| Author | Message | ||
     Annette Cook New member Username: Captaincookrn Post Number: 2 Registered: 08-2005 |
Mark, I did mention "light crisp sail cloth" and probably should have said "mylar" instead of cloth. I will drop my windseeker off sometimes at Aventura. I won't be there this week, but will send you mail when I am able to get ther. You can take it out of the bag and take a look at it. Take it out on the water and try it out--if there's no wind! Annette | ||
| Mark Howe Moderator Username: Unclemark Post Number: 239 Registered: 08-2003 |
Thanks Marc; The original question was simply, what's a Windseeker? asked by one of Annette's crew. I had never heard of one and Annette sent us an explanation. You see in the discussion the type of material was never pinned down. Plus many other details that came up for discussion. Since you agree with the premise, have you seen more than 20% used? How about the bungee idea? | ||
| Marc Hughston Moderator Username: Hughston Post Number: 515 Registered: 03-2003 |
I'm not sure what the original windseeker question was, but... Mathematics aside, there is no dispute among those who use them that windseekers work in very, very light conditions. Think 0-3 knots, you're parked between Todos Santos and Ensenada on a glassy and dark sea, with little wind ripples occasionally illuminated by starlight. Even with string sheets and plastic shackles the spinnaker would hang under it's own weight. The windseeker stands up, responds to the slightest puff, and allows you to build enough boatspeed to begin steering again so you can chase down the zephyrs. Boat speed when it's light means everything, and the windseeker gives you the best chance of attaining it - that's my practical take on it. | ||
| Mark Howe Moderator Username: Unclemark Post Number: 236 Registered: 08-2003 |
The following discussion came from a student mentioning in class an innovative kind of sail. The student might have been Monique or maybe Vicki. It is a fascinating puzzle; thought you might all enjoy. Thanks to Captain Annette Cook -- Mark ********* Monique, Here is my take on the wind seeker question. According to the Bernoulli principle, in order for a sail to produce drive, it requires wind to move across both sides of a curved sail surface in a laminar flow in order to create pressure differences due to speed differences on the two sides, thus creating lift as with a plane wing or forward drive as with a sail. This forward force created on the moveable sail is transferred to the boat by a sheet to a winch. When reaching or beating, If the wind has been reduced to a level where this laminar flow cannot be sustained, there will be no forward movement of the boat, and the dreaded 0.0 speed will be achieved. In light air, a lighter, bigger sail is great, but there is a point that it won't work. Regardless of small or shifting puffs that may be around the boat, it cannot move forward until a sustained laminar flow is achieved. The larger sail becomes an undulating mass of cloth, not very informative and difficult to tack, since its luffing and movement can sometimes create drag and make the windless moment worse. By using a small windseeker, approx. 20-30% of the "J", it is easy to see where and how strong those small puffs are and how they are shifting around the boat. With a string sheet and small area, the volume of wind needed to create laminar flow is much less, and the windseeker can be tacked back and forth easily without creating drag. The sheet should always be secured to the boat and not held in the hand of a crew. An inch of hand movement is an inch of forward momentum lost. Even 0.1 kt of drive created is better than 0.0, and when more speed happens, and it is deemed that the sail's curvature can sustain laminar flow, the windseeker can come down and the genoa up. When running the wind pushes the boat instead of creating drive, if there is enough wind to fill the total volume of the sail (the area created by curvature from luff to clew). A limp drooping larger sail cannot have push, if the total sail if not filled by the random shifting puffs. The puffs will push only on a portion of the large sail's surface. A push on one part of the fabric will only allow another part of the curved fabric to collapse inward, thus no force is transferred to the boat via the sheet to winch. Again, the windseeker takes only a small percentage of available wind to fill and push. When the wind increases to fill the curvature volume and create push on the genoa, it can again be raised. Annette ************* Thanks Annette and Monique; Great explanation! I am still skeptical on two points; the 20-30% seems excessively small and the small spinnaker downwind doesn't compute with me. [also the part about hand held sheet]. Given the kite string for sheet, the lightweight cloth, and if you were to go up to maybe 60 % at least, I