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isobars



Joined: 12 Dec 1999
Posts: 14163

PostPosted: Sat Apr 12, 2014 3:49 pm    Post subject: Reply with quote

If the quarter analogy were valid, a vertical sail would provide no forward thrust.
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coachg



Joined: 10 Sep 2000
Posts: 2001

PostPosted: Sat Apr 12, 2014 5:15 pm    Post subject: Reply with quote

What makes the board go forward and not backward? Is it impossible to sail fin first clew first? Does a vertical sail provide forward thrust? What constitutes a vertical sail?

All valid questions for sure. I don't know how valid the quarter analogy is but it sure helps in understanding the freestyle moves because the board doesn't have to be in the water for the analogy to work. Using the quarter analogy allows us to steer the nose of the board even while in the air.

Coachg
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swchandler



Joined: 08 Nov 1993
Posts: 5816

PostPosted: Sat Apr 12, 2014 6:11 pm    Post subject: Reply with quote

I think that the issue of raising or lowering the boom is getting a bit stretched in this discussion, because boom height more about sail control rather than board control.

Speaking for myself, relatively speaking, there is only the optimum boom height. Any adjustment range is quite small. When it's out of position, things just don't work very well, and I feel limited or constrained. I certainly don't adjust boom height to correspond with different conditions.
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GURGLETROUSERS



Joined: 30 Dec 2009
Posts: 1344

PostPosted: Sun Apr 13, 2014 2:31 am    Post subject: Reply with quote

In Winter in the 80's I used to wear a thick fibre-pile jacket over my wetsuit. It held half a ton (give or take) of water which certainly helped hold down those non-twist sails of yore in overpowered conditions. (Which was just about all of the time.)

And it didn't half improve water-starting technique. There was an imitation waterfall as I levered out of the briny. There's too much fussing over details nowadays, and too little fun! Twisted Evil Laughing

By the way, yachts seem to manage to drive forwards with their vertical masts and sails - sometimes at quite indecent speed!
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Ugly_Bird



Joined: 04 Nov 2008
Posts: 150

PostPosted: Sun Apr 13, 2014 9:24 am    Post subject: Reply with quote

GURGLETROUSERS wrote:
By the way, yachts seem to manage to drive forwards with their vertical masts and sails - sometimes at quite indecent speed!


Including the SailRocket Laughing
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cgoudie1



Joined: 10 Apr 2006
Posts: 1250
Location: Killer Sturgeon Cove

PostPosted: Sun Apr 13, 2014 9:50 am    Post subject: Reply with quote

Yup, what Steve says! I don't change boom height for different conditions.
Find your sweet spot and stick with it and don't overanalyze your gear
set up.

-Craig

swchandler wrote:
I think that the issue of raising or lowering the boom is getting a bit stretched in this discussion, because boom height more about sail control rather than board control.

Speaking for myself, relatively speaking, there is only the optimum boom height. Any adjustment range is quite small. When it's out of position, things just don't work very well, and I feel limited or constrained. I certainly don't adjust boom height to correspond with different conditions.
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U2U2U2



Joined: 06 Jul 2001
Posts: 3056
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Sun Apr 13, 2014 10:06 am    Post subject: Reply with quote

for the majority of time I leave the boom once set, and it gets the same height location on each rig.

I have had experience when the board has JUST NOT enough to plane, and raising it helped, not a cure all, never expected one.

The opposite I have also done, when seriously O/P lowering the boom helps with control, this has helped when out there and getting back is a prime concern

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DanWeiss



Joined: 24 Jun 2008
Posts: 1944
Location: Connecticut, USA

PostPosted: Sun Apr 13, 2014 11:36 am    Post subject: Reply with quote

What Naish is saying about boom height applies in general to setup, but its so simplified as to add some confusion.

Remember, every suggestion to move one dimension for a certain effect assumes that everything else is tuned properly except that one thing. Moving a boom up when harness lines are adjusted for a low boom tends to make the line tension higher, earlier. In this case, one might plane earlier without being in the harness, but may not plane as long during the lulls because the effectively shorter lines will over sheet the sail.

Naish's suggestion about moving booms higher really is code for increasing the triangle between the mast foot, the boom head and the location of the sheeting pressure. Doing that almost always requires corresponding movements of mast foot, boom height and harness line location and length.

Vertical lift of a sail raked to windward does exist but isn't very relevant to why we must move the COE somewhat around the line directly above the mast foot. Brandt's finger-on-quarter demonstrates the effect if not the actual physics. Since the wind blows roughly horizontally, we can lean the sail over us into the wind just as we might lean into the wind when standing on shore. Imagine standing on a frozen pond and leaning into the wind. We would fall down because the force resisting us on land is much lower on ice. This example shows why we move the board by moving the sail's COE around the line extending up from the mast base.

Watch less athletic FW windsurfers who don't pump to a plane in light wind. They rake the sail to windward with the front hand then slowly sheet in. The board turns a bit downwind and slides forward. This motion powers up the board and fin, railing the board a bit. Stepping into the front strap and hitting the harness powers up the board even more, and allows a nice step into the back strap. No pumping, only sail rake.

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windoggie



Joined: 22 Feb 2002
Posts: 2396

PostPosted: Sun Apr 13, 2014 1:34 pm    Post subject: Reply with quote

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



Joined: 10 Apr 2011
Posts: 439

PostPosted: Mon Apr 14, 2014 7:16 am    Post subject: Reply with quote

isobars wrote:


2. I don't see the logic. Unless we're talking about some subtle aerodynamic vertical lift from the sail, whatever weight we hang in the harness lines is opposed by the MFP regardless of the boom height. What am I missing?


There is another effect going on at the same time. The sail COE is located higher than the boom. The wind tries to rotate the sail around the boom, but this is balanced by the mast foot. If you try your sail on the ground before sailing, it's quite noticeable: the mast foot digs in the soil.

When you lower the boom, you move it away from the COE, increasing the wind leverage and decreasing the mast foot leverage. Wind force being constant, more MFP is required to balance the sail.

I think this also explains why larger sails apply much more MFP. Wind force is similar when a small sail and a big sail are used in their proper wind range. Their wieight is not that different. But the COE is higher up on the big sail. This give the wind better leverage that is balanced with more MFP from the board.
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