Where Is the Water in My Bilge Coming From? A Step-by-Step Leak Finder

This is the cheapest clue and often the most useful. Over a week or two, write down:

• After rain only? Points to hatches, deck fittings, windows, scuppers and drains (step 5).
• Only after running the boat? Points to things that leak under way or only when the engine runs: shaft seal, raw-water cooling, exhaust, spray getting in (steps 8, 9 and 12).
• Steadily, even in dry weather at the dock? Points to something below the waterline: through-hulls, hoses, the shaft seal, the hull (steps 8 to 11).
• Only when you use the water system or AC? Points to the freshwater system or condensate (steps 6 and 7).

A bilge pump counter or a "pump running" light makes this easy. See backup bilge pump and alarm. While you hunt, a high-water alarm is cheap protection: Don Casey (BoatUS) describes a simple float switch mounted in the low point but higher than the automatic pump's switch, wired through a fuse to a loud buzzer at the helm, so it only sounds if water outruns the pump or the pump fails. Test it now and then by lifting the float.

If you boat in salt water, find out whether the bilge water is salty. Fresh water means rain, the freshwater tank and plumbing, AC condensate or melted ice. Salt water means the sea is getting in: through-hulls, the shaft seal, raw-water hoses, the hull itself.

Ways to tell, safest first:

• Salinity tester or TDS meter: dip it in a cupful of bilge water and compare with a cup of water from over the side and one from the boat's tap.
• Dry test: let a few drops dry on a dark, clean surface. Seawater leaves a white, crusty salt ring.
• Taste test: many owners touch a fingertip to the water and then to the tongue. Only do this if the water is clean, with no fuel or oil smell and no chance it's from the head or holding tank, and spit and rinse afterward.

On freshwater lakes this test can't separate rain from lake water, so lean on the other clues.

You can't trace a drip in a puddle. Pump down, then sponge or wet-vac out the last of the water (a dry bilge also stops odors; see dry bilge system). Wipe the hull dry along the keel and under the engine.

Then lay white paper towels in the lowest spots of each compartment, and wrap them loosely around suspect fittings and hose ends. Check after a few hours. The wet towel furthest "upstream" (forward or higher) shows you where to look.

Water runs downhill and aft to the lowest point. With the bilge dry, start at the pump and work forward and upward with a flashlight and mirror:

• Look for wet trails on the hull, stringers and hoses.
• Check the limber holes (the drain holes through the stringers) and see which side water is coming from. PassageMaker suggests a coat hanger to clear any that are blocked.
• Look outboard of the engine and stringers. PassageMaker notes that puddles there often come from spray leaking through hullside air boxes or from clogged drains.
• Look for rust trails on clamps and engine mounts, a sign of a long-term drip from above.

On a dry day, with someone below watching with a flashlight, spray one area at a time: hatches, hatch gutters, windows and portlights, deck hardware, rod holders, the hull-to-deck joint, cockpit drains. Start low and work up so you know which area caused the leak.

BoatUS investigators who examined boats sunk at the dock by a heavy rainstorm found the same problems again and again: cockpit scuppers that couldn't keep up, hatches without gaskets or not dogged down, and gutters and drains spilling into the bilge. PassageMaker adds: check that hatch gutter drains are clear and lead overboard, not into the bilge, and that the anchor locker drain isn't clogged.

On a trailer, rain pools wherever the boat sits lowest. Keep the bow raised so water runs aft and out of the open drain plug. See cockpit scupper drains, hatch and portlight leaks and deck hardware rebedding.

If the bilge water tests fresh and it hasn't rained:

• Turn off the freshwater pump at the panel for a day. If the bilge stays dry, the leak is in the pressure side (faucets, fittings, the water heater, the pump itself).
• Listen for the pump cycling when no tap is open. That's a pressure-side leak. See freshwater pump troubleshooting.
• Check the tank fill, vent and overflow, the water heater relief valve, and the shower drain. PassageMaker warns that a shower draining into the bilge instead of a sump, or an overflowing shower sump, is a common source of both water and smell.

Air conditioners and some refrigerators make a lot of fresh water on humid days. PassageMaker lists condensate drips from AC and refrigeration units among the usual sources of bilge water. Find the drip pan under each AC unit and follow its drain hose: is it disconnected, clogged, or simply routed into the bilge? If the water only appears on hot, muggy days with the AC running, this is your answer. Route it to a sump or overboard, not into the bilge (see dry bilge system).

On inboard boats, look at where the propeller shaft leaves the hull.

