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:

Chesapeake and Potomac TIMED Race 2026

Welcome to the Chesapeake Bay, the sailing capital of the United States. Feast on Maryland crab cakes or crabs seasoned with Old Bay spice before sailing down the Bay aboard a Formula 18 catamaran on a 128 nm course that will take you up the Potomac River finishing at Quantico, Virginia. This is a TIMEDrace so you may RE-REGISTER HEREto try again after finishing a run. You will have two weeks to test your skill and decision making after the race opens.
Race #2032
INFOby brainaid.de
Formula 18 catamaran Particulars
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking:
TRQ2 - TRCH - SYC
RACE CLOSE: Saturday,
23 May at 23:00 UTC
Race starts: May 10th 12:00 Registration Open!
▶ Flash
GO TO RACE

Tobruk to Iskenderun 2026

Welcome to the fourth of our nine races across the length and breadth of the Magical Mediterranean Sea, and the second that is not in partnership with a real-life event, this time for good reason, which we won’t get into here. From Tobruk to Iskenderun it’s 670nm in length, with the added complication that the island of Cyprus will be in your way. Hoping, possibly forlornly, for plenty of downwind sailing, we’ll take the Lagoon 55 for the voyage!
Race #1976
INFO by brainaid.de
Lagoon 55 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: MED - SYC
Race starts: May 05th 15:00 Registration Open!

▶ Flash
GO TO RACE

Tasmania to Kamchatka 2026

Welcome to yet another all-new route. Tasmania to Kamchatka, part of our Ocean Championship and also the third leg of the ROW (Round the Old World) series. Be aware of the obstacles on the way, likely to split the fleet. It's like X:mas to X:mas, crossing the same waters, navigating the same archipelagos. This second race of OCQ2 2026 will be sailed in our revised 'version 2' Imoca 60 Foil. Race #2031
INFO by brainaid.de
Imoca 60 Foil v2 PARTICULARS
WX Updates:
0430 / 1030 / 1630 / 2230
Ranking: OCQ2 - ROW - OCCH - SYC
Race starts: May 04th 11:00 Registration Open!
▶ Flash
GO TO RACE

Go to race archive

SYC Ranking

  1. Sailonline Yacht Club Member WRmirekd
  2. Sailonline Yacht Club Member CriticalHippo
  3. Sailonline Yacht Club Member rumskib
  4. Sailonline Yacht Club Member Panpyc
  5. Sailonline Yacht Club Member rafa
  6. Sailonline Yacht Club Member KaSToR
  7. Sailonline Yacht Club Member CollegeFund
  8. Sailonline Yacht Club Member bonknhoot
  9. Sailonline Yacht Club Member vida
  10. Sailonline Yacht Club Member sassy63

View full list

Series

Mobile Client

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

The mobile client