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DanWeiss



Joined: 24 Jun 2008
Posts: 1930
Location: Connecticut, USA

PostPosted: Mon Apr 14, 2014 7:34 am    Post subject: Reply with quote

windoggie wrote:
DanWeiss wrote:
Stepping into the front strap
oh oh.


Tell me about it.

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beallmd



Joined: 10 May 1998
Posts: 1072

PostPosted: Mon Apr 14, 2014 11:46 am    Post subject: Reply with quote

Very nice explanation by Sailboarder. That actually made sense! I thought he was just that close to starting to write equations, I suspect someone is an engineer or physicist (as is Iso).
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allen



Joined: 13 Aug 1996
Posts: 216

PostPosted: Mon Apr 14, 2014 12:28 pm    Post subject: Reply with quote

MFP = F*H/L

F= Force at center of effort dependent on wind velocity and sail area per Bernouli's eqn
H =height of center of effort from board
L= distance from board to boom

Drop L, the boom height, and the MFP goes up for a given CE force. Increase L, MFP goes down.
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isobars



Joined: 12 Dec 1999
Posts: 14003

PostPosted: Mon Apr 14, 2014 1:27 pm    Post subject: Reply with quote

You're overlooking probably the biggest factor: how much weight the sailor is hanging onto the boom via his harness and hands. Your equation also presumes a fixed mast, as opposed to our articulated ones, all further complicated by the sailor's dynamic interaction with the boom and the rig's other anchoring system: our footstraps. Then there's the sailmakers' quest (aided by lots of downhaul) to place the COE at boom height to minimize the pitching moment your equation addresses.

The mind boggles.
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boardsurfr



Joined: 23 Aug 2001
Posts: 538

PostPosted: Mon Apr 14, 2014 1:36 pm    Post subject: Reply with quote

allen wrote:
MFP = F*H/L

Nice formula. I have no clue whatsoever if it is correct or matters. But I certainly move the boom height when the wind strength changes. The reasons I remember are a bit different than those given before. IIRC, they are based on things I heard from Andy Brandt and Matt Pritchard.

In marginal winds, the boom goes up. That makes hanging down into the harness lines easier. Together with a near-vertical, slightly forward mast, this brings almost all my weight into the mast foot. Gets me going.

In strong wind, the boom goes down. That allows the harness lines to be more horizontal, and gets my body further away from the rig. In chop, it also makes it easier to bend the knees more. Both things increase control.

My overall adjustment range is probably about 4 inches for a given sail. On most sails, I may also adjust outhaul by about 1 inch. Together, that often lets me keep the same sail while others go in and re-rig.
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Ugly_Bird



Joined: 04 Nov 2008
Posts: 146

PostPosted: Mon Apr 14, 2014 1:40 pm    Post subject: Reply with quote

allen wrote:


MFP = F*H/L



F=[N]
H=[length]
L=[length]

[MFP]=[N]*[length]/[lenght]=[N]

According to the units, is MFP a force? Or pressure? Then the units are not right.
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isobars



Joined: 12 Dec 1999
Posts: 14003

PostPosted: Mon Apr 14, 2014 1:49 pm    Post subject: Reply with quote

boardsurfr wrote:
I certainly move the boom height when the wind strength changes.

Doesn't your boom head or mast overheat from the friction as you slide your boom up and down as the power in your sail changes over a range of 10:1 (~10 mph holes to ~30 mph gusts) on a typical reach on the Columbia? Smile
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allen



Joined: 13 Aug 1996
Posts: 216

PostPosted: Mon Apr 14, 2014 2:00 pm    Post subject: Reply with quote

Ugly_Bird wrote:
allen wrote:


MFP = F*H/L



F=[N]
H=[length]
L=[length]

[MFP]=[N]*[length]/[lenght]=[N]

According to the units, is MFP a force? Or pressure? Then the units are not right.


MPF is a force (lbs). The load in the sail is a moment (F*H) in inch-lbs which is divided by the length (L in inches) of the force couple reacted by combo of your harness/feet.
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Sailboarder



Joined: 10 Apr 2011
Posts: 409

PostPosted: Mon Apr 14, 2014 2:40 pm    Post subject: Reply with quote

isobars wrote:
You're overlooking probably the biggest factor: how much weight the sailor is hanging onto the boom via his harness and hands. .


These things all go together. The rider is held up by the mast foot, the rider feet and by some sail vertical lift. More MFP also means less weight on the feet if you keep the sail trimmed the same.
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Sailboarder



Joined: 10 Apr 2011
Posts: 409

PostPosted: Mon Apr 14, 2014 2:40 pm    Post subject: Reply with quote

isobars wrote:
You're overlooking probably the biggest factor: how much weight the sailor is hanging onto the boom via his harness and hands. .


These things all go together. The rider is held up by the mast foot, the rider feet and by some sail vertical lift. More MFP also means less weight on the feet if you keep the sail trimmed the same.
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