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Board » General Discussion » Negative VMC!?

Posted by
JB


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2014-11-20 14:54:31 - Negative VMC!?
On the SWR Leg 2 (20.Nov.2014, 14:49 UTC).
With the current HDG and going SE (115,50º) how is it possible to have a negative VMC (see also included picture)!?
Sail Fair.
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Posted by
Lolla


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2014-11-20 16:21:51 - Re: Negative VMC!?
Hi, I can't open your picture, but you're probably going downwind.

Sorry. Correction, for VMC... away from your target.

Cheers.

--- Last Edited by Lolla at 2014-11-20 16:24:53 ---
Posted by
kroppyer


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2014-11-20 19:02:27 - Re: Negative VMC!?
I believe this VMC is calculated using the bearing to next mark, so Cape St Francis. You've probably rounded that mark by now, so if you look now the VMC will be positive again because you're heading roughly in the direction of the next mark: Mauritius.

But.. if that's the case.. then VMC and DTF do not "match" (int[VMC * dt] + DTF =/= race length), which makes me wonder, what's the use of this VMC number then? VMC is very useful, but only if it uses a sensible BRG...

--- Last Edited by kroppyer at 2014-11-20 19:04:29 ---
Posted by
outlaw


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2014-11-20 21:22:12 - Re: Negative VMC!?
Setting aside that DTF would be calculated to Mauritius, and his VMC should indeed be positive...

Setting aside that he probably won't touch Mauritius, and his true VMC towards Musandam is close to zero... (but I'm merely a casual observer and may be horribly wrong on the matter)

Setting aside that DTF and by extension VMC is a big joke... (but of course no better deterministic ranking system exists)

I find it amusing how some SOLers, celebrated winners even, manage to gain DTF at greater than boatspeed.

http://sol.hmm.iki.fi/sollog/boat/765/karriv/

food for thought.
Posted by
kroppyer


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2014-11-20 22:10:11 - Re: Negative VMC!?
DTF is not calculated using the shortest line to the finnish (EDIT: or finish) respecting the marks in between, as far as I've heard/read it's the distance to the rounding line (as depicted here) and then either (a) from that mark to the finish, or (b) from the point on the rounding line that is closest to your boat to the finish.

I guess it's (a), because this would indeed be a gain on computing time, and because we have witnessed jumps in DTF that cannot be explained by (b). I believe hmm's blog has some info on this.

DTF (and VMC to some predefined location/BRG) are just for amusement, as outlaw says ("I find it amusing ...") :)
[Though, it would be nice if the two had a direct relation: "VMC is the speed at which your DTF decreases"]

--- Last Edited by kroppyer at 2014-11-20 22:11:56 ---
Posted by
JB


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2014-11-21 11:54:51 - Re: Negative VMC!?
Thank you all for your comments and apport to this subject.

Lolla
1 - “VMC” can only be negative when:

Pi / 2 rad < cos (alfa) < 3 Pi / 2 rad,

or in decimal degrees:

90º < cos (alfa) < 270º,

where “alfa” is the angle between the BS (Boat Speed) vector direction and the course direction where you want to project your BS (sorry for the redundancy).

2 - In the SWR Leg 2 case psail was sailing between Cape Agulhas and Cape St. Francis making an heading of:

HDG = 115,50º.

3 - To simplify, the SOL bearing for those two marks is 81,13º (leaving Cape Agulhas in a straight line to reach and Cape St. Francis and neglecting the Great Circle calculations).

4 - If we take in account the “logical way” to choose a path for the BS projection, my VMC should have been:

VMC = BS x cos (alfa) = 21,53 Kts x cos (115,50º - 81,13º) = 21,53 Kts x 0,59987 ≈ 17,771 Kts.

5 - Making the backwards calculation the path directions that makes psail VMC < 0,00 Kts are comprehended in the angle interval:

115,50º - 90º > Path direction > 115,50º + 90º or 25,90º > PD > 205,90º.

6 - For having the shown VMC = -6,78 kts the angle alfa = 7,145º (!!??) - see also the included excel file.

Huib and Outlaw
Where the eck came this angle?
A total mystery to me, anyhow, the explanation Huib gave - “the bearing for the next mark (Cape Francis)” - can be the “right” one.

In a pure straight line regatta, from A to B, the calculation of VMC it’s pacific.
The discussion begins when you have to pass on more than two waypoints and you need to maximize your boat VMC.
My method is to simplify things because life itself is already very complicated.

As an example, we have #5 waypoints to make in our path, A, B, C, D and E.
Draw a straight line from A to E, that’s your path direction where you’ll project the BS vector.
The best routing is the one that will make the average VMC bigger.
If, for instance, the finish line is near the starting one, easy, divide your course in two and apply the simplify method two times, one for each course.
Sail Fair.
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Posted by
kroppyer


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2014-11-21 13:39:02 - Re: Negative VMC!?
Here is how your VMC was calculated.

The numbers outlaw linked can maybe be explained if "speed" is at the moment, and decrease of DTF is over an hour. So when you're slowing down you'll see that you've covered more distance the last hour than would be possible with your current boatspeed.

After checking hmm's blog and checking some DTF numbers, I'm back to (b) ("from the point on the rounding line that is closest to your boat to the finish.") in my previous post. Are depicted here

Sol's DTF calculation does not always give the "correct" DTF. But might not be easily improved. VMC can be improved, by calculating the VMC to the rounding line (so that it is in sync with DTF) or every better.

The end of the story is however, as outlaw says, DTF (and VMCs that don't take into account the your routing) are a joke, until you are at the finish.

--- Last Edited by kroppyer at 2014-11-21 13:39:31 ---
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Posted by
JB


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2014-11-21 14:06:16 - Re: Negative VMC!?
Viva Huib.

Passing the blue line, the one used for calculating the "good" VMC, works.
I'm having again a positive VMC - pls see pic.
Thank you Huib.

--- Last Edited by JB at 2014-11-21 14:09:31 ---

--- Last Edited by JB at 2014-11-21 14:10:11 ---

--- Last Edited by JB at 2014-11-21 14:10:58 ---
Sail Fair.
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