Water in the boat’s bilge: where is it coming from?

A little water in the bilge is often nothing to worry about: a weeping tank, rain coming in through a hatch, condensation. But water never gets into a boat without a reason, and a small seawater leak can become a big one. The right method comes in three steps: make sure the boat is not in danger, find out whether the water is fresh or salt, then follow its trail back to the source. The manual from your boat’s builder and your equipment manufacturers always has the last word.

1. What you notice

• A little water at every visit in the marina, always about the same amount.
• Water only after rain, or only after a sail.
• Water when the boat heels or slams into the sea, or after a long run under engine.
• A bilge pump that starts more often than before, or a fresh-water pump that starts on its own.
• Water that is oily, that smells of diesel or oil, or water that smells of sewage.

2. Safety first

If the water is rising fast, do not look for the cause yet: alert the crew, have everyone put on lifejackets, start the electric bilge pump and the manual pump. Meanwhile, look for the leak starting with the most likely places: through-hulls and seacocks, engine cooling hoses, stern gland or shaft seal, saildrive diaphragm, rudder stock. Closing the seacocks one by one (engine stopped for the cooling one) often lets you find and stop the leak. A torn-off hose or a broken through-hull can be plugged with a softwood bung (a tapered wooden plug) or a rag pushed in hard. If the water gains despite the pumps, do not wait: send a distress call on VHF channel 16 (MAYDAY), or in France dial 196 from a phone to reach the CROSS (the maritime rescue centre), and get the tender or the liferaft ready.

If the water smells of diesel, oil or sewage, do not taste it, do not pump it overboard, and ventilate: fuel vapours are dangerous.

3. The probable causes, from most to least common

Start by tasting the water with a fingertip (if it smells of nothing suspicious): fresh or salt, it is not the same family of causes. Be careful: rainwater that has washed a salt-covered deck can be slightly salty, and spray gets in through the deck just like rain.

If the water is fresh

• Fresh-water circuit — a tank fitting, hose, clip or tap that weeps; a water pump that starts without anyone opening a tap gives away a leak in the circuit. This is the most common cause of fresh water in the bilge.
• Tank — an inspection hatch, gauge or weld that leaks, or a tank overfilled and overflowing through its vent.
• Water heater — fittings, or a safety valve that discharges water; it may drip while heating, but a continuous flow is not normal.
• Drains — sink, shower, icebox, fridge condensate that do not end up where they should.
• Rain — window and deck hatch seals, deck fittings (chainplates, stanchions, cleats, a deck-stepped mast), anchor locker, a cockpit that drains badly.
• Condensation — in the cold season, a closed-up boat can collect a surprising amount of water on its inner surfaces.

If the water is salt

• Stern gland or shaft seal — a traditional packed stern gland often drips a little while the shaft is turning, that is intended; it must not leak when stopped. A dripless shaft seal, lip or face type, must not leak at all. A common cause on a boat with a propeller shaft.
• Engine raw-water circuit — raw-water pump, hoses and clips, a raw-water strainer badly closed, heat exchanger, exhaust and its waterlock: the water then only appears when the engine runs.
• Through-hulls and seacocks — toilet, sink and bilge-pump inlets and outlets, depth and speed transducers: a tired seal, sealant that has given way, a corroded seacock.
• Hull windows and hull-deck joint — a hull window or a hull-deck joint that leaks when heeled, chainplates that let spray in when sailing upwind.
• Rudder tube — the tube through which the rudder stock passes through the hull: worn seal or bearing, leaking mostly under helm or under way.
• Saildrive diaphragm — the rubber membrane between the sea and the inside of the boat; a leak there is an emergency (see saildrive: vibrations and milky oil).
• Keel and keel bolts — rarer: weeping around the bolts, often with a crack at the hull-keel joint, especially after a grounding. Have it looked at without delay.

4. What you can check yourself, safely

• Sponge out and dry the bilge completely, compartment by compartment. Without a dry bilge, you cannot track down a leak.
• Note when the water comes back: in the marina without moving, after rain, after a sail, after a run under engine. Each case points to a family of causes.
• Spot the compartment that fills first, then follow the trail upwards and forwards or aft: water follows the slope and can travel several metres before it shows.
• Place some dry paper towel under each suspect (fittings, seacocks, stern gland, windows) and see which one gets wet first.
• Close the main tap or switch off the fresh-water pump while you are away: if the water no longer comes back, the leak is in the fresh-water circuit.
• With the engine stopped and cooled, look at the clips, hoses and seacocks of the raw-water circuit and at the stern gland; work the seacocks to check that they close.
• Hose down the deck area by area, with someone inside to see where the water appears.
• Note the amount (litres per day, or the number of bilge-pump cycles if there is a counter): that is what will tell you whether the leak is getting worse.

5. When to stop and call a professional

Call a professional as soon as the water is salt and you cannot find its source, or the leak is below the waterline: a weeping or corroded through-hull, a seized seacock, the rudder tube, the saildrive diaphragm, keel bolts, a crack. The same goes for a leak in the engine cooling circuit or at a stern gland that you do not know how to adjust, and for leaks through the deck that come back despite your seals. Many of these repairs are done with the boat ashore. Give your observations: fresh or salt water, when, where, how much.

6. Stopping it from coming back

Work every seacock at least once a month, and check through-hulls, hoses and clips below the waterline at every haul-out. Keep a wooden bung next to each through-hull, and test the bilge pump and its float switch regularly. Redo window seals and the sealant around deck fittings when they harden. Adjust the stern gland or repack it, and replace the saildrive diaphragm at the manufacturer’s interval (see inboard engine maintenance). When leaving the boat, close the seacocks that do not need to stay open. The safety equipment guide also reminds you of the bailing means to keep on board.

In the Hullteam log, record every leak found and repaired, the parts replaced (seacocks, through-hulls, diaphragm, seals) and their dates: that is what will make the boat’s value at resale, because a buyer wants to know that the points below the waterline have been looked after (see which proofs count).