Join Date: Feb 2000
Location: Annapolis, Md
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You are probably right that you would not notice the difference between a shelf foot or a loose foot main vs. a traditional attached foot mainsail, if you are sailing a low performance boat like a Challenger 40 ketch, especially in a high wind venue like San Francisco.
But for the rest of us, the reality is that a shelf foot or loose foot, allows the lower portion of the sail to have a closer to proper shape all the way to the foot, whereas a traditional attached foot flattens the lower portion of the sail reducing the performance and increasing heel by over-flattening the largest portion of the sail. This ability to shape the lower portion of the sail offers a real advantage on higher performance boats.
Once you start trying to shape the lower portion of the sail, there are two contradictory arguments that get made; a shelf foot acts as an end plate increasing efficiency by preventing 'pressure leakage' and decreasing tip vortex but loose foot sails have more area and theoretically generate less turbulence at the boom.
In terms of your comment that the bolt rope at the foot of the sail distributes the load far more fairly than points at the tack and clew, with less reinforcing needed at these two connections, that is only true on a low performance sail which lacks a shelf foot. Once the sail has a shelf foot, there should be no load distributed to the bolt rope, and so the load distribution and corner reinforcing is virtually the same between a shelf foot and a loose foot.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay