Sailing to Windward With the Gentry Tuft System - ArvelGentry.com

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. Some Telltale History Somewhere back in time, a competitive sailor placed a yarn telltale about two feet back from the luff. When the leeward telltale twirled it indicated that the sail was stalled and the boat slowed down. But why was it placed two feet back from the luff? In Tom Whidden's book, The Art and Science of Sails, page 196, he states that the telltales "should be between 12 and 24 inches back from the luff--the bigger the boat, the farther aft--to keep them away from the confusing local turbulence, or form drag, of the headstay." Well, this is not exactly correct. "...confusing local turbulence", yes! If the leeward telltale was placed too close to the luff, it would often twirl even though the boat was still sailing fast. Confusing because of the "form drag of the headstay", no! The fact is, if you could magically replace the genoa with a curved sheet of aluminum without a headstay, the flow near the leading edge would still be "confusing." The cause of this confusion is the sail leading edge separation bubble that I discovered and described in the SAIL magazine article (March 1973). After the publication of my tuft system article, grand prix boats, maxis, and even America's Cup boats began to use several telltales in a row instead of the single telltale. They were placed about a foot apart, with the first one a foot or more from the luff. With dacron sails it was usually difficult to see the leeward telltales, and because of their wide spacing, they could not really be used as I had originally described. Instead, sailors started using the first windward telltale as an indication of when the sail is about to luff. In addition to the widely spaced row of telltales that the big boats used, they also usually had a round window with a single pair of telltales. The leeward telltale in this window told them when the sail had already stalled. They had nothing to tell them where they were between the luffing and stalled condition except the experience of the sailor. However, my Tuft System tells you when the leeward separation bubble is formed and how big it is. The size of the bubble tells you when you are close to luffing, when you are close to stalling the entire sail, and how rapidly you are changing from one condition to the other. This helps in both driving the boat and in coordinating sail trim as the wind and sea conditions change. The Leading Edge Separation Bubble The leading edge separation bubble is best understood by studying the tuft system diagrams shown on the first page. These diagrams show only about the first 30 inches of the leading edge of a genoa on a large boat, or about 16 inches on a boat the size of the Ranger 23. The airflow is from the right to the left. Four short pieces of yarn or spinnaker cloth, the tufts, are shown on each side of the sail. Note that the first one is placed as close to the leading edge as possible, and that the subsequent tufts are placed head-to-tail. Each of the drawings show the sail at different angles to the wind. -2- Ranger 23 Newsletter The thin lines with the arrow show the direction of the air as it comes into the sail, and are what aerodynamicists call stagnation streamlines. They represent the dividing line between the air that flows on the lee side of the sail from the air flowing on the windward side (the lower side shown in these diagrams). The separated regions are shown in gray. The angles shown in these diagrams and the changes in the stagnation streamline have been exaggerated for illustrative purposes, as has the thickness of the separation bubbles. Diagram A shows the sail as it is just starting to luff. The stagnation streamline is coming into the leading edge slightly on the lee (top) side of the sail. As the air on the windward (lower) side of the sail tries to make the turn around the leading edge it will separate and form a bubble, shown in gray. The air inside the bubble is very unsteady, and is made up of twists and swirls of very confused air. Any tufts inside of the bubble will twirl wildly. The bubble gets thinner toward the back end, and tufts downstream of the bubble will not twirl but will lie smoothly along the surface of the sail. Except for the fact that the sail is about to luff, the windward bubble has only a small effect on the performance of the sail. On a real genoa, with the headstay swept back at an angle, the general direction of the swirling air within the bubble will be upward. This is why the usual single windward telltale will lift and twirl somewhat upward when the sail is about to luff. Diagram B shows the sail at a slightly higher angle of attack where the stagnation streamline is coming into the exact leading edge of the sail. Precisely at the point where the stagnation streamline hits the leading edge, the air actually comes to a complete stop (that's why we call it the stagnation streamline). The airflow is smooth on both sides of the sail, and in fact starts to accelerate immediately as it splits to flow on both sides of the sail. As we know from modern sail theory, the air on the leeward side accelerates much more rapidly than the air on the windward side. This leads to a lower pressure on the leeward side than on the windward side, giving us the pressure difference and lifting force necessary to make our boat go. Since the flow on both sides of the sail right at the leading edge is accelerating smoothly, no separation bubble is formed. The sail is pointing precisely into the wind. In Diagram C the angle of attack of the sail has been again increased slightly. The stagnation streamline is actually around on the windward side of the sail. As the air flows around onto the leeward side, it separates in the form of a small bubble with only the first tuft slightly agitated or twirling. Aft of this short bubble the flow is again reattached to the sail as shown by the fact that the other tufts are lying down smoothly. The sail is not stalled. For many boats this represents the best condition for windward performance. In Diagram D the angle of attack has been increased even more by heading the boat off slightly. The bubble has
