On my M17, I have a long shaft motor and for most motoring jobs, it works fine. But it too will cavitate when I go forward to raise or lower sails when motoring. Especially in a chop. I would think that would be more evident on the 15. BTW.....Good Old Boat just ran a short piece on a neat trick to use when raising or lowering sails under power. The trick is to tie a bungee from your boom to the tiller. If the boat falls off, as it normally does, the main swings over, dragging the tiller with it to bring the boat back up into the wind. You need enough throttle to keep the boat moving into the wind. If she tacks, it continues to work the same way. If the sails fill and she starts pulling, it should work the same way. My guess is the length of the bungee will have some affect on how close to the wind she stays.