April 2002 — By Mike
|Part 2: Impeller Pumps|
An impeller pump works by creating suction with its flexible impeller, which is generally made of neoprene and turns inside a close-fitting housing. (That’s why lubrication is important.) A cam in the housing deflects the vanes of the impeller as it spins; this creates the suction that draws fluid into the housing. The function of an impeller pump depends on the vanes being intact and not excessively worn. Also, the housing and cover plate must fit snugly against the impeller to preserve the suction.
Impellers can be as fragile as actors’ egos, especially if allowed to run without fluid to lubricate and cool them. If your engine overheats because a plastic bag in the seawater strainer has cut off the supply of water, it’s a good idea to check the impeller before you start up again. Remove the faceplate from the pump housing–it’s usually held on by four or six screws–and open the pump carefully (sometimes you have to slip a knife blade under the cover to persuade it to pop off). The impeller inside will look like a paddlewheel, with the vanes pushed out of shape on one side by the cam. Even if the impeller appears okay, inspect each vane carefully to ensure it’s healthy and not torn across its base. When in doubt, take it out: If a vane comes loose and is pumped into the cooling system, it can block the cooling-water flow and cook a manifold.
Replacing an impeller is easy. If you are lucky, it fits onto a splined shaft with no setscrew holding it in place, and you can pull it out with a pair of slip-joint pliers; their widely adjustable, offset jaws are ideal for reaching into the pump. If the impeller won’t come out, there is probably a setscrew through the body and shaft. Remove one of the hoses, whichever one is easier, and bump the starter until you can see the setscrew through the unoccupied nipple. Remove the screw with a screwdriver, and the impeller will slide right out.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.