Closed vs. Raw Water Cooling: Which System Is Better for Your Boat?

Marine engines use two main types of cooling systems: raw water cooling and closed cooling. Understanding how each system works — and what the trade-offs are — can help you make better decisions about maintenance, upgrades, and long-term reliability. Whether you’re buying a new boat or repowering an older one, the type of cooling system matters.

What Is a Raw Water Cooling System?

In a raw water system, the engine draws seawater or lake water directly into the block to absorb heat. The water flows through the engine, picks up heat, and exits through the exhaust. It’s a simple, lightweight system with fewer components, but it exposes the engine to salt, silt, and biological debris every time it runs.

What Is a Closed Cooling System?

A closed cooling system uses a coolant-water mixture inside the engine, just like in a car. This coolant circulates through the block and heads, while a heat exchanger uses raw water to remove the heat from the coolant. The two fluids never mix. The raw water flows only through the heat exchanger, risers, and manifolds.

Raw Water Cooling — Pros and Cons

Advantages:

• Fewer components — lower initial cost
• Simple to service and easy to understand
• Fewer hoses and parts to fail

Disadvantages:

• Saltwater runs directly through the engine block
• Corrosion builds up quickly without frequent flushing
• Shorter engine life, especially in saltwater
• Seasonal water changes are less effective at controlling internal scaling

Closed Cooling — Pros and Cons

Advantages:

• Protects engine internals from salt and debris
• Coolant contains rust inhibitors and antifreeze properties
• Longer engine life in harsh environments
• Easier winterization — no need to drain the entire block

Disadvantages:

• Higher initial cost
• More parts to inspect and maintain (heat exchanger, coolant, pencil anodes)
• Can mask cooling problems if not properly monitored

Which One Is Better?

It depends on how and where you use your boat.

• Freshwater use: Raw water systems work well and last longer. Corrosion is less aggressive, and flushing isn’t as critical.
• Saltwater use: Closed cooling is highly recommended. Salt doesn’t enter the block, and coolant adds a layer of protection. Heat exchanger maintenance is a small price to pay for engine longevity.
• High-performance engines or diesels: Almost always closed-cooled due to higher operating temperatures and tighter tolerances.

Can You Convert from Raw to Closed Cooling?

Yes. Many engine manufacturers offer conversion kits that add a heat exchanger and loop the coolant internally. However, converting is only worthwhile if the block is still in good condition and not already corroded. If your raw-water engine has been running in saltwater for years without flushing, the damage may already be done.

Conclusion

Raw water cooling may be simple, but it comes at a cost — especially in saltwater. Closed cooling systems add complexity but pay off in protection and engine lifespan. If you plan to keep your boat long-term and run it in brackish or saltwater, a closed system is the smarter choice. If you’re in freshwater and want a simpler setup, raw water can serve you well — as long as you maintain it properly.