Boat engine coolant

Boat engine coolant is critical to engine performance.  Small marine diesel engines operate under sustained loads, variable speed ranges, and corrosive environmental conditions. Because of this, the coolant circulating through the freshwater (closed-loop) side must provide robust protection against corrosion, cavitation, scale formation, and high-temperature breakdown.

Coolant selection is not universal; different manufacturers, Yanmar, Volvo Penta, Beta Marine, Nanni, and others specify particular coolant chemistries that match their metallurgy, operating temperatures, and component designs. Understanding coolant types and their differences is essential for maintaining reliability and longevity in marine applications.

Boat Engine Coolant Fundamentals

Boat engine coolant or marine diesel coolant, sometimes called antifreeze, is a mixture of water (usually demineralized) and glycol: either ethylene glycol (EG) or propylene glycol (PG). Glycol provides freeze protection, raises the boiling point, and acts as a solvent for corrosion inhibitors. The inhibitors themselves—organic acids, silicates, phosphates, borates, nitrites, and others—define the coolant’s behaviour and compatibility with specific engines. Coolants fall into several chemical families, each with unique protective characteristics. Marine engines use all of these types depending on age, design, and manufacturer requirements.

• IAT (Inorganic Additive Technology)
• OAT (Organic Acid Technology)
• HOAT (Hybrid Organic Acid Technology)
• P-HOAT and N-HOAT (Phosphated or Nitrited HOAT variations)
• Si-OAT (Silicated OAT)

Inorganic Additive Technology (IAT)

IAT coolant is the traditional “green” antifreeze, using inorganic inhibitors such as silicates, phosphates, and borates. These compounds create a fast-forming protective coating on metal surfaces. This makes IAT suitable for older marine engines with:

1. Cast iron engine blocks
2. Bronze/brass heat-exchanger components
3. Conventional copper/brass radiators (in some generators)

The principal characteristics are:

1. Provides strong protection for iron, steel, brass, and copper.
2. Relatively short additive life (typically 1–2 years).
3. Silicates can precipitate and cause pump seal wear in some designs.

Use in marine diesels: Older Yanmar, Volvo MD series, and Beta engines originally used IAT coolants, though most manufacturers now recommend long-life OAT or HOAT. IAT is still compatible with engines where corrosion inhibitors must act quickly or where older metallurgy benefits from inorganic films.

Organic Acid Technology (OAT)

OAT coolant uses carboxylate organic acids as inhibitors. Rather than coating all metal surfaces, they protect only the areas where corrosion begins, providing long, stable protection with minimal deposits. Requires engines designed with specific elastomers and aluminium alloys. The principal characteristics are:

1. Very long life (4–7 years depending on concentration).
2. Excellent aluminium protection.
3. Low silicate content (or none).

Use in marine diesels: Modern small diesels, including Yanmar common-rail engines, Volvo D series, and most Nanni or Beta Kubota-based blocks, often specify OAT coolants. These engines use more aluminium in heat exchangers, thermostats, housings, and sometimes cylinder heads. OAT coolants reduce scaling, a significant benefit in marine heat exchangers where coolant film thickness affects heat transfer efficiency.

Hybrid Organic Acid Technology (HOAT)

HOAT combines organic acids with small amounts of inorganic inhibitors, typically silicate or phosphate, giving OAT longevity with enhanced aluminium and iron protection. Common HOAT subtypes:

1. Si-HOAT (silicated HOAT)
2. P-HOAT (phosphated HOAT)
3. N-HOAT (nitrited HOAT)

Use in marine diesels: Volvo Penta frequently specifies yellow or orange HOAT coolants, particularly silicated varieties (Si-OAT or Si-HOAT), because the silicates protect aluminium surfaces rapidly, important for engines that may sit unused for long periods. Marine diesels face condensation and internal corrosion risks when idle; HOAT’s dual protection helps mitigate this. Some Yanmar engines also permit HOAT coolants where aluminium components require enhanced immediate protection at startup.

The Marine Electrical and Electronics Bible 4th Edition and more about engine coolants

Nitrited and Molybdate-Enhanced Coolants

Some industrial diesel engines use nitrited HOAT (N-HOAT) formulations to resist cavitation erosion, particularly around wet cylinder liners. Nanni (Kubota-based) engines and some Beta engines allow nitrited coolants, especially in commercial or high-hour applications. Although most small marine diesels do not use wet liners, some manufacturers still recommend nitrite-containing coolants for:

1. High-vibration conditions
2. Engines operating at continuous high load
3. Rapid temperature fluctuation environments

Boat engine coolant - Manufacturer Variations and Reasons

Yanmar small diesels increasingly specify long-life OAT coolant, usually red or pink. This matches their use of aluminium heat-exchanger housings, compact coolant galleries, and narrow passages where silicate deposits could cause blockages. Older Yanmar engines (1GM, 2GM, 3HM series) originally recommended IAT but are generally compatible with modern OAT coolants if thoroughly flushed. The reasons for variation:

1. Newer engines require aluminium-friendly chemistry.
2. Long-life coolants reduce maintenance intervals for yacht owners.
3. Narrow coolant passages demand low-deposit formulations.

Volvo Penta specifies yellow or orange HOAT (often Si-OAT) coolants for many D-series and newer models. Their use of high aluminium content, combined with occasional long lay-ups, makes rapid-protection silicates desirable. The reasons for variation:

1. Anti-corrosion protection must activate quickly in idle periods.
2. Higher operating temperatures benefit from stable organic acids.
3. Engine-specific elastomers are compatible only with certain additive packages.

Beta Marine and Nanni are engines are based on Kubota industrial blocks, which historically used nitrited or phosphated HOAT in heavy-duty applications. Many marine versions accept either OAT or HOAT depending on model year. The reasons for variation:

1. Kubota block metallurgy (cast iron blocks, aluminium heads).
2. Some models incorporate wet liner technology in industrial variants.
3. Commercial operators often require nitrite-enhanced cavitation protection.

Universal/Common Considerations Across Brands

Elastomer compatibility: Gaskets, pump seals, O-rings, and thermostat housings must be chemically compatible with the coolant.

Flow characteristics: Marine heat exchangers depend on low-deposit coolants to maintain thermal efficiency.

Operating conditions: Marine diesels run at comparatively steady loads, increasing coolant temperature stability and requiring long-term additive performance.

Contaminant sensitivity: Raw-water cooling makes coolant contamination more likely; long-life coolants reduce the effects of small dilution errors.

Why Coolants Should Not Be Mixed. Manufacturers specify coolant types to avoid these failures. Mixing IAT, OAT, and HOAT coolants can lead to:

1. Gel formation
2. Rapid inhibitor depletion
3. Silicate dropout
4. Blocked heat-exchanger tubes
5. Water pump seal wear
6. Overheating and corrosion

Boat engine coolant

Boat engine coolant for small marine diesel engines vary widely in chemistry and performance characteristics, reflecting the needs of different engine designs, materials, heat-exchanger geometries, and operating environments. Whether using traditional IAT, long-life OAT, or specialised HOAT formulations, each manufacturer’s specification is engineered to provide optimal corrosion protection, thermal stability, component compatibility, and service life. Correct boat engine coolant selection, following the manufacturer's guidance, is essential for ensuring reliability, reducing maintenance costs, and extending the life of a marine diesel engine.