Little or no water at the exhaust

On an inboard engine cooled by seawater, the water drawn in by the pump comes out at the exhaust, mixed with the gases: it cools the engine (directly, or through a heat exchanger) and then the exhaust itself, often made of rubber and plastic. A steady flow, with its usual spurts, means everything is circulating. Little water, or no water at all, means the engine and the exhaust are no longer being cooled — and you need to act fast. Your engine manufacturer’s manual always has the last word.

1. What you observe

• No water at the exhaust outlet after starting, only gases.
• A few irregular spurts, then nothing.
• A weaker flow than usual, or one that weakens as you motor along.
• Steam at the exhaust, a drier, more metallic sound.
• The temperature alarm following a few minutes later.

Know your installation: on some boats the water comes out through a different outlet from the gases (water-gas separator), or below the waterline. Note once and for all where and how it comes out when all is well: that is your point of comparison.

2. Safety first

• Look at the exhaust in the seconds after every start. The water should arrive very quickly. If it is not there after a few tens of seconds, stop the engine.
• Do not let an engine run without water: the pump impeller, then running dry, is damaged in a short time; the rubber exhaust hose and plastic muffler heat up and can melt, letting gases and water into the boat.
• At sea, if you need the engine to get to safety, keep it at idle for strictly as long as necessary, watch the temperature, and switch to sail or anchor as soon as possible.
• Do not keep cranking. This is the trap with this fault: see the box below.

Why you must not keep cranking

On a “wet” exhaust, seawater is injected into the exhaust elbow and then falls into a muffler (often called a waterlock) placed lower than the engine; it is the gases that then push it towards the outlet, through a hose that rises in a gooseneck before dropping back down to the hull. When the engine is turned over by the starter without firing, the pump still sends water, but the gases are not there to drive it out: the muffler fills up, then the water can rise through the elbow to the cylinders. On engines installed below the waterline, an anti-siphon loop (a loop topped by a small valve) also stops seawater filling the engine by gravity alone; if it is dirty or stuck, it no longer does its job. Remember: after two or three failed attempts, close the raw-water seacock during the following attempts, and reopen it as soon as the engine runs.

3. Likely causes, from most to least common

• Raw-water seacock closed — after laying up, cleaning a strainer, or as a precaution at anchor.
• Blocked intake or through-hull — plastic bag, weed, mussels and shells after a long stay in harbour, or an intake painted over at the last haul-out.
• Raw-water strainer dirty, or lid not tight, seal missing or damaged: the pump sucks air and loses its prime.
• Impeller worn, with broken blades, or stuck to the pump body after a long lay-up.
• Air leak on the suction side — a loose clip or a split hose between the seacock and the pump.
• Belt slack or broken, on engines where it drives the raw-water pump.
• Raw-water pump itself — leaking shaft seal, worn cam or body.
• Heat exchanger blocked by pieces of impeller or by scale.
• Exhaust elbow blocked by carbon and corrosion: the water no longer gets through.

4. What you can safely check yourself

Engine stopped, in this order:

• The raw-water seacock: open?
• The raw-water strainer: seacock closed first, lid open, basket cleaned; water should rise in the bowl when you reopen the seacock. Seal in place, lid properly tightened, seacock reopened.
• The intake: if water does not rise in the strainer with the seacock open, the blockage is before it — through-hull or intake. Clearing it from outside is only done in harbour or at anchor in a calm sea, engine stopped and key removed, and only if you know how to do it safely.
• The clips and hoses between the seacock and the pump: tight, with no splits.
• The belt, if it drives the pump: present, tensioned, with no cracks.
• The impeller, if you have done the job before and have a spare impeller: seacock closed, pump cover removed, impeller checked. A missing blade has gone off towards the heat exchanger: it will have to be found.
• On restarting, seacock open, the water should come back within seconds: watch the outlet before doing anything else.
• If the engine has been cranked a lot, the oil level: higher than usual, or milky, is a sign that water has got in. Do not restart.

5. When to stop and call a professional

Call a marine mechanic if the water does not come back despite an open seacock, a clean strainer, a cleared intake and a sound impeller: the pump, the heat exchanger or the exhaust elbow is to blame, and taking them apart is their job. Call one straight away — and do not touch the starter again — if it stopped dead, if the oil is milky or its level has risen, or if the exhaust got hot enough to melt or leak. Give them the make and model of the engine, its hours, when it started (first start of the season, after a haul-out, under way), how long the engine ran without water or was cranked, what you have already checked, and the date of the last impeller and the last check of the elbow.

6. Stopping it from happening again

• Get into the habit of looking at the exhaust every time you start.
• Change the impeller every year or on the schedule in the manual, keep an impeller and its gasket on board. Some owners remove it during lay-up so that it does not stick: follow the manual.
• Clean the strainer, work the seacock regularly, check the intake at every haul-out without painting it.
• Have the exhaust elbow and the anti-siphon loop checked as the manual says.

The due dates are in the guide to maintaining an inboard engine, and refilling the circuit in spring in the one on laying up and recommissioning; if the temperature has already risen, see also the temperature alarm. Once the fault is sorted, record it in the Hullteam log — cause, parts, date, engine hours: it is this record that will give the boat its value at resale, see which proof really counts.