I always have at least two, and I abuse the hell out of 'em. The biggest job is dust, mostly from fiberglass, teak, metal shavings and some kind of terrible black, original glue dust that the builder used on walls and roof panels. I use them to clean out head clogs, bilge, thru-hulls, vents, and the occasional wasp nest. Here is a shot of my smaller unit drawing moisture out of the core of my transom. When I drilled the holes for the swim platform brackets, one of them was weeping moisture. I found the lowest place on the transom that was still cored and drew a vacuum on it while leaving the other holes open for air intake. Turns out that there wasn't much in it, but many yards use the same process to dry wet cores.