How to Size a Marine Box Cooler for Fishing Boat Engine & Generator

Step-by-step professional sizing guide, fully applicable for fishing boats, CuNi 90/10 box cooler, main engine + generator cooling (fresh water / lube oil / charge air cooling).

1. Basic Data You Need First

Collect these 6 data points before sizing:

Main engine / generator model & rated power (kW / HP)

Number of engines / generators to cool

Cooling medium: Jacket Fresh Water / Lube Oil / CAC Charge Air

Max inlet temperature (engine outlet): typically 85–90°C

Required outlet temperature: target 45–50°C

Local seawater average temp: 15°C / 25°C / 30°C (tropical / temperate)

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2. Calculate Heat Rejection (Core Step)

Rule of thumb for marine diesel engines:

Heat to be dissipated ≈ 35% ~ 40% of engine rated power

Formula:QCooling​=Engine Power (kW)×0.38

Example:Fishing boat engine 450 kW

Q=450×0.38=171 kWBox cooler must be sized for ≥171 kW cooling capacity (add 10–15% safety margin).

With 15% margin:

10% margin: 188.1 kW

15% margin: 196.65 kW → select ~200 kW box cooler

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3. Select Tube Material & Standard Dimension

Standard for Fishing Boats

Tube material: CuNi 90/10 (C70600)

Tube OD: 19 mm / 25 mm

Wall thickness: 1.2 mm – 1.5 mm (fishing boat coastal/mud water use 1.5 mm)

Tube type: U-tube 2-pass (most efficient for box cooler)

Heat Transfer Area Rule

1 kW cooling needs approx 0.08 ~ 0.10 m² CuNi tube area

Example 200 kW:

200×0.08∼200×0.10=16∼20 m2Choose box cooler with 16–20 m² heating surface area.

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4. Match Sea Chest Size (Critical for Fishing Boats)

Box cooler must fit your existing sea chest:

Measure internal sea chest: Length × Width × Height

Leave 50–100 mm gap on all sides for seawater flow

Water flow speed inside sea chest: 0.3 ~ 0.5 m/s (not too fast, not too slow)

If sea chest is small → increase tube length, use dense CuNi 90/10 bundle.

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5. Design Pressure & Temperature Sizing

Tube side (fresh water / lube oil): 1.0 MPa design pressure

Sea water side: 0.4 MPa

Design temp: Inlet 90°C, outlet 45°C

Test pressure: Hydro test 1.25 × design pressure

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6. Fishing Boat Special Sizing Rules

Frequent idle / anchored → choose Box Cooler NOT Keel Cooler

Shallow muddy water → thicker CuNi 90/10 tube (1.5 mm wall)

Tropical seawater >28°C → increase cooling area by 15%

Multiple generators → size separately or combine total heat rejection

Always add 10–15% safety margin for aging, fouling, mud buildup