Guide
Why Marine Antifreeze Is Not the Same as Automotive Antifreeze
One of the most common winterization mistakes is reaching for the green jug of automotive antifreeze instead of proper boat antifreeze when preparing a vessel for cold storage. Automotive antifreeze is typically based on ethylene glycol — a compound that is toxic to humans, animals, and aquatic life. It is designed for closed cooling systems in cars and trucks where it never contacts potable water or the environment directly.
Marine raw water cooling systems are different. Raw water is drawn from the lake, river, or ocean and pumped through the engine to absorb heat before being discharged overboard. If you flush this system with ethylene glycol antifreeze, residue ends up in the water. The same problem applies to freshwater holding tanks and hose systems on board.
The correct product for raw water passages and any system that could contact potable water is non-toxic propylene glycol marine antifreeze — typically pink or blue in color. Disposal and discharge rules still vary by location, so always follow product labels and local regulations. Never use automotive antifreeze in raw water cooling passages, freshwater tanks, or any system that may contact drinking water or be discharged overboard.
Guide
Propylene Glycol vs Ethylene Glycol: The Difference That Matters
Understanding the chemistry helps you make the right purchase at the marine supply store.
Propylene glycol is the basis for non-toxic marine antifreeze. It is widely used in RV and marine winterization products because it is much less toxic than ethylene glycol and is generally preferred where fluid may contact potable-water or raw-water systems. In a boat context, propylene glycol antifreeze is used to flush raw water cooling passages, protect freshwater onboard systems, and treat water lines in heads and livewell systems. It typically comes in a pink color, though some brands use blue.
Ethylene glycol is the active ingredient in standard automotive antifreeze and is also found in some marine closed-loop coolants. It is significantly more toxic and should only be used in sealed, closed-circuit engine cooling systems — the kind with a dedicated coolant reservoir that never contacts raw water or the outside environment. Many inboard and sterndrive engines have a closed freshwater cooling system (sometimes called a heat exchanger system) that circulates a coolant mix through the engine block, similar to a car. This is the appropriate application for ethylene glycol-based products, at the concentration your engine manufacturer specifies.
The practical rule is simple: if it touches raw water or potable water, use propylene glycol. If it stays inside a sealed closed loop, follow the engine manufacturer's specification — which may call for ethylene glycol at the correct concentration.
Guide
Where Boat Antifreeze Is Used: The Three Systems
Most boats have up to three distinct systems that require attention during winterization, and each has different antifreeze requirements.
The raw water cooling system draws water from outside the hull and pumps it through the engine (and in some designs, through a heat exchanger) before discharging it overboard. This system must be flushed with non-toxic propylene glycol antifreeze to displace any remaining water in the passages, pump, hoses, and exhaust manifold. Water left in these passages can freeze, expand, and crack metal components.
The freshwater onboard system includes potable water tanks, pressure pumps, hoses, and faucets. This system should be drained as completely as possible, then flushed with diluted propylene glycol to protect any residual water in lines that cannot be fully drained.
The closed engine cooling loop — present on many inboard engines, sterndrives, and some larger outboards — circulates a dedicated coolant solution that does not mix with raw water. This system works like a car's cooling system. It requires the appropriate coolant concentration for your climate, as specified by the engine manufacturer. Checking the freeze protection level with a hydrometer or refractometer before winter is a straightforward step that takes only a few minutes.
Guide
How to Flush the Raw Water System with Boat Antifreeze
Flushing the raw water system is the central antifreeze task for winterizing most boats. The general approach is to introduce non-toxic propylene glycol antifreeze into the raw water inlet so that it flows through every passage — water pump, thermostat housing, exhaust manifold, risers — and exits at the exhaust outlet, confirming full coverage.
The two most common methods are the flush fitting method and the bucket method. With a flush fitting or muff attached to the raw water intake, the engine can draw antifreeze directly from a supply bucket as it runs. With the bucket method, the raw water intake hose is disconnected and submerged in a bucket of antifreeze, and the engine is run until the antifreeze color appears at the exhaust. Both approaches follow the same principle: antifreeze in, antifreeze out the other end.