would be more optimistic. I would also think that a bit of give from a hand held sheet would not lose power but gain. And that the bigger the chute, the more [cross section of] air it catches, hence more power. The idea that the give and take of a properly shaped sail is lost power bothers me because that seems counter to Newton. If something gives forward there is a decrease in force in some vector, true, but an equal and opposite reaction occurs when it recovers. Plus there is gain by keeping the shape optimal. But the real question is, do those who use this technique win races? The more I think about this the more arguments I can come up wth.. But mostly it is a matter of remembering the "feel" of it. If I got a little pufff I would let out my sheet the inch or three you mention and then very slowly bring it back in feeling the power as I did so. Use 40# test fish line and wear gloves. I'm thinking more like 80% J and cut high but full. For dwnwind, use 3 spinnakers; a huge one in the center and two "studding" chutes to the sides to catch and funnel air into the center. These would each have laminar lift at their outer edges producing a force vector at outward angles but counteracting each other. :-) [yeah, I'm crazy!! you recall Mad Max?] Thanks, Mark *********** We aren't talking about spinnakers, but now that you mention them, there is no way a puff of air will lift a spinnaker up out of the water and fill it to move the boat. We aren't talking about five kts of wind, but NO wind, but as you know, the water is never without some air movement, and the windseeker can find these small whisps or puffs. The windseeker is very light crisp sail cloth. It doesn't droop and sag. Argument for an attached-to-the boat sheet--The sail doesn't propel the boat, but transmission of the energy or drive via a solid attachment to part of the boat it is supposed to move. It the sheet pulls the sailor, that does not propel the boat. I haven't seen a sailor yet who could anticipate a gust of wind and be ready to brace himself so that the energy is transferred to the boat instead of to his body. Consider, that there is really only one point of trim that is optimal for creating the greatest drive. If during the gust this is altered, you no longer have laminar flow, but allow the wind to break away from the sail. The person holding the sheet is attached to the boat so whatever force is absorbed by the person is transferred to the boat; just not so suddenly. One of the other students suggested a bungee. I was reminded of the difference between a sudden hard throttle power or a more protracted lower gradual throttle setting. Which has the greater effect on a boat. The sudden hard power is largely wasted because of the mass it is trying to get in motion. The other thing would be, you sheet out on a puff because the apparent wind shifts aft on a puff. Then it shifts forward again as the boat accelerates. This would argue for the bungee, but when you're starting from 0.0 that's probably not an issue. But because the sail will be expanding its shape on a puff, I might think the principle would still apply. - Mark Newton wasn't talking about drive/lift. Drive only continues when the laminar flow continues along the curvature of the sail. The undulation of an unfilled sail won't allow the wind to flow, but will cause it to lift off the surface, therefore drive stops. The same thing happens when pinching too close to the wind. At the moment of luff, boat speed drops drastically, because the laminar flow has stopped. And back to Newton, if you mean action Vs reaction, then consider that an undulation of the sail that allows laminar flow will move the boat forward, so therefore an undulation of the sail fabric in the opposite direction will actually create the opposite reaction, i.e., move the boat backward.???? My allusion to Newton would involve several of his propositions having to do with mass in motion [or not] and forces. If a force into the cloth downwind is absorbed by an undulation, I think that will still translate to the boat as a force downwind; no laminar flow involved. Across or upwind it all depends on maintaining sail shape exactly as you say, and that will depend on this light crisp cloth you speak of. And perhaps it is very smooth too; I am visualizing a sail made of crisp saran wrap. But my "gut" still wants 60-80%. - Mark Yes, we do win races, because the boat goes faster. Anything better that 0.0 is good. We douse sails that can't stay full and put up lighter smaller sails that can maintain shape in the little bit of air available. Your arguments require wind to keep your spinnakers full. The windseeker is not for conditions that support a spinnaker or a 150% sail, but when it won't. 40# test is too small, but the smallest non-nylon line----1/8 in. or 3/32. Keep thinking, the world needs a new mathematician! Annette |
