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How to Set a Genoa Sail Car

Jan 1, 2006
4,204
Marblehead Skiff 14' Greenport, NY
The key is at 4:28 and on. Not there yet but will watch again and again ... The pressure gradient above the wing is lower toward the foil surface. The pressure gradient below the foil is higher towards the foil surface. I can't reconcile. The flow should be nearly the same. Don't get it.
 
Sep 6, 2010
1,298
Jeanneau 349 port washington, ny
The key is at 4:28 and on. Not there yet but will watch again and again ... The pressure gradient above the wing is lower toward the foil surface. The pressure gradient below the foil is higher towards the foil surface. I can't reconcile. The flow should be nearly the same. Don't get it.
The coanda effect is the reason why the air (or fluid) curves around a surface and causes the pressure gradient. But the key to understanding why the boat moves forward is a simple application of Newtons second (F=ma) and third laws (action/reaction).
 
Jan 1, 2006
4,204
Marblehead Skiff 14' Greenport, NY
Yeah and you just wanted a relaxing sail. Please get back to us about that continental divide sail.
 
Mar 23, 2015
201
Catalina 22 MK-II Dillon, CO
yes ... but I'm still trying to get the boat moving (damn inertia) .. :banghead:
 
Jan 11, 2014
4,155
Sabre 362 113 Fair Haven, NY
One of the unique problems that sailors have at 9,000 feet that we sea level sailors don't have is that the air is less dense. The amount of energy in 10 kt winds at sea level is greater than the energy in wind at 9,000 feet. Also As air temperature increases air density. As altitude and temperature increase, air density decreases resulting in less energy available to drive the boat.

One thing you may need to do is adjust your expectations. If you read about a sea level sailor having a delightful cruise at 12 knots of breeze, your 12 knots of breeze is going to be more like a sea level 6 knots of breeze. When sea level sailors talk about reefing at 20 knots, for you that is more like 12 knots, which is a nice comfortable sailing wind.

To get your boat moving, throwing bricks off the stern might work, but an easier way is to let your sails luff, then gradually bring them in until they stop luffing while holding the same course. Do this a few dozen times and you'll get a good feel of the dynamics and basic sail trim. Get this down first and learn the rest over the next 30 years.

[First paragraph edited for clarity.]
 
Last edited:
Nov 13, 2015
45
Hunter 290 Toronto Ontario
I think Wienie and his first video and I agree that the simplest physics that accurately explains sail/wing lift is based on Newton, not Bernoulli. Simple ("High School") Bernoulli is fun - who doesn't love blowing across the top of a piece of paper and seeing it lift up? - but it simplifies to the point of falsehood and paradox, like proving that planes (or keels and rudders!) with symmetrical airfoils canNOT generate lift, or that a plane with asymmetrical wings can't fly inverted! But simple Newton is still true, so it gets all my focus.
And the reason that attached flow is more effective than disturbed turbulent flow is that it deflects more air in a helpful direction (towards "aft"), so it develops more lift. E.g., if you over-trim and lose the attached flow around the outside of the jib, you're no longer deflecting or "bending" that airflow towards "aft".
Newton also explains why going downwind - the speediest way to sail according to landlubbers - is so dead: there IS no way to deflect the air flow "aft"! All we can do is slow/stop as much air/wind as possible so it pushes us before slipping by. Simple Newton there too, and Bernoulli doesn't help us much.
But Bernoulli was right (and complex) - it's the simplifying High School teachers and sailors and authors who got it wrong.
And nobody has to know ANY Physics to sail well, or even very well, as 1 or 2 fine racers in my fleet keep proving. But for many of us it's a helpful and illuminating way of "getting" it.
 
Nov 13, 2015
45
Hunter 290 Toronto Ontario
Dlochner's
"As air temperature increases air density decreases resulting in less energy available to drive the sailboat."
is accurate but off topic. But if he'd written "As ALTITUDE increases" the whole thing would be spot on.
 
