Cooling Systems

Raw Water vs. Freshwater — The Two-Circuit System

Almost every marine diesel on a cruising sailboat uses a two-circuit cooling system — and understanding how these two circuits interact is essential to diagnosing every overheating problem you'll encounter. The system works on the same principle as a car's radiator, except there's no radiator and no airflow. Instead, seawater does the job that air does in a car.

The freshwater (internal) circuit is a closed loop, identical in concept to your car's cooling system. Engine coolant — a 50/50 mix of propylene glycol antifreeze and distilled water — circulates through passages cast into the engine block and cylinder head, absorbing heat from the combustion chambers, cylinder walls, and exhaust ports. A belt-driven freshwater circulation pump (sometimes called the internal water pump) keeps the coolant moving through the circuit. A thermostat controls flow, keeping the engine at its optimal operating temperature (typically 75–85°C / 167–185°F) by bypassing or directing coolant through the heat exchanger.

The raw water (external) circuit is an open loop. Seawater enters through a through-hull fitting below the waterline, passes through a raw water strainer that catches debris, and is drawn through the system by the raw water pump — a positive-displacement impeller pump. The raw water flows through the heat exchanger, where it absorbs heat from the freshwater circuit without the two fluids ever mixing. The now-heated raw water exits the heat exchanger and is injected into the exhaust elbow, where it mixes with exhaust gas to cool it before the wet exhaust exits the stern.

Why two circuits? Because salt water is corrosive, and you don't want it flowing directly through your engine's internal passages. The freshwater circuit contains corrosion inhibitors and antifreeze that protect the engine block, head, and gaskets. The raw water circuit handles the corrosive seawater but only contacts components designed for it — the strainer, impeller pump, heat exchanger tubes, and exhaust system. The heat exchanger is the bridge between the two worlds, transferring heat without mixing the fluids. When this system works correctly, the engine runs at a stable, optimal temperature regardless of seawater temperature or ambient conditions.

Heat Exchangers — How They Work and How They Fail

The heat exchanger is a tube-and-shell assembly, typically 25–50 cm long, mounted on the side of the engine. Inside the cylindrical shell, a bundle of small-diameter copper-nickel or cupro-nickel tubes carries raw seawater. Engine coolant flows around the outside of these tubes within the shell. Heat transfers from the hot coolant through the tube walls to the cooler seawater. It's the same principle as any radiator — surface area and temperature differential do the work.

Failure modes are predictable and almost always related to corrosion or blockage. The raw water tubes develop scale and mineral deposits over time, especially in warm, high-mineral waters. This scale insulates the tube walls and reduces heat transfer efficiency — the engine gradually runs warmer over seasons, not overnight. In severe cases, tubes can corrode through entirely, allowing raw water to leak into the freshwater circuit. You'll know this has happened when the coolant overflow bottle fills with milky, rust-colored fluid, or when the engine starts overheating despite adequate raw water flow.

Internal zincs (sacrificial anodes) are installed inside the heat exchanger end caps to protect the copper-nickel tubes from galvanic corrosion. These pencil-shaped zinc anodes corrode preferentially, sacrificing themselves to protect the more expensive tubes. When the zinc is consumed, the tubes become the sacrificial element and start corroding. Inspecting and replacing heat exchanger zincs every 6 months — or more frequently in warm saltwater — is one of the most important maintenance tasks on a marine diesel. It takes five minutes and costs $5–$10. Ignoring it costs $800–$2,000 for a new heat exchanger.

Cleaning the heat exchanger involves removing the end caps (typically held by 4–6 bolts each) and physically rodding out the tubes with a brass brush or wooden dowel to remove scale and debris. On heavily scaled exchangers, a chemical descaling solution (dilute muriatic acid or a commercial marine descaler like Rydlyme) can dissolve mineral deposits. This should be done annually or whenever the engine's operating temperature creeps upward despite a clean strainer and a good impeller.

