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Author Topic: can you make a wind-driven boat/vehicle go faster than the wind 'pushing' it?  (Read 9790 times)

Offline MarkV

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The answer turns out to be that the numbers appear to have been put on the wrong lines with 4 actually being 3, 5 being 4, etc., so either the speeds are wrong or the graph is distorted,
Yes, I see what you mean. If you extrapolate the scale, you end up with 1kts in the center, not 0kts.

The most impressive VMG preformance on water, that I'am aware of:
newbielink:http://en.wikipedia.org/wiki/USA_17_%28yacht%29 [nonactive]
"USA 17 reached the windward mark in 1h29, so her velocity made good was about 13.5 knots (25.0 km/h; 15.5 mph), or about 1.8 times wind speed. USA 17 took 63 minutes to reach the downwind mark, so her velocity made good downwind was about 19 knots (35 km/h; 22 mph), or about 2.5 times wind speed."
« Last Edit: 25/03/2013 09:56:31 by MarkV »
 

Offline imatfaal

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Great posts MarkV - thanks
 

Offline David Cooper

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The most impressive VMG preformance on water, that I'am aware of:
http://en.wikipedia.org/wiki/USA_17_%28yacht%29

One particularly good link from there is to this:-

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

As a child I used to sail Mirror dinghies in a handicap fleet which contained at the top end Nacra 5.2 catamarans (crewed by people who raced at national competition level). We all sailed more or less directly downwind, not knowing any better, though the p.y. handicap ratings for the boats would have been based on the way people sailed back in those days. Things have clearly moved on a lot since then, though I still can't find a polar for the Mirror dinghy. I imagine that it's quite hard to work them out though, and different people measuring speeds and angles in different and varying windspeeds are going to produce different polars. It would take GPS and windspeed and direction measurements to do the job properly by putting such devices an all boats and merging the data to create the most accurate polars, although they'd still vary just through weight and weight distribution of different crews, plus steering and sail handling style, so even then it won't be possible to get anything definitive. It will of course be able to distinguish between the top sailors and the rest, so it would help everyone up their game, indicating where they're doing something wrong.
 

Offline David Cooper

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[Edit: the following is a comment on two posts which have now been deleted as someone had posted them to the wrong thread.]

That's an unfortunate hazard with this forum if you open another thread in another tab and then try to reply to a previously opened tab.
« Last Edit: 26/03/2013 05:45:17 by David Cooper »
 

Offline MarkV

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Things have clearly moved on a lot since then,
Yep. When you think about, what the USA17 VMG performance actually means: You can release a balloon at the upwind mark, when the boat starts at the downwind mark. The boat will go to the upwind mark, then back to the downwind mark, and arrive there before the balloon.

In terms of absolute speed, things have also move a lot recently. 121 km/h is the new record from last year:

203 km/h  for landyachts:
 

Offline dlorde

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Thanks MarkV, for the detailed clarification and explanations; I have only a simple understanding of the principles, not the details necessary for a clear explanation. Your contribution helped me too  :)
 

Offline MarkV

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Great posts MarkV - thanks
Thanks MarkV, for the detailed clarification and explanations; I have only a simple understanding of the principles, not the details necessary for a clear explanation. Your contribution helped me too  :)
You're welcome!

If you find high-performance windpowered stuff interesting, check out dynamic soaring:

newbielink:http://en.wikipedia.org/wiki/Dynamic_soaring [nonactive]
"As of March 2012, the highest reported speed for radio control dynamic soaring was 498 mph (801 km/h)"

This is achieved using winds of about 50 mph.


 

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