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    Posted: 22 Apr 14 at 3:46pm

PY = 1800 - 144*Length - 37*Sail Area + 1.2*Weight
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iGRF View Drop Down
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 3:55pm
Righting moment?

Originally posted by Peaky


PY = (1800 - 144)*Length - (37*Sail Area) + (1.2*Weight)

+ftfy

or
PY = (1800 - ((144*Length) - (37*Sail Area) + (1.2*Weight)))


+ftfy

Edited by iGRF - 22 Apr 14 at 4:00pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 4:10pm
Bless My Dear Aunt Sally

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iGRF View Drop Down
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 4:30pm
Please Excuse My Dear Aunt Sally


So now which is it, and what about the righting moment? I'm still waiting for all those who promised me the spreadsheet to ante up.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 4:42pm
Good question about RM. For the 21 boats I looked at there was, suprisingly, no correlation between RM and PY. This might be because all the boats I looked at where kiteless hikers (Laser, Solo, Icon, Streaker etc), and so had similar RM.

I initially had a much more complex formula with things like RM to weight ratios and based the natural log of the PY number, but it did not give a much better fit and was harder to get input numbers for (wetted surface area, sail carrying power etc).

So, for boats with no spinnaker, hydrofoil or trapezes:

PY = 1800 - (144 x Length) - (37 x Sail Area) + (1.2 x Weight)

where Weight is total displacement of boat plus crew


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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 4:55pm
It's windsurf race night tonight so I'll be back off the air for a bit, but let me have a go with that and compare it with one I wasted hours on trying to work out myself (Was nothing quite as simple as yours). We definitely need something, I've been away from my club for a week and already I can see some very dispirited chaps not wanting to even bother as a result of the latest PY carnage.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 5:26pm
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Posts: 857 		Post Options Post Options   Quote AlexM Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 5:32pm
Have you just proved the RYA numbers correct?
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Posts: 392 		Post Options Post Options   Quote iitick Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 5:44pm
Remarkably close!
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 14 at 5:50pm
The formula has been taken from the numbers, so you'd expect it to work for the majority, wouldn't you?

Interesting that according to that, the Europe punches above its weight, which is what I'd expect from a boat a foot shorter than the ones it has a comparable PY to, and that the Pico and Topper, mainly sailed by beginners, have PY's more generous than the formula suggests.

Interesting stuff. Now we will see how the Zero and Aero do compared to the theory.

I assume it is waterline length we are dealing with?
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