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 » Technical Support » wrong VMG

Page: Previous 1 2 3

Posted by
dunbur


Users avatar

Posts
9
Registered at
2009-02-18 06:03:58
2009-04-22 16:53:17 - Re: wrong VMG
NZL_PaulR said @ 2009-04-22 10:24:24:
If the courses are all "COMPASS" courses, then the predictor is very accurate and the margin of error is indeed small.

I doubt that. Of course, you know your boat will be somewhere on that line, but the speed of the boat within the next 6 hours is uncertain and therefore the location on that line. Using TWA the speed might be more predictable since then it only depends on the wind speed and not the direction, but of course you are not necessarily on that line but the difference on average is similar to when sailing via CC.
Posted by
NZL_PaulR


Users avatar

Posts
311
Registered at
2008-07-22 12:21:30
2009-04-22 20:39:22 - Re: wrong VMG
dunbur said @ 2009-04-22 16:53:17:
I doubt that. Of course, you know your boat will be somewhere on that line, but the speed of the boat within the next 6 hours is uncertain and therefore the location on that line. Using TWA the speed might be more predictable since then it only depends on the wind speed and not the direction, but of course you are not necessarily on that line but the difference on average is similar to when sailing via CC.
REPLY:
I disagree.

When on a Compass Course, direction of travel is FIXED. Speed certainly will change with the change in wind speed, However changes in wind "direction" have minimal effect on boats course, subject to any SOL leeway factors that seem to be minor or NIL.

So you will be somewhere along the predicted course line, dependant mainly upon only ONE factor being the wind strength and resultant boat speed.

BY CONTRAST, if a TWA course is set, the the predictor immediately "BENDS" dependant upon wind DIRECTION changes as ONE factor AND in addition, progress along that "Bent" course is FURTHER determined by changes in wind strength. This make TWO factors variable and so the predicted positions has less accuracy.

In dead reckoning navigation, your area of probability of your position increases. The likely position you will end up in will be somewhere in a larger circle. The circle may be squeezed into an ellipse if one of the factors (wind speed or direction) has greater variance.

So yes your comment about boat speed on TWA being more predictable is correct, as long as the wind direction is fairly constant. However if sailing through a low pressure centre, that is moving quickly as well, your wind speeds and wind direction will be changing a lot. By contrast, many SOL races have been stuck in high pressure zones, with minimal progress and no matter what changes steering commands in direction are made, the resulting position is seldom in the predicted position.

In SOL, as I understand, leeway, sea state, tides and currents are not used in the formulae.

So TWO factors changing must result in less accurate position estimates than when only one factor is changed.
If it breaks, it's not strong enough!

Page: Previous 1 2 3

Please login to post a reply.

Races

Next Race: 00d 00h 00m

Current Races:

Tall Ships 2026 - Aarhus to Harlingen


Welcome to the first of four virtual Tall Ships Races in European waters which are concurrently being organized in-real-life by Sail Training International . The course for this first race is from Aarhus in Denmark, through the Skaggerak and down the North Sea to Friesland’s Harlingen – a proper tall ship training adventure of circa 450nm, which online this year you will race in Sailonline’s classic Clipper 240.
NOTE: Starts and Finishes in tall ships racing are always offshore to avoid conflict with shipping and shipping lanes; online and real-life may not match exactly.
Race #1992
INFO by brainaid.de
Clipper 240 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: TSI - SUPBUD - SYC
Race starts: Jun 27th 16:00 Registration Open!
▶ Flash
GO TO RACE

Viareggio Bastia Viareggio 2026

Our sixth race of nine to celebrate all things Mediterranean - fickle winds, history and olive oil - was planned to be our third buddy-up with another incredible Italian event, the Viareggio Bastia Viareggio (or BVR for short). Unfortunately, in-real-life the BVR has been cancelled this year. But online we shall race it never the less. It’s a quickie – a 160nm roundtrip there-and-back in classic Class 40. Have fun, and if you’re planning a BBQ, piccola isola Capraia halfway across is simply lovely!
Race #2041
INFO by brainaid.de
Class 40 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: MED - SUPBUD - SYC
Race starts: Jun 24th 12:00 Registration Open!

