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Board » Technical Support » wrong VMG

Posted by dunbur 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 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!

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