• A traditional stuffing box (packing gland) is designed to drip a little while the shaft turns, so the packing stays cool. It should drip much less, or not at all, when the shaft is still.
• A dripless shaft seal shouldn't leak at all.
• Check the rubber hose (the shaft log hose) and its clamps for cracks and rust too.

PassageMaker notes that an old-style stuffing box admits water that must be removed. Also grab the shaft log itself (the tube the hose fits onto) and make sure it is solid. BoatUS points out that a steady leak there can come from a cracked shaft log tube rather than the packing, and owners have found older boats with a separate metal shaft tube that had worked loose. A steady drip with the boat at rest, or water spraying while under way, means adjustment or repacking: see stuffing box and dripless seal. Do the same check where the rudder post goes through the hull.

These leaks often appear only with the engine running, which makes them easy to miss. With the boat secured and someone at the helm ready to shut down, look (don't reach) at:

• The raw-water strainer lid and its gasket (see raw-water strainer).
• The raw-water pump and its seal; on some engines the pump is belt-driven and only leaks while it's turning.
• Every hose and clamp from the intake through-hull to the exhaust.
• The exhaust hoses, risers and elbows.
• On sterndrives, the engine block drain plugs and the exhaust Y-pipe.

Shut down before touching anything. Replace any rusty clamp and any hose that feels soft, cracked or swollen.

Go through each through-hull fitting: engine intake, generator, AC, head, livewell, washdown, transducers and speed sensors, cockpit drains. At each one:

• Wipe it dry and wrap it with a paper towel.
• Look for weeping where the fitting meets the hull and where the hose meets the fitting.
• Work each seacock handle; it should move.
• Look for cracked plastic, green or pink corrosion on bronze, and double clamps where they're needed.

This is where BoatUS's "small part below the waterline" sinkings start. See seacocks and through-hulls.

On boats without inboard shafts, the usual suspects are at the stern:

• The drain plug and its tube or fitting.
• Screws and bolts through the transom below the waterline (transducer, trim tabs, swim ladder, motor mounting bolts).
• Livewell and washdown plumbing.
• Hatches in the splashwell or motor well; BoatUS found unsealed hatches there sank several boats in a rainstorm.
• On sterndrives, the rubber bellows and the transom assembly (see sterndrive bellows and gimbal).
• On aluminum boats, loose or weeping rivets (see aluminum rivet leaks).

BoatUS's claim files point to the same three culprits again and again for boats that sink at the dock: sterndrive bellows (it suggests looking at them every couple of months and replacing them every three to six years), leaking cockpit and livewell plumbing, and leaking stuffing boxes. Check those first.

On a trailer, fill the bilge partway with a hose (plug in) and look for drips outside; then raise and lower the bow with the tongue jack to see whether the leak changes with the water level.

If the bilge only fills when running:

• Spray: water forced in through vents, air boxes, a leaky hull-to-deck joint, or the anchor locker.
• Bilge pump discharge: if the hose doesn't loop high above the waterline, water can siphon back in as the boat heels or squats. BoatUS suggests the loop be at least 18 inches above the waterline (see bilge pump).
• Scuppers and drains that sit near the waterline when the boat is loaded or squats at speed.
• The shaft seal, which may only spray when the shaft spins.
• Fittings that go under at speed. BoatUS explains that at displacement speeds the waterline rises at the bow wave and the stern quarter-wave, which can submerge through-hulls that sit dry at the dock. Note at what speed the leak starts.
• Sailboats that leak only when heeled: change tacks and see if it stops. A leeward fitting siphoning, or a keel bolt or keel sump working under load, are the usual suspects.

Have a helper drive in calm water while you watch the bilge (wearing a life jacket, away from moving parts).

Water can enter through the hull itself: cracks, a damaged keel joint, loose keel bolts on a sailboat, or a soaked core around fittings. Mast steps and chainplates let rain in on sailboats. If none of the above explains the water, have the boat hauled and the bottom and keel inspected. BoatUS's tip for that haul-out: leave the bilge full of water and look outside for the telltale trickle. And when a through-hull or other bedded fitting is the culprit, smearing sealant around its edge rarely works; the lasting fix is removing the fitting and rebedding it, which usually means a haul-out.

Fix one source at a time, dry the bilge, and watch the pump counter or paper towels for a few days. Record what you found and fixed in BoatMend. Keep oil-absorbent pads in the bilge; BoatUS Foundation reminds boaters that pumping oily water overboard can bring hefty fines.