written by Matias Collins on November 15 of 2004 and read by 1146
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The tighter the uppers the less the effect - on mast bend - of moving or tensioning the lowers - and vice versa. When the uppers are eased (400lbs seems ideal for all conditions) slight changes in the position or tension of the lowers make big changes in the mast bend.
To induce lateral mast sag (mid-mast sagging to leeward) and increase mainsail fullness and power in light to moderate air and/or waves, the uppers should be forward and the lowers should be loose so that they can easily be moved by hand (thru a 15 cm range in 8-10 knots). In winds greater than 12-14 knots to eliminate the mast sag (make the mast straight laterally) and flatten the mainsail, the lowers should be tightened progressively and the mast bend checked.
Moving the uppers (and the lowers) forward (of their neutral position) increases their tension, bends the mast fore-and-aft ("pre-bend"), flattens and moves the draft aft in the mainsail, and increases jibstay sag - trim that is fast in smooth water and light to moderate air. .
Moving the upper shroud cars aft to their neutral position decreases the tension in the uppers, allows the tip of the mast to fall to leeward, straightens the mast fore and aft, opens the leech, and thereby flattens the mainsail. The degree of tip fall-off (lateral bend) will depend on the tension in the lowers (the effect of tightening the turnbuckles and/or the movement of the lower shroud cars aft). The tip should fall off (with the upper cars forward) in strong gusty winds and smooth water, but the mast should be straight or, with a heavy crew, sagging to leeward in strong winds and waves.
I never move my upper shroud cars aft of the mid-mast position (about 4" aft of all the way forward).
The only way to get the lateral mast bend right for the conditions (and equal on each tack) is to check it by sighting up the front of the mast when going to windward with two crew hiked.
Shroud car ramp angles vary. You should check to see that the tension in the uppers decreases when the cars are pulled aft to the mid-mast position and that the tension in the lowers increases.
Please let me know how and why you trim your shrouds differently! and write me at
Article by Stuart Walker
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