Back in 1992 I put a loose-footed mainsail on my racing boat, a Moore 24. Performance over the slug-footed main proved to be significantly better, with upwind speed easily 1/4 kt faster and pointing nearly 5 degrees higher in "normal" conditions. On a Moore 24 that's a lot of improvement. At the time I was a partner in one of the local sail lofts (now Doyle) and we were starting to build an increasing number of loose-footed mains for both racing and cruising boats. I recall that almost all of these customers were more than pleased with the performance of their new sails once they learned the (few) tricks to trimming them. Most of these had the top 2 battens full and the lower 2 longer than usual until the boats got over 32 feet where 4 full battens were the norm.
We also converted a number of slug-footed mains to loose-footed. While this gives the handling advantages of a loose footed sail, the aerodynamics (read "shape") is a compromise and not quite up to the performance level of a designed-from-scratch loose-footed sail.
One of the things we learned early on with loose-footed mains is the necessity of a proper sail slide at the clew. A standard slide, regardless of its material, does not slide freely enough in the track under load. We furnished our loose-footed sails with a 4-inch long stainless slide that was Teflon coated. The higher aspect ratio and lower unit pressures of this slide made for much easier adjustment as it did not "cock over" and jam in the sail track. This simple improvement along with sufficient outhaul purchase (at least 3:1) enabled easy adjustment under load.
One of the fallacious arguments you'll hear against loose-footed mains is that the force is concentrated upward at the very end of the boom. The fear is that the boom will tend to be bent by having the ends "fixed" with the mainsheet and/or vang pulling downward in the middle. In reality, this where the forces occur anyway even on a slug-footed sail as the interim slides are under almost no load at all. The dynamics of a loose-footed sail are nearly identical to a slug-footed one and the boom is designed to withstand that. This "old wives tale" sort of eyeball engineering simply doesn't hold up to the physics involved.
FWIW, I have a brand new Doyle loose-footed main on
Rhiannon that is getting ready for its first sail even as we speak!
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