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Bigger rig triangle for slalom and Formula ?
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zirtaeb



Joined: 03 Jul 2009
Posts: 2278

PostPosted: Mon Aug 27, 2012 6:06 pm    Post subject: Bigger rig triangle for slalom and Formula ? Reply with quote

Hey...
Was wondering, what happened to the turn of the century idea of moving the mast track back further on slalom and course race boards?
Lots of talk then of track back for less wetted surface, more rig upright for efficiency, and more twist to avoid the catapults.
But the last two years, seems almost every slalom and course racer pushes the mast track farther forward than ever before.
Standard measurement from center of front straps to center of mast track USED to be about 23" for narrow slalom boards.
Now, for slightly wider slalom boards, about 23-24.5", almost the same as in 1997.
I ask, because I have some 1997 boards.
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bamer



Joined: 23 Jul 2014
Posts: 57

PostPosted: Wed Aug 29, 2012 3:44 pm    Post subject: Reply with quote

Dom,

Cracks me up a bit that you would ask this here. I have never seen a question this technical asked on this forum.

A few things:

-Twisting force increases on a foil as rake increases.

-More rake allows for lower COE placement in a windsurfing sail.

-More rake generally lowers induced drag through smaller tip vortex.

-Wider boards with higher aspect lifting surfaces require more power and/ or pumping to pop them on a plane. Once planing they accelerate more quickly than their narrow counterparts.

From non-planing, to planing, to initial acceleration the sail is generally in an upright and efficient orientation before the rider transitions to foot straps. The open gap at the bottom is not ideal from an efficiency standpoint, but is tolerable for a limited time and because of the relatively low rate of flow.

-More rake generally allows nose to ride higher/ more free because it lowers downward mast foot pressure.

So let's put it together for the different racing formats:

Slalom= Wide boards which are quicker to plane and allow for lower wind minimums (compared to older designs), accelerate faster in corners, are easier to jibe because of lower planing threshhold and increased stability.

Formula= Wide boards that plane off early allow for lower wind minimum, gives rider leverage to carry big fins which allow the kit to be sailed higher upwind and deeper downwind which generally means better VMG. Big boards and big fins require big sails to generate power needed to get planing and stay high upwind and deep downwind, especially in marginal conditions.

In both formats we need a big sail to get planing and accelerate quickly but once we are moving we want to maximize efficiency. So we rake the sail back which lowers the induced drag and increases the twisting moment on the sail which allows excess power to bleed off AND, AND, AND presents a more efficient, easier to manage foil (more lift, less drag, more control) because apparent wind rotates aft as you move towards the top of the sail due to the boundary layer on the water. TWIST ON A SAIL IS NOT ONLY FOR SPILLING EXCESS WIND/ POWER.

Further, these sails are manageable largely because of a fairly low center of effort and built in twist. If the COE on the sails was higher and/ or they were less twisty the range would be much more limted.

Further even more ... these wide boards have excellent range and speed potential largely because of refined rockers and tail cut outs. Once powered, they sit on a small amount of wetted surface with the rest of the board relatively high out of the water. Less downward mast track pressure allows them to consistently ride high which is integral to their functionality.

That is my basic explanation. A sail or board designer would likely come up with something much better.
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zirtaeb



Joined: 03 Jul 2009
Posts: 2278

PostPosted: Wed Aug 29, 2012 4:02 pm    Post subject: Reply with quote

Holy smokes, my head is spinning.
As you, I often sail pre 2000 slalom boards, but have a couple from '07. They seem to like the same rig triangle placements.
I'm constantly stuck trying the track back, low boom idea, tons of downhaul for twist, but it don't work at all for me.
The I go track more forwards, but if underpowered, it don't work at all for me.
Much don't work.
When you coming back to the windsurf scene? I need a punching bag in slalom, so I'll make you rig the same size as me... Very Happy
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georges



Joined: 10 Apr 2000
Posts: 266

PostPosted: Wed Aug 29, 2012 8:23 pm    Post subject: Reply with quote

Wow your WAY smarter than isobars. Cool
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zirtaeb



Joined: 03 Jul 2009
Posts: 2278

PostPosted: Wed Aug 29, 2012 8:45 pm    Post subject: Reply with quote

You should see the carbon sandwich fins that Ben makes. Unreal.
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pointster



Joined: 22 Jul 2010
Posts: 197

PostPosted: Thu Aug 30, 2012 11:46 pm    Post subject: Reply with quote

bamer wrote:

A few things:

-Twisting force increases on a foil as rake increases.

-More rake allows for lower COE placement in a windsurfing sail.

-More rake generally lowers induced drag through smaller tip vortex.

-
That is my basic explanation. A sail or board designer would likely come up with something much better.


1. What do you mean by "twisting force"?

2. Planform and camber profile of the sail determine the COE of the sail. You could have a sail with a hig rake angle and high COE, and low rake angle and low COE. Depends on the design of the sail.

3. More rake does not lower induced drag. Rake , or sweep, increases spanwise flow, which effectively increases the angle of attack of the top of the sail, wich causes more tip vortexing, thus more induced drag.
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zirtaeb



Joined: 03 Jul 2009
Posts: 2278

PostPosted: Fri Aug 31, 2012 1:18 pm    Post subject: Reply with quote

Strange game we're playing here...theory vs real world human application.
Rake...I'd think, more rake, the whole rig get's lower, lowering the COE as well as the overall height of the unit.
Twisting forces..since you can hold the sail easier, the the amount of twist is fully lowered, while the span from trac to mast tp s stl the same have no 's.....
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pointster



Joined: 22 Jul 2010
Posts: 197

PostPosted: Fri Aug 31, 2012 3:00 pm    Post subject: Reply with quote

Ok Lee, short answer: people are sailing shorter and larger sails with the center of effort lower and farther back. That means the mast foot has to be farther forward to keep the center of effort over the center of lateral resistance.
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zirtaeb



Joined: 03 Jul 2009
Posts: 2278

PostPosted: Fri Aug 31, 2012 4:00 pm    Post subject: Reply with quote

Good stuff, thanks.
I notice the new North sails are much flatter in front, draft much further back. And the longer boom trend tends to move the draft farther back from the mast.
Since I seldom use more modern sails (newer than '00), I had to ask.
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mac



Joined: 07 Mar 1999
Posts: 4960

PostPosted: Fri Aug 31, 2012 5:43 pm    Post subject: Reply with quote

Poinster--you are correct. The 2013 North 10.0 is 8 cm longer than the 2012, and the mast track needs to be further forward and the harness lines much further back. COE is further back, I've not tried to measure how far. Steve Sylvester and Jason Ward are pretty happy with the 2013 North 9.4's performance, compared to the 2012 9.3, and mast tracks are way forward. But the extreme mast bend ....
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