Sep 6, 2010
1,298
Jeanneau 349 port washington, ny
I think Wienie and his first video and I agree that the simplest physics that accurately explains sail/wing lift is based on Newton, not Bernoulli. Simple ("High School") Bernoulli is fun - who doesn't love blowing across the top of a piece of paper and seeing it lift up? - but it simplifies to the point of falsehood and paradox, like proving that planes (or keels and rudders!) with symmetrical airfoils canNOT generate lift, or that a plane with asymmetrical wings can't fly inverted! But simple Newton is still true, so it gets all my focus.
And the reason that attached flow is more effective than disturbed turbulent flow is that it deflects more air in a helpful direction (towards "aft"), so it develops more lift. E.g., if you over-trim and lose the attached flow around the outside of the jib, you're no longer deflecting or "bending" that airflow towards "aft".
Newton also explains why going downwind - the speediest way to sail according to landlubbers - is so dead: there IS no way to deflect the air flow "aft"! All we can do is slow/stop as much air/wind as possible so it pushes us before slipping by. Simple Newton there too, and Bernoulli doesn't help us much.
But Bernoulli was right (and complex) - it's the simplifying High School teachers and sailors and authors who got it wrong.
And nobody has to know ANY Physics to sail well, or even very well, as 1 or 2 fine racers in my fleet keep proving. But for many of us it's a helpful and illuminating way of "getting" it.
Was not implying Bernoulli's is not involved here. Bernoulli's is the reason why air moving faster over the top (leeward) causes a decrease in pressure with an upwards resultant force. This pressure causes a downturn in the stream and in accordance with Newtons 3rd law, the upwards force on the air on the wing is equal to the downwards force of the wing on the air. They are one and the same only opposite in direction.
The point is, is that you do not have to understand Bernoulli's equation or fluid dynamics to see how lift propels the boat forward. One needs only to see the acceleration of the wind down to calculate the force on the wing up. That's the lift.
 
Jan 11, 2014
4,155
Sabre 362 113 Fair Haven, NY
Dlochner's
"As air temperature increases air density decreases resulting in less energy available to drive the sailboat."
is accurate but off topic. But if he'd written "As ALTITUDE increases" the whole thing would be spot on.
Edited the original post for clarity.

As temperature and altitude increase, air density decreases. That's why spring and fall winds are stronger for any given speed than during the summer at sea level.
 

Joe

Jun 1, 2004
6,769
Catalina 27 Mission Bay, San Diego
Lift....by itself...doesn't propel the boat forward. The lift generated by the sail is perpendicular to the chord shape. Without the boat's foils. ... hull, keel and rudder.... providing resistance and redirection of the sail's lifting force the vessel would just skitter sideways.

When air flows over a sail's curved surface....the entire mass of wind does not change direction, per se. The curve of the sail forces the adjacent airflow to travel a longer distance in order to maintain the same direction as the unaffected air. So....the attached air must speed up to do this...which creates the pressure difference we call lift.
 
Nov 13, 2015
45
Hunter 290 Toronto Ontario
Lift....by itself...doesn't propel the boat forward. The lift generated by the sail is perpendicular to the chord shape. Without the boat's foils. ... hull, keel and rudder.... providing resistance and redirection of the sail's lifting force the vessel would just skitter sideways.

When air flows over a sail's curved surface....the entire mass of wind does not change direction, per se. The curve of the sail forces the adjacent airflow to travel a longer distance in order to maintain the same direction as the unaffected air. So....the attached air must speed up to do this...which creates the pressure difference we call lift.
You must be puzzled every time a balsa wood glider flies, with wings as flat as a board generating enough lift that they fly pretty well! Paper planes do the same! Newton explains it. The breeze leaving a trimmed sail's leach IS deflected, moving in a different direction than the wind hitting the luff.
 
Aug 1, 2011
3,492
Catalina 270 Wabamun - on the orange ball
and then you add the effects of an inland lake, on the prairies. If you tack on every windshift, you'll be the last one over the line more often than not.
 
Oct 28, 2008
24
Catalina 320 Lake Texoma
Been following this thread since its inception with some amusement and incredulity. Clearly, the bar fight over how a sail provides thrust, (that is what makes one move, is it not) will not be settled here. Crawled out of my office the other day after flying it back from SNA to Dallas at 45,000 feet, and not once did the thought of Mssrs. Bernoulli or Newton come to mind. I think Catalanc asked how to set the jib fairleads, not the intro to aerodynamics 101. As several have mentioned, sailing on jib alone is a very practical and low work load form of no-compete sailing and I do it regularly on my C320. Windy day, old guy, why keep score? It promotes cheating! Buy Dons book, learn the function and effect of "all" the sail controls. Get a hand held GPS, go sail, move stuff around and see what happens. Empirical study has debunked more than one "theory". And go sail with some of the guys that finish up front in the beer can races. They may not know why they win, but they sure know how. And the next thing you know you'll want a bigger boat.
 
Oct 28, 2008
24
Catalina 320 Lake Texoma
#3 is pretty easy: For every action there's an equal and opposite reaction. Throw any matter over the transom, and the boat gets pushed forward.
Actually, throw anything you want off the boat, It gets lighter. And if you throw it off the bow, will the GPS show it.