Facebook

Login

Support Sailonline

If you haven't already - join the SAILONLINE YACHT CLUB!

Please also consider making a donation - all amounts are greatly appreciated!

Board » General Discussion » Negative VMC!?

Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
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.
Attachments
Posted by
Lolla


Users avatar

Posts
32
Registered at
2011-11-03 01:28:52
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


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
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


Users avatar

Posts
33
Registered at
2012-06-01 13:37:55
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


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
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


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
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.
Attachments
Posted by
kroppyer


Users avatar

Posts
347
Registered at
2010-02-05 15:25:55
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 ---
Attachments
Posted by
JB


Users avatar

Posts
305
Registered at
2013-03-07 08:54:34
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.
Attachments

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Great Nicobar to Marion Island 2025

The Adventure Continues – Leg #6 of the RWW Series.
We’re setting sail on the next stage of the RWW Series: a 4,400-nautical-mile passage from Great Nicobar to Marion Island. This demanding leg will test endurance, strategy, and seamanship at the highest level. As part of the 2025 Ocean Championship, this marks the ninth legendary challenge in the series — and one of the most exhilarating yet. The race will be contested aboard the Archer 78’ ketch, a vessel built to push sailors to their limits across the vast Indian Ocean.
PRIZE: SMPF
Race #1900
INFO by brainaid.de
Archer 78 PARTICULARS    
WX Updates:
0430 / 1030 / 1630 / 2230    
Ranking: OCQ3 - RWW - OCCH - SUPSOL - SYC
Race starts: Sep 01st 11:00 Registration Open!
▶ Flash
GO TO RACE

SSANZ Triple Series 2025 - Race 3

Welcome to the grand finale of this year's SSANZ Triple Series! The stage is set for the Lewmar 50, a thrilling 39nm challenge tracing the southern shores of the stunning Waiheke Island, pushing out to Tarahiki before the sprint back to Islington Bay. Expect fast legs, tactical battles, and no shortage of drama as our SOLers take on the waters in their Young 88s, lining up once again against the talented IRL members of the Young 88 Association. With the series on the line and everything still to fight for, it all comes down to this final race!
Race #1934
INFO by brainaid.de
Young 88 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
SSANZ - SYC
Race starts: Aug 29th 21:15 Registration Open!
▶ Flash
GO TO RACE

Red Eye - Myth of Malham 2025

In 1947, Hugh McLean & Sons of Gourock, Scotland built a radical racing yacht for John Illingworth, soon-to-be Commodore of the RORC, to a design by Laurent Giles. With a masthead rig, giving her ‘free’ sail area per the rules of the time, and short overhangs reducing pitching, Myth of Malham went on to win The Fastnet that year and again in 1949, and to compete in numerous Admiral's Cups. In 1958, in honour of the yacht, the Myth of Malham race was conceived - a mini-Fastnet to the first lighthouse on a rock west of Cowes, Eddystone, rather than the second, to be held in non-Fastnet years. IRL this year it has been raced already, but this stops us not from “Red Eye” racing its online 230nm in Fareast 31Rs, leaving how to deal with the Isle of Wight open to your own discretion.
Race #1847
INFOby brainaid.de
Fareast 31R PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC - RED
RACE CLOSE: Monday,
September 1 at 2300 UTC.
Race starts: Aug 25th 12:00 Registration Open!
▶ Flash
GO TO RACE

Africa by Sea 2025 - Maputo to Mogadishu


The fifth leg of our exploration of Africa By Sea takes us north along the length of the Continent’s succulent sub-tropical and tropical Indian Ocean coast. Much fought over by traders and colonists both well-before and after a Portuguese expedition under the command of Vasco da Gama first sailed there from Europe back in the 16th century, it is yet to be properly explored as a yachting destination. With plenty of islands off- and inshore, small and very large, there is much to interrupt your 1900nm voyage and visit in your Swan 65, from Maputo to Mogadishu! Race #1858
INFOby brainaid.de
Swan 65 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC - ABS
RACE CLOSE: Saturday,
September 6 at 2300 UTC.
Race starts: Aug 20th 17:00 Registration Closed
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member WRmirekd
  2. Sailonline Yacht Club Member vida
  3. Sailonline Yacht Club Member CriticalHippo
  4. Sailonline Yacht Club Member KaSToR
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member Kipper1258
  7. Sailonline Yacht Club Member Sax747
  8. Sailonline Yacht Club Member BRENTGRAY
  9. Sailonline Yacht Club Member CollegeFund
  10. Sailonline Yacht Club Member FreyjaUSA

View full list

Series

Mobile Client

SYC members have the benefit of access to our mobile/lightweight web client!

The mobile client