Pointing and pulling was: Sailing a Cal 29

Some many months ago we, at the Womens Sailing Association, had a presentation by a sailmaker from one of the local major lofts. He demonstrated how sails work: they create vorteces, which is what actually powers the boat. If you think the jib does all the powering and the main is there just to balance the boat, think again. The main actually changes the directiion of the wind as it approaches your jib, giving the jib a more favorable wind. It actually 'lifts' the wind. This is why getting onto another boats lee bow is advantageous, their sails are actually lifting the direction of the wind so you can point up and luff them. This vortex thing also explains why it takes so long for a sailboat to get going again after it stops dead in the water. When the boat stops moving foward it shuts down its 'vortex generator'. The sailmaker showed how you can see this if you take a foil shape and drag it through the water, even your bathtub. you can see the vortexes shedding of the foil on the bottom of the bathtube, as shadows of darkness, spining around as you pull the shape through the surface of the water. Fascinating! Spinning, interesting, gyroscopes, electrons whirling around the nucleus of the atom, solar systems around the black hole at the center of the universe, tornados and cyclones. the magnetic field that follows electricity in flux, etc. Ha, knowing this has got to make you a better sailor! Jerry

--- In Ca… [at] yahoogroups.com, Gerald Sobel <sobel_solar@...> wrote: > > Some many months ago we, at the Womens Sailing Association, had a presentation by a sailmaker from one of the local major lofts. He demonstrated how sails work: they create vorteces, which is what actually powers the boat. If you think the jib does all the powering and the main is there just to balance the boat, think again. The main actually changes the directiion of the wind as it approaches your jib, giving the jib a more favorable wind. It actually 'lifts' the wind. This is why getting onto another boats lee bow is advantageous, their sails are actually lifting the direction of the wind so you can point up and luff them. MIke says-- Back in my previous life as an aeronautical engineer, we learned that vortices are not what you are after. It is attached flow. A sail is a wing on end. LIft is simply the difference in pressure because the upper, or windward, surface is longer than the bottom, or leeward, surface. Air has to travel a longer distance and this causes the pressure to drop on the lifted side as the air molecules spread out. Flattening the sail as the wind picks up is analagous to retracting the flaps as air speed increases. When you lose the attached flow, the wing or sail stalls. The vortices occur as the two streams of air rejoin at the trailing edge. There is also a vortex effect at the wingtip and the endplate, now exaggerated on some 737 wingtips, seems to reduce that. Here is a nice article on vortex reduction. It explains (although it doesn't mention specifically) the winged keel that cost the US the America's Cup. It is vortex reduction that makes things go fast. Planes or sailboats. http://www.ikarus342000.com/Avorart.htm Mike Kennedy Conquest Cal 40 # 96

Mike, if I tighten the halyard, it puts more curvature in the leading edge of the sail (loosening flattens the front). It would seem to me that: 1. More curvature leads to lower pointing due to the angle of incidence of the wind. 2. More curvature makes it more difficult for the air to adhere to the sail for the length of the sail. I'm really guessing on the second one. Guidance appreciated. I've also heard theories on "squeezing the seed" for forward motion. Does that ring any bells? Cheers Charlie From: Ca… [at] yahoogroups.com [mailto:Ca… [at] yahoogroups.com] On Behalf Of mtkennedy1 Sent: Friday, December 01, 2006 9:40 AM To: Ca… [at] yahoogroups.com Subject: [Cal_Boats] Re: Pointing and pulling was: Sailing a Cal 29 --- In Ca… [at] yahoogroups.com, Gerald Sobel <sobel_solar@...> wrote: > > Some many months ago we, at the Womens Sailing Association, had a > presentation by a sailmaker from one of the local major lofts. He demonstrated how sails work: they create vorteces, which is what actually powers the boat. If you think the jib does all the powering and the main is there just to balance the boat, think again. The main actually changes the directiion of the wind as it approaches your jib, giving the jib a more favorable wind. It actually 'lifts' the wind. This is why getting onto another boats lee bow is advantageous, their sails are actually lifting the direction of the wind so you can point up and luff them. MIke says-- Back in my previous life as an aeronautical engineer, we learned that vortices are not what you are after. It is attached flow. A sail is a wing on end. LIft is simply the difference in pressure because the upper, or windward, surface is longer than the bottom, or leeward, surface. Air has to travel a longer distance and this causes the pressure to drop on the lifted side as the air molecules spread out. Flattening the sail as the wind picks up is analagous to retracting the flaps as air speed increases. When you lose the attached flow, the wing or sail stalls. The vortices occur as the two streams of air rejoin at the trailing edge. There is also a vortex effect at the wingtip and the endplate, now exaggerated on some 737 wingtips, seems to reduce that. Here is a nice article on vortex reduction. It explains (although it doesn't mention specifically) the winged keel that cost the US the America's Cup. It is vortex reduction that makes things go fast. Planes or sailboats. http://www.ikarus342000.com/Avorart.htm Mike Kennedy Conquest Cal 40 # 96 Yahoo! Groups Links

--- In Ca… [at] yahoogroups.com, "Husar, Charlie" <husar_charlie@...> wrote: > > Mike, if I tighten the halyard, it puts more curvature in the leading > edge of the sail (loosening flattens the front). It would seem to me > that: Halyard tension, as I understand it, pulls the draft forward, making the shape more like a wing. A wing has the maximum depth about 1/3 to 1/4 of the way from the leading edge. When the wind picks up (or airspeed increases), the greater lift tries to pull the curve back to the center. That's not a problem in rigid wings but may have been in early flight and is certainly a problem in sails. You want to keep the optimum shape and that requires increasing halyard tension with increased wind force. > > 1. More curvature leads to lower pointing due to the angle of incidence > of the wind. I think the sail tends to stall with deeper curve. Look at a B 17 wing and then at a B 52 wing. High speed means flatter shape of the drag rises too high. It gets harder to keep the flow attached. It has been nearly 50 years since I thought about this stuff so I don't want to get too deeply into it. Marchaj's book is the bible and I used to have a copy. > > 2. More curvature makes it more difficult for the air to adhere to the > sail for the length of the sail. Right. > > I'm really guessing on the second one. Guidance appreciated. That is the flat wing theory above. There are a number of books on sail design. Of course, one way to deal with it is to use hard sails, like the C class cats do. Here is a nice forum: http://www.boatdesign.net/forums/showthread.php?t=457&page=4 > > I've also heard theories on "squeezing the seed" for forward motion. > Does that ring any bells? Nope. Mike Kennedy Conquest Cal 40 # 96 > > Cheers > Charlie >

Husar, Charlie wrote: > > Mike, if I tighten the halyard, it puts more curvature in the leading > edge of the sail (loosening flattens the front). > Just for clarity here, I assume that you mean the foil-shape along the luff is deeper with tight halyard, and flatter with loose halyard. Or do you mean that the catenary on the sail's leading edge is more curved with tighter halyard (which does not make sense to me)? By the way, for all you warm-latitudes sailors, we've got the wind out of the NE at about 25 knots, snow diminishing visibility to about 1/2 mile max. ofyen less at ground level, and a big ka-whump of snow since 9 a.m. Chris Campbell