: Wouldn't that be great if they would?

There would certainly be a brewsky of their favorite choice awaiting (or two or ten) if they stumbel across me somewhere:)

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