My gaff rigged main sail is 200 square feet and the club footed self-tacking jib is another 100 square feet.
For a 13,000 pound motorsailer, the 300 square feet I have works well. (46 square feet/ton)
It is very effective for roll reduction with 10 knots of wind or more, at any angle except +/- 15 degrees on the nose.
For propulsion, it is effective at the same angles, giving me 2-3 knots of speed (or reducing my engine rpm significantly) in 10-15 knots of wind.
In 20 knots of wind, I can shut down the engine and sail at about 5 knots, or cruise at 7 knots with reduced rpm.
For roll reduction only, I'd say a steady sail would need to be at least 30 square feet per ton to be effective.
Retrofitting a mast, standing & running rigging and sails for the average trawler wouldn't be cheap. For most, it wouldn't make financial sense. The exception may be boats that have factory sailing versions available. (Gulfstar, Willard etc) It may make sense if the a full compliment of mast, boom, rigging, sails, can be purchaed cheap from old boat.
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