Submitted on November 29, 2006 by Rich Magner

Question: I have occassionaly come across comments about mast sag, where the recommendation is to adjust the mast so the middle sags to leeward in light air and conversely in heavier air, set it up to pop to windward.

- is this a legitimate consideration?
- if what is the theory?

Regards,
Rich Magner

Answer: Rich,

Thanks for your question. Mast sag, like mast bend, is used to add or reduce power to the sail plan. It is used predominantly on dinghies and smaller keelboats. On larger boats we work to minimize side bend.

Mast sag (to leeward) adds power as it adds depth to the main and narrows the slot. Tuning to have the middle bow to windward reduces power as it flattens the main and opens the slot. Side bend is controlled by tuning the lower shrouds: eased in light air to allow sag, and tightened in a blow to pop the mid-mast to weather.

Beware: The impact of side bend/ sag is proportionately greater than the impact of fore and aft mast bend. A little goes a long way, and too much can make the mast fall down – resulting in a dramatic loss of power.

Good Luck,
Bill Gladstone
www.northu.com
203 245 0727

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