Short shaft
5hp should be sufficient to push you to hull speed. However, I would worry about using a short shaft model. In positioning outboard mounting brackets, manufacturers are trying to reconcile two opposing functions: the engine has to be low enough to keep the prop in the water, and yet high enough so that the helmsman can reach the controls from the cockpit. In order to accomodate both, sailboat manufacturers almost universally locate the brackets with the assumption that a long-shaft model outboard will be used. I use a 20" long shaft model on my Oday 23, and I keep the bracket in the lowest position all the time. In order to keep the prop out of the water in the slip, I tilt the engine up. Unless you are planning on keeping your prop in the water 100% of the time, you will have to raise the bracket and/or kick the engine anyway, even with a 15" short shaft. I don't see how switching to a short shaft will do anything to help your neck on that front. Not only that, but if you use a short shaft model, you can bet that your prop will be coming out of the water in anything other than the flattest, calmest conditions. This will not only be inefficient and annoying, but will also cause excessive wear and tear on the engine. If you are determined to use a short shaft model, I would recommend a couple of things: first, before you buy, measure carefully to make sure that the prop on the short shaft will actually be in the water. The cavitation plate (that flat plate above the prop, parallel with the water line) on short shaft models will be 5" higher than your existing engine. If your measurements show that plate won't be completely submerged on your new short shaft engine, you will have to lower the mounting bracket. Probably the easiest way to do that is to remove the bracket and the spacing block (that hunk of fiberglass between the bracket and transom), redrill new mounting holes through the transom, and just reposition the whole shebang however many inches lower you have to. You may need to reinforce the transom at the new attachment points (if the old attachment points had extra backing). Also, take care to make sure that it will be feasible for you to operate your engine when it is hung from its new, lower position. I have found the gear shift is hard enough to reach as is. If the controls will be lower than you can comfortable reach, you may want to consider installing remote controls. Also, if you can't raise or lower your new engine to keep it out of the water while its in the slip, be prepared to inspect and change your zincs, as well as to paint the submerged portions of the engine with bottom paint. One other option: most powerboats have systems installed that raise and lower their outboard engines. you may want to explore installing one of those instead of going through the above. good luck.
