How to Keep a Dry Bilge, Including Dry Bilge Pump Systems

Every bit of water you stop is water you don't have to pump. Work through where is the water in my bilge coming from. The common ones on cruisers and sailboats, from PassageMaker's list: spray through hullside air boxes, a dripping stuffing box or rudder post, leaky hoses and fittings, deck and hull joints, clogged hatch drains, overflowing shower sumps, and AC or refrigeration condensate.

A traditional stuffing box is meant to drip a little while the shaft turns, and that water ends up in the bilge. PassageMaker suggests considering a dripless shaft seal at your next haulout if you want a dry bilge. If you keep the stuffing box, make sure it isn't dripping more than it should, especially at rest. See stuffing box and dripless seal.

PassageMaker warns never to let shower water drain into the bilge: soap, hair, dirt and sunscreen quickly make it reek, and hair can clog the bilge pump.

• Shower and sink drains should go to a sump box with its own pump and float switch, then overboard. If the sump pump fails, fix it; don't disconnect the hose and let the shower drain into the bilge. See sump and macerator pumps.
• AC condensate from each unit's drip pan should run to a sump or an overboard fitting above the waterline, not into the bilge. A condensate line that just ends in the bilge can add a lot of fresh water on humid days.
• Refrigerator and icebox drains need the same treatment.

Limber holes are the drain openings through stringers and frames where they meet the hull, so water can run to the pump. PassageMaker recommends keeping them clean and clear, using a coat hanger to unclog them. Run the wire through each one and flush with a little fresh water. A blocked limber hole leaves a pocket of standing water the pump never reaches.

A dry bilge starts as a clean one. PassageMaker's method: wet-vac out the standing water, wipe surfaces with a household spray cleaner and rags, then dry by hand. Don't flood an oily bilge with sudsy cleaner: it spreads oil over more surfaces, and the BoatUS Foundation notes many bilge cleaners just suspend oil so it gets pumped overboard anyway. Use oil-absorbent pads under the engine and next to (not touching) the pump float. See bilge cleaning.

Knowing how your pumps work helps you choose the fix:

• Centrifugal pumps (most submersible bilge pumps) move a lot of water and can pass small debris, BoatUS explains, but they're designed to work submerged and they lose output as they push water higher.
• Diaphragm pumps are self-priming: they suck water up through a hose like a small vacuum, so the pump can be mounted up and away from the wet bilge. They move less water and cost more, and BoatUS notes their internal valves can clog if too much debris gets in.

That's why a dry bilge setup uses a small diaphragm pump for the last inch, while the big centrifugal pump stays in place for real volumes of water.

Before installing anything, see how dry you can get the bilge by hand. A sponge and bucket, or a small wet-vac, done after each trip may be all you need.

BoatUS describes an owner whose 1986 engine compartment still looks new because the bilge under the engine gets towel-dried after every trip: less humidity, less rust, and any new leak shows up right away.

For hard-to-reach pockets, Practical Boat Owner showed a simple portable setup: a cheap 12-volt diaphragm pump on a board that sits on a bucket, with a suction hose to reach into corners and a plug for a 12-volt outlet. Small diaphragm pumps like these, or a hand-operated one, reach water that never flows to the main sump.

If the water keeps coming back (a stuffing box drip, condensation), an installed system does the work for you. A typical kit, like the Rule Dry Bilge Pump described by Xylem, includes:

• A self-priming diaphragm pump, mounted high and dry, that can run dry without damage.
• A pickup hose that goes to the lowest point of the bilge.
• Built-in backflow prevention so water can't run back into the bilge.
• Its own small thru-hull fitting for the discharge.
• A panel switch.

The maker says this type of pump draws the bilge down to about 1/8 inch. Some systems have an inline strainer; the Rule kit is designed without one. Others run on a timer or a delay switch. Read the kit's instructions for wiring and hose limits before you buy, and confirm it suits your bilge depth and the distance from pickup to pump.

• Pickup: at the very lowest point of the bilge, usually just aft of or beside the main pump, secured so it can't wander. Make sure it doesn't foul the main pump's float switch.
• Pump: in a dry, easy-to-reach spot above the bilge water, within the suction distance the maker allows. Diaphragm pumps aren't waterproof.
• Discharge: through its own thru-hull well above the waterline. BoatUS warns that low discharge fittings let water siphon back in when the boat is loaded or heels; on sailboats, think about where the waterline sits when heeled. Don't add check valves to bilge discharge lines; BoatUS advises against them because they can stick or clog.
• Hoses: as short and direct as possible, supported so they can't chafe or kink, with stainless clamps.

• Drill the thru-hull hole, seal the fitting with marine sealant and tighten it as the instructions say. If the hull is cored, seal the exposed core first or have a yard do it.
• Wire the pump on its own fused circuit, using the fuse size the maker specifies, with tinned marine wire and heat-shrink connectors.
• Keep every connection above the bilge water. Corroded connections down in the bilge are the most common cause of bilge pump failure, BoatUS notes.
• Wire it so it doesn't interfere with the main pump's automatic float switch. Your main pump must still work with the battery switch off.
• A very dry bilge makes a high-water alarm even more useful, because any water that does collect is news. Don Casey (BoatUS) mounts the alarm float above the main pump's switch so it sounds only if the pump can't keep up or fails; keep the dry bilge pickup well below it so the two never fight.

See bilge pump wiring for connection details. If you're not comfortable with 12-volt wiring, have a marine electrician do it.

Pour a bucket of water into the bilge. Let the main pump do the bulk, then run the dry bilge pump and watch it pull the last of the water. Check outside that it's discharging, that nothing drips at the thru-hull or hose connections, and that the bilge is down to a damp film. Then check the main pump still cycles automatically when you add more water.

Even a pumped bilge stays damp without air. PassageMaker's tips: on breezy dock days, open hatches, doors and drawers so air reaches every space; lift lazarette hatches if the underside is damp; check that hatch gutter drains are clear and run overboard; and dry the anchor rode once or twice a season. A small dehumidifier or a solar vent helps on a boat that sits closed up. See mold and mildew prevention.

• Run the dry bilge pump and look at the discharge monthly. BoatUS suggests the same monthly routine for the main pump: rinse the bilge with fresh water, clear debris from the pump and float, and test it from the helm switch and by lifting the float.
• Inspect the diaphragm and valves at least twice a year; BoatUS notes rubber parts in pumps crack and deteriorate with age even when the pump isn't used much.
• Keep oil out of the bilge so the pump never discharges a sheen.
• Before freezing weather, follow the maker's winterizing instructions so water left in the pump head and hoses can't freeze.

Log each check in BoatMend alongside your main bilge pump tests.