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Board » Sailonline Yacht Club » VMC or "by the numbers" in oscillating winds

Posted by AGage Posts 411 Registered at 2008-12-27 10:31:57	2011-04-11 07:43:01 - Re: VMC or "by the numbers" in oscillating winds
	Try these webpages Tom, it is about a technique you describe known as 'wallying the windshifts': http://www.ockam.com/vmc.html and http://wcsailing.blogspot.com/ - near the bottom for this specific issue, but there is also more good stuff.... If you are required to sail the non-favoured tack (ie land forces you to), then you often want to sail higher than VMG - by how much is the magic question, and I cant really give you any general answers. One good way to get a feel though is experimenting with the predictor line. Set your boat at the max VMG angle, and get to 100% performance. Then point up by 1 degree. After you click 'Send' place the crosshair over the point for 1 hour, and hold it there. When the new course is established and the predictor redraws itself, compare the new 1hr point to the old. For simplicity say you are trying to maximise your course N, and are on Port so want to minimise your Eastward travel (which is further from rhumbline). Well what you want to compare here is the 2 points' latitudes. If the new point is no further S than the original vmg point, then 1 degree higher is better. Now do it again... Eventually you will find pointing higher slows you too much. Also using the ruler to record the various points is a good idea, and zoom in so you are measuring as accurately as possible. Also be aware that as you are actually moving N, the points will tend to be more N anyway each time, so factor this in if possible. And of course you can also do the exact same thing pointing progressively lower than vmg angle, to see visually how sailing a bit faster (but also remember as 76T points out - also further)... Also download my polar butterfly overlay and experiment using some card as 76T suggest, or a tool I also mention on the blog: MB-Ruler for drawing and moving lines on the screen. Regarding books, the best I have ever read is "High Performance Sailing" by Frank Bethewaite - he is a meteorologist and aeronautical engineer, has (IIRC) 5 world championships between his 2 kids and he and his son were largely responsible for the development of the 18" Skiffs here in Aus as well as many other classes. He was also official met for the Aus Olympic sailing team for decades - so while the book is largely detailed toward dinghies, there are also excellent sections on weather reading and waves etc... When he retired from his job, he and a likeminded group set about trying o understand boats better and to make them sail faster (this was decades ago, before much of what we now know was known), and (one of) the thing i really like about his book is he talks alot about the actual experiments they did to sail faster - and not just 2 boat testing or similar, but actual emperical scientific experiments, like drag tests on foil designs and he even built his own wind tunnels - probably the advance he is most known for inventing/discovering is the wing-mast. I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by AGage Posts 411 Registered at 2008-12-27 10:31:57	2011-04-11 07:55:49 - Re: VMC or "by the numbers" in oscillating winds
	BTW, The Ockam site is chock full of great info - when it mentions using 'strip charts' these are available for SOL by using brainaid's NMEA proxy and NavMonPC, which gives you stripcharts for both wind data and boat performance I help develop the client interface for the best online ocean racing sim there is... __/)/)_/)__
Posted by sol Posts 296 Registered at 2008-08-14 11:08:14	2011-04-11 09:54:17 - Re: VMC or "by the numbers" in oscillating winds
	Thank a lot Aaron. I have seen "Wallying" mentioned, but have put it on the backburner for the time being. Now I find that I already know what it is Thanks for the links, and the trick with the predictor and markers. I have your Butterfly, but having been a SOTP sailor for almost 50 years - I havn't really figgered out what to use it for. I just print a paper copy of the polar. I will look more into it though. At my age - I'm a slow learner.
Posted by WhyNot Posts 51 Registered at 2011-02-03 18:39:54	2011-04-11 09:57:16 - Re: VMC or "by the numbers" in oscillating winds
	Ooopss! That was me answering. To many instances of SOL running at once. Sorry
Posted by 76Trombones Posts 362 Registered at 2009-08-03 18:04:56	2012-02-18 20:15:03 - Re: VMC or "by the numbers" in oscillating winds
	I'm not actually sure that this is working properly, but here is a quick spreadsheet that I worked up a while ago, allowing you to compare VMG and VMC courses for different shifts and target angles/mark locations. You want the target that'll get you to the mark the quickest, which depends more-or-less on average wind direction. The amount you can foot varies with the amount of shift. Attachments VMCdemo.xls (35.5 KB)
Posted by 76Trombones Posts 362 Registered at 2009-08-03 18:04:56	2012-02-18 20:23:45 - Re: VMC or "by the numbers" in oscillating winds
	Updated to show the effect of sailing VMG on the wrong tack. Footing on the wrong side is even worse, obviously. Attachments VMCdemo2.xls (44.0 KB)

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