Raw Water Strainer Maintenance

The raw water strainer is a see-through filter housing installed between the through-hull and the raw water pump. Its job is to catch debris — seaweed, jellyfish, sand, plastic bags, barnacle fragments — before it reaches the impeller pump, where it would cause damage or blockage. The strainer is a simple device, but neglecting it is one of the most common causes of overheating on sailboats.

Inspection should be part of your pre-start routine. Before every engine start, glance at the strainer. The basket should be visible through the clear bowl with no accumulated debris. In clean marinas, you might go weeks without seeing anything. After motoring through weed beds, a jellyfish bloom, or shallow water with suspended sand, the basket can clog in minutes. A clogged strainer restricts raw water flow, which means the heat exchanger can't dissipate heat, and the engine temperature climbs. If you're motoring and the temperature gauge starts rising, the strainer is the first thing to check.

Cleaning the strainer requires closing the raw water through-hull seacock first — this is non-negotiable. Remove the strainer lid (typically a clear cover secured by a bail latch or threaded ring), lift out the basket, clean it, and replace it. The entire job takes 60 seconds if you've practiced it. On offshore passages, you may need to do this while underway, which means closing the seacock, cleaning the basket, reopening the seacock, and confirming water flow — all while the engine is off and the boat is rolling. Practice this at the dock until it's second nature.

The strainer gasket (the O-ring or flat gasket between the lid and the housing) is a common source of air leaks. If the gasket is cracked, compressed, or missing, air can be sucked into the raw water circuit on the suction side of the pump. This air reduces cooling flow without causing an obvious leak — the engine runs warmer than it should, and you may see air bubbles in the exhaust discharge. Replace the strainer gasket annually or whenever it shows signs of deterioration. Keep a spare aboard.

Impeller Replacement

The raw water impeller is a flexible rubber impeller inside the raw water pump that pushes seawater through the cooling circuit. It's the component most likely to fail in the entire cooling system, and it fails in a way that's both predictable and catastrophic if ignored. The impeller is a wear item — it has a finite life, and replacing it on schedule is non-negotiable.

How impellers fail: the flexible rubber vanes are pressed against the pump housing's cam plate as the impeller rotates, creating the pumping action. This constant flexing causes the vanes to take a set (permanently bend in one direction), crack at the base, or break off entirely. A set impeller loses pumping efficiency — water flow decreases and the engine runs warmer. A cracked vane may hold together until the engine is shut down and restarted, at which point the weakened vane snaps off. Broken vane pieces travel downstream and lodge in the heat exchanger tubes, blocking raw water flow. One broken impeller can simultaneously eliminate pumping capacity and block the heat exchanger — double failure from a single $25 part.

Replacement interval: most manufacturers specify every 200 hours or annually, whichever comes first. Some cruisers change them every 100 hours when cruising in warm, sandy, or debris-heavy waters. The impeller is cheap; the engine is not. This is not a part to stretch. A new impeller costs $15–$40 depending on your pump model. A seized engine from overheating costs the price of the boat's engine.

The replacement procedure is straightforward on most sailboat engines, though access is the main challenge — the raw water pump is often behind the engine or under another component.

1. Close the raw water through-hull seacock

   Confirm it's fully closed. On some boats, the seacock handle is stiff or obscured. Verify by cracking the strainer lid slightly — no water should enter. If it does, the seacock isn't sealing and needs service before you proceed.

2. Remove the pump cover plate

   The raw water pump (typically a Jabsco, Johnson, or Oberdorfer) has a face plate held by 4–6 screws or bolts. Remove them and carefully pry off the cover plate. Note the orientation of the impeller and which direction the vanes are bent — the new impeller must go in the same way. Photograph it before removal.

