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 Post subject: To Point or Not To Point
PostPosted: Mon Apr 15, 2019 4:25 pm 
faltbootemeister

Joined: Sun Jan 11, 2015 4:10 pm
Posts: 172
Watching the last America's Cup I was exposed to Velocity Made Good [VMG]. My better half Swampgirl, who introduced me to sailing and sailing kayaks, had already explained the concept but I had been unaware of the term.

So, let's assume that my destination is directly into the wind. Am I better off pointing as close to the wind as I can and still have the boat moving at say, 3.3 knots, or should I fall off a bit where I can achieve, say, 3.8 knots? At what point do I maximize VMG?

I can say that sailing at 3.8 knots is more fun than sailing at 3.3 knots and often I have no particular place to go so I choose the greater absolute velocity over seeking to maximize VMG.

A pretty big deal in the small-boat-racing world, Stuart Walker, was an acquaintnace of Swampgirl when she was much younger. He gave her a copy of a 1960 book he edited called THE TECHNIQUES OF SMALL BOAT RACING. Swampgirl retrieved this book after a discussion of when using two leeboards would be better than using just one.

In the chapter on sailing in strong wind, the author points out that at faster speed the boat side slips less. This would be a vote for falling off a bit.

Elsewhere in the chapter the author talks specifically about small catamarans. For these boats in strong wind, the author claims describing a scalloped pattern maximizes VMG. Fall off the wind a bit to gain momentum, then point up until the boats slows significantly, perhaps even pinching a bit. Then fall off once again to regain momentum.

For those of us who use akas and amas, aren't we trimarans? Perhaps this method might work for us. Then again, it would seem this method assumes the boat can glide. Most folders do not seem so good at this.

Anyway, just food for thought on a day when the water is still too cold for me to want to go out.

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PostPosted: Wed Apr 17, 2019 8:20 am 
faltbootemeister

Joined: Mon Jul 10, 2017 3:08 pm
Posts: 148
An interesting question for any sailboat, I've often mentally debated it underway. The answer depends on the math: how far do you have to bear off and how much speed do you gain. My simple answer is to buy a handheld gps, or install an app on your phone which wil give you speed over ground and velocity made good. Warning: trying to sail a very small light boat and consult electronics can result in swimming lessons.


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PostPosted: Thu Apr 18, 2019 5:29 pm 
lord high faltbotmeister

Joined: Thu Sep 06, 2007 8:05 am
Posts: 855
Location: atlanta, georgia
Thanks for this very interesting topic, I had no idea how much time and how many words could be devoted to VMG...just ask google! I look forward to digging in to learn more but Tim, you have the key issues teed up for us. Yep, speed over ground will be faster on a tack off the wind, and it also results in better VMG as opposed to sailing as close to the wind as our boats will go. How much off the wind makes optimal VMG? Stay tuned, or chime in, because I have not found the answer yet! There are so many factors, even with a single sail. Introduce a headsail or mizzen and, well, oh my. But Tim's assumption that we (with amas) are really trimarans is correct, and the importance of that is related to the amount of heeling we do/don't do. And then there is the issue of rudder angle, which provides both lift (who knew) and drag (I DID know that). The trimaran aspect (minimal heeling) and the rudder issue are both factors that go into the weather vs. lee helm, which also varies depending on wind strength and tack angle...and so on! And these both factor into VMG...somehow. Oh, and btw, the hydraulic drag (caused by hull moving through water) that needs to be overcome is LINEAR with respect to speed, which is completely different that the drag in air, which increases exponentially with speed. This is important because you get to "keep" all of the increase in speed you get by sailing a bit off the wind. If that was not the case one would experience diminishing benefit from falling off the wind to increase ground speed, as some of that speed would be spent overcoming the non-linear drag you would experience sailing through a sea of air.
Well, I'm no physicist but I am intrigued by the sailing geeks I have begun to read on the sailing blogs. If anyone is interested in a deeper dive from one of the better sources I found you can look at this string: http://www.cruisersforum.com/forums/f48 ... 02231.html

Best,
g

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