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getaway helm problems
http://www.hobie.com/au/en/forums/viewtopic.php?f=9&t=12976		 Page 1 of 1
Author:  Roy [ Sun Jul 05, 2009 9:30 pm ]
Post subject:  getaway helm problems
OK so I think I know the whole concept of upwind or down wind helm. If the center of pressure (sails) is ahead of the center of resistance (hulls) we will have down wind helm effect. So the opposite is also true, that is if the center of pressure is behind the center of resistance we will have an upwind helm pull. What I can't understand is with my getaway in light air (under 10 mph) will have a very slight downwind helm tendency. But as the wind picks up the boat starts pulling more and more upwind, given the same crew sitting in the same place with sail trim the same how can the center of pressure or the center of resistance be moving?? I have put about 3/16” spacers behind the upper pintlel so the upwind effect is not so bad but with the new style rudder there is no other way to adjust the rake. I have also checked to be sure the rudders aren't creeping up either. My 16 never did this, once I had the rake set helm control was fairly neutral. Anyone have an answer??
Author:  srm [ Mon Jul 06, 2009 7:23 am ]
Post subject:  Re: getaway helm problems
The center of effort of the sailplan can definitely move. Since the sail is a flexible membrane (as opposed to say a rigid airplane wing) it tends to deflect and change shape as the windspeed increases. Typically, the CE moves aft (which is what you experienced) and the helm "loads up". One signal of this would be noticable luffing along the leading edge of the mainsail- a sure sign that the CE has shifted back a lot.

The way to counteract this is to tighten the downhaul. Doing so increases mast bend which flattens the sail and encourages twist in the upper section of the mainsail. It also encourages the CE to move forward and down. Increasing mainsheet tension will also flatten the main and help to lock-in the draft location (you may need to travel out slightly as well).

Last, keep in mind that the trim of the jib needs to compliment the trim of the main in order to maintain a balanced sailplan. If you ease the main, then you want to ease the jib accordingly (and vice versa).

sm
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