Bilge Pump Replacement

By Jan Mundy

Boatbuilders often mount bilge pumps in inaccessible areas deep in the bilge, usually before the deck is in place, unmindful of replacement or servicing. Particularly in small powerboats, replacement demands crawling through a small hatch, often working blind. (Use a mirror for a rearview look if you’re adept at working backwards.)

Submersible bilge pumps typically run for nine years before failure and less for leaky boats where the pump regularly cycles or dry boats, where lack of use causes corrosion or grit, sand or salt build-up. Failure also occurs when water gets into the pump through wires or shaft seal — the pump fills with water then goes through several freeze-thaw cycles. This was the case with a Mayfair pump in one of DIY’s test boats when it failed to run after 10 years of normal use in freshwater. Replacement is an easy one-hour job, after removing the old unit, drying the bilge and sealing the original mounting holes.

1 If you’re lucky, the original pump has a detachable base, so removing mounting screws is easy. The Mayfair unit didn’t; removal meant dangling in a narrow stern locker, groping with hands, trying to locate three Phillips-head screws. For bilge work, an extra-long handled screwdriver comes in handy. Note the corroded hose clamps in the photo (above).

2 Since pumps rarely have the same footprint, the next step is to seal the original mounting holes with 3M Premium filler, 3M 4200 (I prefer the Fast Cure) 5200 or equivalent brand. Be sure holes are dry before filling. While you’re down there, inspect the hull for other openings in the hull and properly seal. I found a few extra holes drilled by the manufacturer, none were sealed. (The very first item on your to-do list after purchasing a boat, either new or used, should be to rebed every piece of hardware, caulk voids, holes, etc.)

3 Placement of pump and switch is critical. Even when replacing, it’s best to double-check the location. Position the pump base (strainer) so the nozzle aligns with the discharge hose and locking tabs line up with the pump body. The float switch should mount at least 1/4" (6mm) above the strainer so it’s not automatically running all the time. Rule pumps come with an adapter that connects the switch to the pump base and positions the switch at the proper height. Pumps are prewired and include fasteners; you’ll need to supply sealant and heat-shrink connectors.

4 Carefully drill a pilot hole in the hull for the pump base, liberally caulk around the hole, then fasten base with supplied screws. Don’t drill through! Never mount base directly on a cored hull — a leaking fastener can quickly delaminate cores below the waterline. Instead, install a hardwood (teak or mahogany) backing block, then attach base to block.  

5 Connect the hose, double clamp (if enough clearance on hose barb) and install the pump switch.

6 You have an option of wiring the pump for manual or automatic operation. Since this boat already had an automatic-manual pump control, we stripped the lead ends, connected negative and positive ends to the original wires leading from the panel and battery, routing the wires high, just underneath the deckhead. Ends were crimped together using adhesive-lined, heat-shrink connectors. If you don’t have any, seal the connectors with sealant. Note: positive lead from bilge pumps should connect directly to the positive battery post so as not to accidentally disconnect with the master switch. Also, all pumps must be fused and with the proper size fuse or a pump may overheat and blow a circuit. The original pump installation included an in-line 6-amp fuse, the new Rule pump required a 9-amp fuse.

7 Not included but recommended is a protective float switch cover. For extra reliability, add a second switch mounted slightly higher. Better yet, add a second pump.