Raw Water Cooling on Marine Diesels: Strainers, Heat Exchangers and Overheating

A marine diesel does not overheat because it is old. It overheats because something between the sea and the engine has narrowed, and the water that should be carrying the heat away is not getting through. Almost every overheating call we take from the marinas at Wallaroo and Port Vincent traces to one of half a dozen places in the raw-water circuit, and the sensible way to think about the system is as a single path from the hull to the exhaust, with a known weak point at each stage.

How does a marine diesel cool itself?

Most engines you will meet — the Volvo Penta D-series, the KAD and TAMD families, the Yanmars and the Cummins — are closed-circuit cooled. There are two loops. The inner loop is coolant, like a car, circulating through the block with a thermostat. The outer loop is raw water: seawater drawn in through the hull, pushed through a heat exchanger where it takes the heat out of the coolant, and then injected into the exhaust to cool it before it leaves the boat.

The raw-water loop is the one that fails, because it is the one full of salt. Follow it from the start.

The seacock and the strainer

Water enters through a seacock and goes first to a strainer, a basket in a clear bowl that catches weed, plastic and the odd jellyfish before they reach the pump. The strainer is the single most common cause of an overheat that comes on suddenly in an otherwise healthy engine, and it is also the easiest thing on the boat to check: look at the bowl before you leave the berth, every time. A strainer half full of weed does not cause a problem at idle and causes one at cruise, when the engine wants three times the water. That pattern — fine slow, hot fast — is a restriction somewhere in this circuit almost every time.

The seacock itself should be exercised, opened and closed, every few weeks. A seacock that has not moved for a year will not move when you need it to.

The raw-water pump

Next is the pump, and it is an impeller pump, the same rubber-vaned design as an outboard water pump, doing the same job with the same weakness. It runs dry for a few seconds and loses vanes; it sits over winter and takes a set; it ages and stiffens. The impeller is an annual replacement on a diesel, not a repair-when-it-fails item, and the reasoning is set out in our guide to when to replace an impeller. When an impeller does shed vanes, the pieces travel downstream and lodge in the heat exchanger inlet, which is the next stage.

The pumps themselves wear too: the seal behind the impeller weeps, the bearing gets rough, the cam plate scores. Replacement JMP raw-water pumps for Volvo Penta and the other makes are a straightforward swap when the original is past repair.

The heat exchanger

The heat exchanger is a bundle of small tubes with raw water flowing through them and coolant flowing around them. Over the years salt and scale build up inside those tubes and the fine passages narrow, and the engine's temperature creeps up a degree or two a season until one hot day it does not have enough margin left. This is the slow overheat, the one that owners live with for years by backing off the throttle.

The fix is to pull the tube stack, clean it — usually an acid bath, occasionally rodding — and refit it with new end-cap seals. While it is apart, check the sacrificial pencil anodes that screw into the exchanger body; they protect the tube stack and they are consumed like any other anode. On an engine that has never had it done, the difference afterwards is startling. Where a stack is too far gone, the Mr Cool heat exchanger range covers most common engines as a complete replacement.

The exhaust elbow

The last stage is the one nobody sees: the raw water is injected into the exhaust at the elbow or riser, and that elbow is the hardest-working piece of metal on the engine, hot gas on one side and salt water on the other. It corrodes from the inside. As it does, the water injection passage narrows, the exhaust runs hotter, and eventually it either blocks — overheat — or perforates, which puts exhaust gas and water into the engine bay. On a salt-water boat a cast iron elbow has a life of years, not decades, and the exhaust smoke guide explains how the colour of the exhaust tells you it is on the way; see what your marine diesel's exhaust smoke is telling you.

Reading the symptoms

• Hot at cruise, fine at idle: restriction — strainer, heat exchanger, or an impeller down a vane.
• Sudden overheat: strainer blocked, impeller failed, or a plastic bag over the intake.
• Less water from the exhaust than usual, steam with it: the flow has dropped. Do not wait for the alarm.
• Temperature creeping up over seasons: the heat exchanger is scaling.

The one thing not to do is keep resetting the alarm and pressing on. A diesel run hot warps heads and blows head gaskets, and that is a rebuild. Hoses and pipes for the Volvo Penta circuit are in Volvo Penta hoses and pipes. Strainer cleaning, impellers, heat exchanger servicing and elbow replacement are the core of the annual work we do through inboard servicing and repairs, at the workshop or alongside in the marina. If the temperature has started to climb, ring 0412 532 984 before it becomes a head gasket.