Jon, there are different types of impact screw drivers. The simplest is a rather heavy piece of metal, about an inch diameter, with a spring inside. Generally there are two settings, one for tightening and one for loosening(why one would need the former is beyond me). When you whack the end with a hammer, the spring compresses and the screw fitting on the end(blade, Philips etc.) converts the whack to a twisting motion. Bob's method is similar, though in that case your hand does the twisting. I would tend to put some penetrating oil on or WD 40 and let it stand for a good bit of time before I used an impact driver. As it has been in marine environment, go judiciously as metals don't do well there.
Erich in Seattle
Erich in Seattle