Calculate engine cooling requirements and component sizes
Common mid-size diesel
V8 gas engine
Auxiliary sailboat engine
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Download NowA marine engine dumps a startling amount of heat into the water around it, and the cooling system's job is to carry that heat away reliably at every rpm the engine will see. Get the flow wrong and the engine runs hot, warps heads and corrodes, or worse, overheats silently until it seizes on a long climb.
The sizing flows from heat rejection. A diesel sheds roughly 2,500 BTU per horsepower-hour and a petrol engine more, and the raw-water flow needed is that heat divided by the temperature rise you allow, using water's well-known capacity to carry heat. A heat exchanger adds its own area requirement on top.
Enter horsepower, engine type and your desired operating temperature, and the calculator estimates heat rejection, the raw-water flow in gallons per minute and the exchanger size so the cooling system is matched to the engine rather than left to chance.
Heat Rejection = HP × factor (2500 diesel, 3500 petrol BTU/hp-hr); Flow GPM = Heat ÷ (ΔT × 500); HX Size = Heat ÷ (150 × ΔT)| Engine | BTU/hp-hr | Flow at 100 hp | Notes |
|---|---|---|---|
| Diesel (naturally aspirated) | 2500 | ≈ 25 GPM | Lower heat, robust |
| Diesel (turbo) | 2500 - 2800 | ≈ 26 GPM | Charge-air heat adds load |
| Petrol inboard | 3500 | ≈ 35 GPM | Runs hotter, more flow |
| Central cooling loop | as above | thermostat-controlled | Common on modern boats |
Heat load and flow by engine type and size.
Formula verified against standard naval-architecture references · Updated October 2026
Roughly heat rejection divided by the allowable temperature rise and water's heat capacity, which for a diesel works out to about a quarter of a gallon per minute per horsepower, so a 100 hp diesel wants on the order of 25 GPM and a petrol engine more. Enter horsepower, type and temperature rise here and the calculator gives the flow your pump must actually deliver at full load.
A diesel converts more of the fuel's energy into work and rejects less of it as waste heat per horsepower-hour, roughly 2,500 BTU against a petrol engine's 3,500. That lower heat load means less cooling water and a smaller exchanger for the same output, which is one reason diesels run cooler and are more tolerant of a marginal cooling system.
One whose capacity exceeds the total heat it must reject divided by the temperature difference available, including the engine, any aftercooler, transmission and generator loads it serves. Undersizing is common and shows as a warm-running boat in summer. The calculator reports the exchanger duty from heat rejection and temperature rise so you can check the installed unit has real margin.
Because the cooling water starts warmer and the air that helps the exchanger is thinner, so the system's capacity to shed heat drops precisely when the engine is working hard. Warm inflow reduces the temperature difference the exchanger relies on, and a system sized for cold water runs hot when the sea is warm. Add flow margin if you cruise warm waters.
It is the single most frequent cause of marine overheating. A weed or film across the raw-water strainer throttles flow, exhaust risers and blocks overheat, and the damage is uneven and costly. Favour a large, accessible, easily cleaned strainer and check it whenever raw-water flow matters, because a little routine attention prevents the worst failures.
A closed fresh-water loop cools the engine, and a separate raw-water loop cools that loop through a heat exchanger, so the engine block sees clean treated coolant rather than raw seawater. It improves corrosion and lets the engine run at a steady, efficient temperature. Sizing still comes down to total heat rejection, which is exactly what this tool computes.
Confirm the engine holds its rated temperature at full load in the warmest water you sail, with a tachometer-verified pump delivering at least the calculated flow and a clean strainer. If it runs cool and steady there, the system is right; if it creeps up under load, add flow, clean the loop, or enlarge the exchanger until margin exists.