. grown in length as indicated by the fact that the first three tufts are twirling. If the first tuft is very close to the leading edge, the windward tuft may even twist around to the lee side of the leading edge. In Diagram E the angle of attack has been increased even more. But now, the bubble has burst and extends clear past the trailing edge of the airfoil. The whole sail has stalled. Lift goes down and drag goes up. The boat slows down and does not point well. By now you should have the idea as to how the Gentry Tuft System works. Not only does the system tell you how close you are to luffing (all the leeward tufts lying down), how close you are to stalling the sail (the first two or three leeward tufts twirling), but how rapidly you are changing between these two conditions. If you need to point high, make all the leeward tufts lay down, with only the first one occasionally twirling. For better speed, bear off slightly so that the first leeward tuft is agitated. For more power, briefly head off allowing the first, second, and maybe the third tufts to twirl. The key in using this system is to watch the tufts on the leeward side of the sail. Communication between the helmsperson and the genoa trimmer is obviously very important. If the trimmer sees the first, second, then third tuft twirl in succession, he/she must either let out on the sheet some, or ask the helmsperson to head up to avoid stalling the genoa. If all of the leeward tufts are lying down, the sail is about to luff. The boat should be headed off slightly or the genoa trimmed in some. Experience with the system also has a side benefit in that it provides more precise steering. It also allows the helmsperson to spend more time watching for waves and chop instead of watching for the sail to luff or watching a single telltale to indicate a stall, and all the time wandering back and forth between the luffing and stalled condition. The standard single telltale only tells you when the sail is stalled, and that's too late. With the tuft system you can avoid stalling the sail because you can see the stall coming before it happens. Tuft System Installation The actual length of the tufts and their placement on the genoa will depend upon the size of the boat. On a 23 foot boat the tufts should be only 3 inches in length. Placed head-to-tail, usually three leeward tufts will twirl just before the sail will stall. On a 35-foot boat the tufts should be about 6 inches in length. It may take some experimenting to find the proper length and placement for the sails on your boat. One approach is to install several rows of tufts with different lengths and spacing. You might also attach a regular telltale about four feet back on the sail to help in calibrating the tuft system for the fully stalled condition. Then go sailing to find out which row works as described in this article, and then remove all but the best row of tufts. The tufts on the port side of the sail should be red, those on the starboard side green. On a lightweight Mylar genoa -3- Ranger 23 Newsletter you may be able to see the tufts without a window. However, on some polyester-Mylar sails and on older dacron sails, you will want to install an elongated window in the sail so that all of the leeward tufts can be seen. The front edge of the window should be right up against the bolt rope or luff tape. The photo below shows the tuft system installation on Kittiwake's heavy number 1 polyester-Mylar genoa. If you have a good sewing machine you can install the window yourself. A Window Kit is available from Sailrite for $6 (Toll free at 1-800-348-2769). The key is to sew the window onto the sail first, then carefully cut away the dacron sail material with a sharp X-acto knife without nicking the plastipane. I installed one on Kittiwake's 130% genoa. It was easy and works great. For the Ranger 23, first make a cutting pattern on a piece of paper. Draw a 5" x 11" rectangle and, with a drawing compass, scribe a 5" diameter circle inside the rectangle at each end. This rounded rectangle becomes the template for cutting out the plastic window. Next, decide where you want the window on the sail. For a tall rig R-23, put it about 9 feet up the luff from the tack. You may have to adjust this up or down some to keep the window away from any sail panel seams. With a pencil mark a reference point on the luff tape at the chosen location. With a protractor measure a 72 degree angle down from the luff tape. This will be the centerline of the window and will allow the tufts to flow nicely in the streamwise direction when the sail is set. Lay the sail out smooth and, using the double stickybacked tape provided in the kit, attach the window to the sail. The front edge of the window should be right up against the edge of the luff tape. Now, carefully sew a row of zig-zag stitches around the outer edge of the plastic window. Turn the sail over and draw a line on the dacron that is 0.5" inside of the outer edge of the plastic window. This is your guideline for cutting out the dacron material. I used a sharp X-acto knife to cut out the dacron. I made an incision in the center, then carefully continued the cut to the guideline and around
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Sailing to Windward With the Gentry Tuft System - ArvelGentry.com

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