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!?

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.
Hi, I can't open your picture, but you're probably going downwind.

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


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


food for thought.
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 ---
Thank you all for your comments and apport to this subject.

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.
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 ---
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.

Please login to post a reply.


Next Race: 00d 00h 00m

Current Races:

Victoria to Maui 2022
The Victoria to Maui International Yacht Race, first contested in 1968, is the pinnacle of Pacific Northwest ocean racing, running every two years. This year, Sailonline invites the SOL fleet to race the 2300nm course in Delphia 47s, from Victoria, British Columbia to Hawaii, finishing off Lahaina Maui.
Race #1596
INFOby brainaid.de
WX Updates:
0430 / 1030 / 1630 / 2230
Race starts: Aug 09th 19:00 Registration Open!

▶ Flash

Brisbane to Hamilton Island Yacht Race 2022

Time once again to head for the warm waters inside The Great Barrier Reef, but this year not only online but together with our buddies at the Royal Queensland Yacht Squadron and their friends from across the world, in the iconic Brisbane to Hamo! Online we’re racing maxi’s to give you a chance to cover the 550 glorious blue water nautical miles in a time to best some of the fastest monohulls on the planet including Andoo Comanche, Black Jack and Hamilton Island Wild Oats . Fair winds, all!
Race #1606
INFOfrom brainaid.de
Super Maxi 100 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SYC
August 14 at 2300 UTC.
Race starts: Aug 05th 01:00 Registration Closed
▶ Flash

Sailonline’s 15th Birthday Balloon Race
Sailonline offers a huge variety of boats to sail in. One of the more unusual ones has got to be SOL's Balloon, which was polarized by past SOLmaster 76Trombones. Notably, the SOL Balloon will actually ‘sail’ in other directions other than straight downwind, although it will not 'sail' over land or beyond a 'beam reach'. Like in 2013 and 2018, we will be taking the Balloon across the Atlantic from St. John's and 'land' it in the sea off the Cliffs of Moher.
Race #1605
INFO by brainaid.de
WX Updates:
0430 / 1030 / 1630 / 2230
Race starts: Aug 01st 19:00 Registration Open!

▶ Flash

Simonstown to St Helena 2022
This is a copy of a traditional South African December yacht race, but this time we will be racing in the middle of European summer. SOLers are invited to race 1712nm in our GB90s from Simonstown, RSA, to the exotic and remote island of St Helena - an island paradise with warm, friendly, hospitable islanders!
Race #1593
INFO by brainaid.de
WX Updates:
0430 / 1030 / 1630 / 2230
August 12 at 2300 UTC.
Race starts: Aug 01st 18:00 Registration Closed

▶ Flash

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member WRmirekd
  2. Sailonline Yacht Club Member Pit8008
  3. Sailonline Yacht Club Member rafa
  4. Sailonline Yacht Club Member FreyjaUSA
  5. Sailonline Yacht Club Member Sax747
  6. Sailonline Yacht Club Member GREATSKUA
  7. Sailonline Yacht Club Member Vida_Maldita
  8. Sailonline Yacht Club Member Kipper1258
  9. Sailonline Yacht Club Member HappyHour
  10. Sailonline Yacht Club Member sassy63

View full list


Mobile Client

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

The mobile client