Michael,
I agree that the “rule of thumb’ would mean that the headsail would be at about a 60 degree angle to the centerline of the boat. And I also see your point about the headsail at 90 degrees projecting the maximum area for force to be generated. The “barn door” being pushed by the wind. But if you note, in the area just above the sections labeled “drag predominate” are equations for the coefficient of lift and drag, and a graphical representation of the coefficient for various angles of attack (with sail area and apparent wind speed being constant) For a constant sail area and apparent wind, the maximum coefficient of drag is about 1.20 for a “barn door” at 90 degree angle of attack. With the angle of attack at 30 degrees, which is what you have with the headsail at 60 degrees to the centerline of the boat (and neglecting the impact of the aspect ratio of the sail), the coefficient of lift reaches about 1.50. So the sail at 60 degrees to centerline vs. 90 degrees to centerline, you would see about 25% more driving force from the sail. Now the force generated is also proportional to the square of the apparent wind speed. As the boat going dead down win accelerates to its maximum speed, the apparent wind speed will decrease. So the driving force will drop off significantly. With the angle of attack at 30 degrees, the apparent wind will stay higher as the boat accelerates, and so the driving force will stay higher. This impact is shown in the polar diagrams shown as part of the discussion you cite. Notice that the max VMG downwind is generated for a course of about 30 degrees off DDW for wind speeds lower than 10 knots, going up to about 10 to 15 degrees off DDW as the true wind speed approaches 30 and boat speed approaches 10. Bottom line, maintaining a curved shape to the headsail and flow over the sail generates more drive than a flat “barn door” sail. Rick Brass Washington, NC From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Michael Brown via CnC-List Sent: Wednesday, August 24, 2016 10:50 AM To: cnc-list@cnc-list.com Cc: Michael Brown <m...@tkg.ca> Subject: Re: Stus-List Assymetrical spinnaker pole Hi Rick, I am not familiar with the rule of thumb for optimum whisker pole length, though it looks like the geometry is setting the pole at right angles to the boat, and the jib holds a 60 degree angle at the tack. For light air dead run I set the jib at 90 degrees to the centerline to maximize projected sail area and ignore flow around the sail. As you say a barn door. About 3/4 the way down there is a chart in the section "Drag Predominate ( separated flow )": https://en.wikipedia.org/wiki/Forces_on_sails The driving force ( Fr ) in the right hand side diagram is at maximum with the sail at 90 degrees. Also the drag force is directly in line with the boat, presumably where the mark is, where as if the sail was at a 60 degree angle to the wind the lift would also be at 60 degrees. I am in an area where PHRF allows long whisker poles and no adjustment for length. In light air and flat water the barn door trim seems to be the fastest. Michael Brown Windburn C&C 30-1 From: "Rick Brass" <rickbr...@earthlink.net <mailto:rickbr...@earthlink.net> > If your J is 13?6?, the LP of your 155 is 20?11?. By the rule of thumb, the optimum whisker pole length is about 16?9?. With the 13?4?pole at the mast and perpendicular to the centerline of the boat, the line from the tack fitting to the end of the pole is 19?1?. Perpendicular to the center line would project the sail out as far as possible (which is what the whisker pole is supposed to do). There should have been some curvature to the sail, which you want so you get the maximum pressure on the sail (a cupped shape generates more pressure than a flat barn door shape). I find I usually run the pole about 15 to 20 degrees forward of the mast to promote that cupped shape. The LP of your 135 would be about 18?3?, so the spin pole would stretch the sail flat and still be a bit forward of perpendicular. (Optimum whisker pole would be 14?7?) You might need to run the spin pole at a forward angle to get the proper sail shape, though that doesn?t get you the maximum projection of the sail. And the essence of a whisker pole is that the length is adjustable, so you and match the length to the size of the headsail you are flying. Mike Hoyt?s comment that a pole longer than J costs you seconds in most PHRF areas is true. But not in all areas. And some of us are cruisers who want to get the most out of the boat, even though too short handed to fly the spin except on a log run. Plus I have always thought the use of J for max pole length in non-spin racing to be arbitrary and slightly inequitable in those regions that give a separate rating for spin and JAM racing. Seems like if you race JAM, you ought to be able to optimize the boat for JAM racing. Rick Brass Washington, NC
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