3. Extract the old impeller

   Grip the impeller hub with an impeller puller tool or large pliers and pull straight out. On a well-maintained pump, it slides out with moderate force. On a pump that hasn't been serviced in years, the impeller may be bonded to the shaft by corrosion — this is where the puller tool earns its keep. Count the vanes on the old impeller. If any are missing, they're somewhere downstream — in the hose, the heat exchanger, or the exhaust. You must find them, or they'll block raw water flow.

4. Inspect the pump body and wear plate

   Look inside the pump cavity for scoring, corrosion, or debris. Check the cam plate (the flat or curved surface the impeller vanes press against) for deep grooves. Light wear is normal; deep scoring means the pump body or wear plate needs replacement. Remove any broken vane fragments from the cavity.

5. Install the new impeller

   Coat the new impeller vanes lightly with glycerin, dish soap, or the manufacturer's recommended lubricant — never petroleum-based grease, which attacks the rubber. Bend the vanes in the direction of rotation and push the impeller onto the shaft, ensuring it seats fully. The vanes should flex against the cam plate. Replace the cover plate gasket with a new one, reinstall the cover plate, and tighten the screws evenly in a star pattern.

6. Restore raw water flow and verify

   Open the through-hull seacock. Start the engine and immediately check for raw water discharge from the exhaust — a steady stream of water should be visible exiting the stern within 5–10 seconds of starting. If there's no water flow, shut down immediately and verify the seacock is open, the strainer is clear, and the impeller is installed correctly. Also check the strainer for the pump cover gasket you may have dropped inside (it happens).

Thermostats and Temperature Regulation

The thermostat is a wax-pellet valve in the freshwater circuit that regulates engine operating temperature. When the engine is cold, the thermostat is closed, blocking coolant flow to the heat exchanger and forcing it to recirculate through the engine block. This allows the engine to warm up quickly to its optimal operating temperature. Once the coolant reaches the thermostat's rated opening temperature — typically 71–82°C (160–180°F) depending on the engine — the wax pellet expands, opening the valve and allowing coolant to flow through the heat exchanger for cooling.

A stuck-closed thermostat causes overheating because coolant can't reach the heat exchanger. The engine temperature climbs past normal operating range and into the danger zone. This failure mode is sudden — the engine runs fine one day and overheats the next. A stuck-closed thermostat is easy to diagnose: the heat exchanger and the hoses leading to it will be cold to the touch even though the engine temperature gauge reads high. If the block is hot but the heat exchanger is cold, the thermostat isn't opening.

A stuck-open thermostat (or a missing thermostat — yes, some owners remove them) causes the engine to run too cold, which is less dramatic but equally damaging over time. A diesel that runs below its designed operating temperature produces excessive combustion byproducts, the oil doesn't reach temperature to evaporate water contamination, cylinder walls experience accelerated wear from incomplete fuel burn, and fuel consumption increases. Cold-running diesels develop cylinder glazing — a polished, hard surface on the cylinder walls that prevents piston rings from sealing properly, leading to oil consumption and loss of compression.

Testing a thermostat is simple: remove it from the engine, suspend it in a pot of water with a thermometer, and heat the water on a stove. The thermostat should begin to open at its rated temperature (stamped on the body) and be fully open 10–15°C above that. If it doesn't open, or doesn't open fully, replace it. Thermostats cost $15–$40 and should be replaced every 3–5 years as preventive maintenance, or immediately if the engine shows temperature regulation problems.

Overheating Diagnosis — A Systematic Approach

When the temperature alarm sounds or the gauge climbs past normal, do not keep running the engine. Reduce to idle immediately. A marine diesel that overheats for even a few minutes can warp the cylinder head, blow the head gasket, seize pistons, or crack the block — any of which is a catastrophic, boat-ending repair. The first 60 seconds of your response determine whether you're dealing with a $25 impeller replacement or a $15,000 engine swap.

Work the system in order, from the outside in. The most common causes of overheating are external and simple; the rare causes are internal and expensive. Start with the easy checks.

