Raw Water vs Keel Cooling

There are three cooling arrangements in common use, and they are frequently confused with each other. The distinction matters because it changes what can go wrong.

Direct raw water cooling

Seawater is pumped straight through the engine block and out the exhaust. No coolant, no heat exchanger.

Now largely confined to older and smaller engines. It is simple and it is hard on the engine: salt water through cast iron passages deposits scale, corrodes, and forces the engine to run at a lower temperature than is ideal for combustion efficiency and engine life.

Failure modes: blocked passages, internal corrosion, salt scale, and no practical way to inspect the inside of the block.

Indirect (heat exchanger) cooling

The engine runs a closed circuit of coolant, exactly like a car. That coolant is cooled by seawater passing through a heat exchanger. Raw water then continues on to cool the exhaust and is discharged.

This is the standard arrangement on almost every modern recreational marine diesel.

Advantages: the engine internals see treated coolant at a proper operating temperature, corrosion inhibitors work, and the raw water side is confined to components you can service and replace.

Failure modes: impeller wear, blocked strainer or intake, silted heat exchanger, corroded coolers, mixing elbow blockage, and the tube bundle eventually leaking raw water into coolant.

What owners get wrong: treating the coolant side as the whole cooling system. Most overheats in this arrangement are raw water side problems.

Keel cooling

Coolant is circulated through pipes, channels or a plate in contact with the water outside the hull. No seawater is pumped into the boat at all for engine cooling.

Common on commercial vessels, workboats, and boats operating in shallow, silty or debris-laden water.

Advantages: no impeller, no strainer, no raw water pump, no seawater inside the boat, nothing to block with weed or sand. Very robust, very low maintenance, and well suited to river and estuary work where an intake would constantly clog.

Trade-offs: the cooler is external and exposed to grounding and fouling. Marine growth on the cooler reduces capacity, so antifouling and inspection matter. Capacity is fixed by the installation, so the engine must be matched to it. Repairs usually mean a haul-out.

Keel-cooled installations often pair with a dry exhaust, since there is no raw water available to inject into a wet one — though hybrid arrangements with a small raw water circuit purely for the exhaust do exist.

Which one do you have

If there is a seacock, a strainer and a rubber impeller pump, you have a raw water circuit — direct or indirect. If there is also a coolant header tank with a pressure cap and antifreeze in it, you are indirectly cooled. If there is no seawater intake for the engine, you are keel cooled.

Frequently asked questions

Is keel cooling better than a heat exchanger? For workboats and silty water, generally yes. For recreational boats, the heat exchanger arrangement is lighter, cheaper, and entirely adequate.

Do keel-cooled engines have impellers? Not for engine cooling. Some installations still run a small raw water pump for exhaust cooling or a gearbox cooler.

Can I convert to keel cooling? It is done, particularly on commercial conversions, but it means a hull-mounted cooler sized to the engine and usually an exhaust change. It is a design job.

Why does my heat exchanger keep silting up? Sandy or muddy operating water, an intake positioned low, or an ineffective strainer. Frequent silting is one of the arguments for keel cooling.

Does direct raw water cooling damage an engine? It shortens its life relative to indirect cooling — scale, corrosion and a low operating temperature. It is why almost nothing is built that way now.