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Sail size vs. wind speed
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boardsurfr



Joined: 23 Aug 2001
Posts: 1120

PostPosted: Sat Jan 01, 2011 12:30 pm    Post subject: Sail size vs. wind speed Reply with quote

Winter = less windsurfing, more thinking. So:
Windsurfing sails work like air foils, similar to airplane wings. The lift generated by an airfoil is proportional the wind velocity squared (see, e.g., en.wikipedia.org/wiki/Lift_(force)).

However, real life experience and sail size calculators show a linear relation between wind strength and sail size. For example, I'm typically using an 8.5 m sail in 15 knots, and a 4.2 sail in 30 knots.

But according to the lift equations, a 4.2 m sail in 30 knots should produce about 2 x as much lift as an 8.5 m sail in 15 knots. However, actual pressure in the sail feels similar. What's missing here?
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dllee



Joined: 03 Jul 2009
Posts: 4208
Location: East Bay

PostPosted: Sat Jan 01, 2011 12:45 pm    Post subject: Reply with quote

Nothing in real life matches up exactly with theory, so possibly theory is really an and excersise meaning nothing in reality.
Try using the same board and fin.
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coachg



Joined: 10 Sep 2000
Posts: 2713

PostPosted: Sat Jan 01, 2011 1:01 pm    Post subject: Reply with quote

An 8.5 at 15 knts feels similar to a 4.2 at 30 knts? Are you insane or superman? Try boosting a jump and you should quickly feel the 2X difference in power.

Coachg
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isobars



Joined: 12 Dec 1999
Posts: 18980

PostPosted: Sat Jan 01, 2011 1:09 pm    Post subject: Reply with quote

Probably the biggest single reason is that apparent wind is closer than the 15-kt ambient difference would imply. Then there are more complex factors such drag (board, fin, rig, and rider), terrain, perceived inertia (due to rig weight and maneuverability), and the fact that one calculator's graph is a curve even if not a purely squared relationship ( http://tinyurl.com/2apoy3e ).

Mike \m/
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benkaufman@pobox



Joined: 19 Feb 2001
Posts: 3

PostPosted: Sat Jan 01, 2011 2:30 pm    Post subject: Re: Sail size vs. wind speed Reply with quote

Here are a few other factors:

1. I doubt that the OP's 8.5 and 4.2 are of the same design, and even if they were, how do we know that the sail's size proportionally corresponds to foil area?

2. Is the OP using the same board and fin? I observe a big difference in how much sail pull I feel using the same 6.0 sail but a 107L versus a 81L board.
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sergem



Joined: 24 Oct 2003
Posts: 393

PostPosted: Sat Jan 01, 2011 2:42 pm    Post subject: Re: Sail size vs. wind speed Reply with quote

boardsurfr wrote:
...
However, real life experience and sail size calculators show a linear relation between wind strength and sail size. For example, I'm typically using an 8.5 m sail in 15 knots, and a 4.2 sail in 30 knots.

But according to the lift equations, a 4.2 m sail in 30 knots should produce about 2 x as much lift as an 8.5 m sail in 15 knots. However, actual pressure in the sail feels similar. What's missing here?


There is also the angle of attack. Are you sheeting in your 4.2 all the way in 30kts? Are you sailing more upwind/downwind on 8.5 in 15kts?

Also, I hazard to point that your body accounts for more drag when sails become smaller.
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boardsurfr



Joined: 23 Aug 2001
Posts: 1120

PostPosted: Sat Jan 01, 2011 4:46 pm    Post subject: Reply with quote

isobars wrote:
the fact that one calculator's graph is a curve even if not a purely squared relationship

Haha, that's funny, especially coming from a rocket scientist! I'd think you would recognize an inverse linear relationship. Look at the numbers in the spreadsheet, and you'll see. Or just download Jim's spreadsheet and look at the formulas.
isobars wrote:
Probably the biggest single reason is that apparent wind is closer

I think that's right. Looking at GPS speeds on regular (freeride or FSW) boards, board speeds tend to be very close in 15 and 30 knot winds - typically around 22 knots. That gives apparent winds of 27 and 37 knots, respectively, and, in turn, a 1.9-fold difference in lift. Much closer to the typical 2-fold difference in size than the factor 4 from the true wind.
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isobars



Joined: 12 Dec 1999
Posts: 18980

PostPosted: Sat Jan 01, 2011 7:10 pm    Post subject: Reply with quote

boardsurfr wrote:
isobars wrote:
the fact that one calculator's graph is a curve even if not a purely squared relationship

Haha, that's funny, especially coming from a rocket scientist! I'd think you would recognize an inverse linear relationship. Look at the numbers in the spreadsheet, and you'll see. Or just download Jim's spreadsheet and look at the formulas.


You're right, but you also mistakenly presumed that I cared enough to look that closely. Wink

Anyone advanced enough to sail short boards knows by now that sail size selection is far too personal for an equation to help, and the beginner curve recommends 1 and 2 meter sails. Jim's diligent and erudite labor of love is thus more an academic exercise than a valuable tool, as we've often discussed here before.

Mike \m/
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windward1



Joined: 18 Jun 2000
Posts: 1098

PostPosted: Sat Jan 01, 2011 7:38 pm    Post subject: linear vs. curve Reply with quote

boardsurfr,

What do you use for your sail size at 22.5 knots (half-way between 15 and 30 knots)? I bet it is not a 6.3 sq mtr sail (half-way betweeen 8.5 and 4.2 sq mtrs).

You only took two points and drew a straight line. fill in a few points in-between and you will get a curve. Maybe not quite a reversed square, because of the apparent wind parameter and drag and such, but a curve non-the-less.

Windward1
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boardsurfr



Joined: 23 Aug 2001
Posts: 1120

PostPosted: Sat Jan 01, 2011 7:56 pm    Post subject: Reply with quote

isobars wrote:
Anyone advanced enough to sail short boards knows

Once more, isobars thinks he knows exactly what all advanced sailors think. Has he not gotten enough responses to learn that not everyone agrees with you? Can he learn at all?

There certainly are many advanced sailors who do not use sailsize apps or similar. I have seen plenty of them end up being unhappy with the sail sizes they chose to pick by whatever other choice processes they used (quite often the "What are you using?" approach).

But there are also many advanced short board prefer to use the data they can get, from wind sensors and wind meters to sail size apps or spreadsheets. I have talked to plenty of windsurfers who think that this works better than whatever they did before. Admittedly, all of them were smart enough to understand that the data and sail size suggestion where just a starting point that then could be modified according to preferences and conditions.
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