Step 1 — Check raw water discharge at the exhaust. Go to the stern and look at the exhaust outlet while the engine idles. You should see a steady stream of water. If there's no water, or only a trickle, the raw water circuit is blocked or the impeller has failed. If water flow looks normal, the problem is in the freshwater circuit or the heat exchanger.

Step 2 — Check the raw water strainer. If exhaust water is absent or reduced, close the through-hull, open the strainer, and inspect the basket. Seaweed, a plastic bag, or jellyfish wrapped around the basket is the most common cause of sudden overheating on a coastal passage. Clean the basket, reassemble, open the seacock, and restart.

Step 3 — Check the impeller. If the strainer is clean but water flow is still absent, the impeller has likely failed. Open the raw water pump cover plate and inspect. Replace the impeller and account for any missing vanes — they're in the heat exchanger now.

Step 4 — Check the thermostat. If raw water flow is normal but the engine is hot, feel the heat exchanger and its hoses. Cold heat exchanger plus hot engine equals a stuck-closed thermostat. Remove and test or replace it.

Step 5 — Check coolant level and condition. Low coolant in the freshwater circuit means there's a leak — check hoses, hose clamps, the heat exchanger end caps, and the water pump seal. Milky, discolored coolant suggests raw water has breached into the freshwater circuit through a corroded heat exchanger tube or a failed oil cooler (on engines with a coolant-to-oil heat exchanger).

Step 6 — Consider less common causes. A collapsed raw water hose on the suction side of the pump (old rubber hoses collapse under vacuum), a fouled or blocked exhaust elbow (carbon and salt scale restrict flow over years), a slipping or broken water pump drive belt, or a failed head gasket allowing combustion pressure into the cooling jacket. These are progressively less common but are the diagnoses that remain after the simple checks are eliminated.

Keel Coolers — The Alternative System

Keel coolers (also called grid coolers, skin coolers, or external heat exchangers) eliminate the raw water circuit entirely. Instead of pumping seawater through the engine compartment, a keel cooler mounts a network of copper-nickel pipes to the outside of the hull below the waterline. Engine coolant circulates through these external pipes, and the surrounding seawater absorbs the heat directly through the pipe walls. There is no raw water pump, no impeller, no strainer, no raw water through-hull, and no seawater inside the boat.

The advantages are significant for certain applications. With no impeller to fail, no strainer to clog, and no internal raw water plumbing, the number one cause of marine diesel overheating is eliminated. There are no sacrificial zincs to replace inside a heat exchanger (the external pipes are cathodically protected by the boat's bonding system and hull zincs). There are no rubber raw water hoses to age and collapse. Maintenance on the cooling system is reduced to checking coolant level, replacing coolant on schedule, and keeping the external pipes clean of marine growth during haul-outs. For long-range cruisers, particularly those sailing in debris-heavy, silty, or weed-choked waters, keel coolers offer a meaningful reliability improvement.

The disadvantages are practical. Keel coolers add drag — the external piping creates additional wetted surface area and, on some installations, protrudes enough to affect sailing performance. They're vulnerable to grounding damage and must be positioned carefully to avoid impact. Installation requires hull penetrations and is typically done during a major refit or new build, not as casual retrofit. The external pipes must be accessible for cleaning during haul-outs, and in tropical waters they foul with marine growth that reduces cooling efficiency. The system also relies on boat speed or ambient water movement for optimal heat transfer — in flat calm with no way on, cooling efficiency drops compared to a pumped raw water system.

Keel coolers are most common on steel and aluminium boats (where welding external piping to the hull is straightforward), on long-range cruising boats that prioritize reliability over performance, and on motorsailers with large engine compartments where the plumbing simplification is valuable. They're rare on production fibreglass sailboats, where the conventional two-circuit system is standard. If you're repowering or building a custom boat, a keel cooler is worth evaluating — but for most owners with a conventional setup, maintaining the raw water circuit properly is the more practical path.