▶ Flash
GO TO RACE

Shetland Race 2026 - Leg 2


After the thrill of the first leg, we don't linger long in Lerwick, Shetland. Following brief but joyous celebrations, it's time to set sail back across the North Sea to Bergen, Norway, for the gripping second leg of the Shetland Race 2026!

Covering another unpredictable 190 nautical miles, tactical depth and fierce competition await. This year, we take the helm of the Linjett 33. Designed by Mats Gustavsson in 1994 for Sweden's historic Rosättra Båtvarv, this sturdy, quality cruiser-racer has been a Sailonline favourite in northern waters since 2010. Whether you are chasing victory or savoring the voyage, hoist your sails and let the journey home begin!
Race #2040
INFO by brainaid.de
Linjett 33 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
SHE - RED - SUPBUD - SYC
Race starts: Jun 20th 16:00 Registration Closed
▶ Flash
GO TO RACE

Brisbane to Keppel Tropical Yacht Race 2026


When the winter weather turns cool in southern Australia, sailors head up to the warmer tropics. Sailors from all over the world make their way to The Great Barrier Reef for the big winter regattas at Hamilton Island, Airlie Beach and Magnetic Island.
This year's B2K will start on 20 June to offer an event suited to a broader range of boats. Sailonline is, once again, joining with the racers from RQYS to race the 343 nautical miles from Brisbane to Keppel Island, this time in our First 44.7. It might be winter down under, but all you will need is shorts and t-shirts.
Race #2026
INFO by brainaid.de
First 44.7 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SUPBUD - SYC
RACE CLOSE: Sunday,
June 28 at 2300 UTC.
Race starts: Jun 20th 01:00 Registration Open!
▶ Flash
GO TO RACE

Newport Bermuda Race 2026


The 636-mile Newport Bermuda Race is one of the oldest regularly scheduled ocean races, and with the Fastnet and the Sydney Hobart Race, it is one of the three great classic races of the yachting world. First raced in 1906, now in its 54th biennial, the Race can again also be participated in online. In real life, the race is open to entry in four different division – Double-Handed, St. David's Lighthouse, Gibbs Hill Lighthouse, and Finisterre Division – which in the past we have also offered online. This year, however, our online race will be all-in one-design in silky swift Club Swan 50 yachts, so that your results can count towards SOL’s SUPerBUDdy title, whilst hopefully still giving you a chance of overall line honours against the fastest yachts of the real-life Gibbs Hill Division, especially if you can find some forecast winds that don’t quite work out that way on the real water!
Race #2038
INFOby brainaid.de
Club Swan 50 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: SUPBUD - SYC
RACE CLOSE: Wednesday,
July 1 at 2300 UTC.
Race starts: Jun 19th 17:05 Registration Open!
▶ Flash
GO TO RACE

Bluefin Tuna - Mexico Gulf to North Sea 2026

Join us and follow the Bluefin Tuna, as they migrate from their spawning grounds in the Gulf of Mexico to their feeding grounds in the North Sea off the coast of Norway. This 4300 nm race is part of our Ocean Championship and the final race of OCQ2 2026, and also the second leg of our Migration series. We will be sailing in our Gunboat 90. Race #2035
INFO by brainaid.de
Gunboat 90 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - MIG - OCCH - SYC
RACE CLOSE: Saturday,
July 4 at 2300 UTC.
Race starts: Jun 08th 11:00 Registration Closed
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member WRmirekd
  2. Sailonline Yacht Club Member KaSToR
  3. Sailonline Yacht Club Member CriticalHippo
  4. Sailonline Yacht Club Member bonknhoot
  5. Sailonline Yacht Club Member Panpyc
  6. Sailonline Yacht Club Member rafa
  7. Sailonline Yacht Club Member cdhinman
  8. Sailonline Yacht Club Member HoratioPugwash
  9. Sailonline Yacht Club Member YANN
  10. Sailonline Yacht Club Member ms0689

View full list

Series

Mobile Client

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

The mobile client