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Sail Trim and Aeronautic Theories.

Oct 19, 2017
6,226
O'Day 19 Littleton, NH
Through all of these discussions about airfoil lift, it is understandable and reasonable to compare a sail to an airplane wing, but they have a major difference that is seldom addressed.

The purpose of an airplane wing is usually to provide lift perpendicular to the direction of travel and further, that lift, caused by air flowing across the foil, is generated by the driving force provided by either the aircraft itself, or gravity, in the case of a glider.

For a sailboat, the purpose of generating lift is to take that driving force from the wind itself. The dynamics are different even though the mechanics are similar. There isn't a straight 1:1 analogy between the two.

-Will (Dragonfly)
 
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Jan 1, 2006
4,693
Slickcraft 26 Greenport, NY
Well, I finished the article. Overall it seems well reasoned. But in some areas I'm still a bit at sea. Some areas seem contradictory. To avoid major recriminations at home, and in the interest of household peace and not to mention hours of chasing my tail on the internet, does anyone know of a video of the bathtub experiment or better yet a water or air stream demonstration of the circular flow phenonmena?
 
Sep 22, 2018
1,257
Hunter 216 Kingston
John
A little curious about the red lining? :)

“I think what he is trying to tell the reader is the tufts provide visual clues so the sailor can tread the fine line between steering too high (luff) and too low (stall) so that windward progress is optimized without adjusting sail trim.”

This is the section that prompted me to make my comment

30D2518E-356D-45E5-9526-0C33A2A1BDC8.jpeg


He is talking about visual cues helping beginning sailors to sail to windward effectively. They have yet to develop what I think Jon refers to as “the ride”

To me there is a small angle between pointing too high where the sail will luff and too low where the separation bubble forms and the sail starts to stall.

If the boat is headed at the optimal angle to the wind the tufts are stable, too high or low and they start to swirl around. Visual warning to the beginner that they need to adjust something quickly or the “problem” gets bigger.
 
Oct 22, 2014
12,095
CAL 35 Cruiser Portland OR, moored EVERETT WA
Yes. That is my understanding.
Note that Bethwaite calls this the “Natural method” of Sailing. You set the course, then set the sails, and alter the boat heading to adjust the angle of attack for the sails. No trimming needed. It is all about the “Set-Up”. Easy to do. With Gentry’s insight you make smart adjustments to the boat to adapt to your readings of how the “angle of Attack” is affecting the sails. Trying to sail the boat faster by balancing it between luff and stall.

Bethwaite challenges the “Natural Method” by suggesting you can make your boat sail faster if you also adjust the sail trim to address changes of angle of attack as you sail through gusts. He calls the Natural Method as the lazy mans sailing (and it is SLOW).

Gentry expands on the balancing when you are using 2 sails (main and jib). In the day there was a lot of talk about the “Slot” and the “Bernoulli Principle”. Gentry explains that there is no increased air movement in the “slot”. That the trim needed to keep the laminar flow over each of the sails is dependent on the sails, the sails angle of attack to the apparent wind, and that you may adjst the main differently to the jib as the jib will affect the trim of the main.
1589910694823.png
 
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Oct 19, 2017
6,226
O'Day 19 Littleton, NH
Gentry explains that there is no increased air movement in the “slot”.
... and that you may adjst the main differently to the jib as the jib will affect the trim of the main.
So... if there is no increased air movement in the slot, and I can get there with that, what is happening that makes the second statement necessary?

-Will (Dragonfly)
 
Jan 1, 2006
4,693
Slickcraft 26 Greenport, NY
I'll sleep on it. Probably wake up at 2 am and not be able to get back to sleep because the Loin King is in prison.