Unbelievable the details I have learned about M-boats from all your postings. This is stuff generally accumulated through hard knocks and a lot of years. One of the obvious facts is that 15 and 17 masts can be different, and I suspect the years have changed "factory" boats a lot. My main appears to be factory stock (albeit tanbark) with the first slug 18" up, and the remainder spaced about 26". Along the foot, the sail has slugs only at the tack and clew, so I imagine it would be called "loose-footed". The reef cringle is 36" from the tack. If I ride the boom some 12" from the deck (?), the first slug will sit about 2" below the mouth (which is located 32-36" up the mast) and will not be affected by reefing. The second slug could fall out during reefing or shaking out a reef, and the uppers could escape during lowering or raising, but I suspect that my mast design might prevent this. The mouth is made by cutting a narrow trapezoid from one side of the extrusion, just large enough to pass the gooseneck and slugs if they are properly positioned. Unless Murphy is around, It seems likely that slugs would tend to pass right by without exiting. In any case, a trapezoid cut from lower down for a new opening could perhaps be welded into the upper opening, and voila! By the way, how far from the base are you making the new mouth? Tom Jenkins M17 #426 (2004)