This is an overview only. The exact procedure — flow path, hose routing, valve positions, idle duration — varies significantly between engine makes, models, and years. Always consult your engine manufacturer's service manual or a qualified marine technician before performing this procedure. Using the wrong approach for your specific engine can leave sections of the cooling system unprotected.
Guide
Freshwater System: Tanks, Pumps, and Lines
The freshwater system on a cruising boat or liveaboard typically includes one or more holding tanks, a pressure water pump, hoses running to sinks and showers, a water heater, and potentially a watermaker. Each component can trap water that freezes and causes damage if not treated.
The recommended approach is to drain as much water as possible from the tanks first — some owners use the boat's water pump to push most of the water out through the faucets. Once the tanks are mostly empty, diluted propylene glycol antifreeze is introduced into the system and pumped through by running the water pump and opening each faucet in turn until the antifreeze color appears. The water heater bypass valve, if present, should be used according to the heater manufacturer's instructions to avoid diluting or contaminating the heater tank.
The concentration of propylene glycol used in freshwater systems is generally lower than what is used in raw water cooling passages, because freeze protection is the only requirement — the antifreeze is not pumped through a hot engine. Check the product label for the dilution ratio that achieves the freeze protection temperature you need for your storage location.
Guide
Closed Engine Cooling Loop: Using the Right Coolant Concentration
Engines with a closed freshwater cooling system circulate a dedicated coolant mix through the block and cylinder head. This coolant is isolated from raw water by a heat exchanger. Because it is a sealed system, the coolant is not discharged overboard, and the same coolant typically remains in the engine for one to five years depending on the product type and the manufacturer's service interval.
For this system, the antifreeze product and concentration are specified by the engine manufacturer. Some marine engine makers specify a silicate-free or OAT (organic acid technology) coolant to avoid corrosion in aluminum components. Others specify conventional green coolant. Using the wrong formulation can cause corrosion, gel formation, or seal damage over time.
Before winter, check the freeze protection level of the closed coolant using a hydrometer or refractometer. If the protection level is not adequate for the coldest expected temperature at your storage location, drain and refill to the correct concentration. The engine manual will give the minimum freeze protection rating required and the correct coolant-to-water ratio.
Guide
Antifreeze Concentration and Freeze Protection Ratings
Marine antifreeze products are sold at varying concentrations. Understanding the difference between burst protection and freeze protection helps you choose the right product for your storage conditions.
Freeze protection is the temperature at which the solution begins to form ice crystals — at this point it becomes slushy but has not yet frozen solid. Burst protection is the lower temperature at which the solution is cold enough that a frozen block could exert damaging pressure on pipes or passages. Burst protection temperatures are typically 10–20°F lower than freeze protection temperatures on the same product.
For most cold-weather storage in the continental United States, propylene glycol marine antifreeze rated to -50°F burst protection is commonly sold, but that level of protection is overkill in many climates. Protection to -20°F is generally adequate for storage in most northern US and Canadian locations, though you should verify the expected minimum temperature at your storage facility. Products labeled -100°F are concentrated solutions intended to be diluted on site — read the label before pouring undiluted product into your system.
In any case, the goal of boat antifreeze in raw water passages is to displace water, not necessarily to freeze-proof the antifreeze itself. A well-flushed system with properly drained low points has very little remaining liquid volume, so the protection level of the residual antifreeze is an additional safety margin rather than the primary defense.
Guide
Storage Posture and Drainage Slope
The angle at which a boat sits on its stands during winter storage affects how liquids drain — and how antifreeze settles — inside the cooling passages.
Many engine manufacturers specify that the hull should be positioned with a slight bow-up tilt when introducing antifreeze into the raw water system. This tilt helps ensure that antifreeze flows toward and covers the low points of the cooling circuit — particularly the exhaust manifold and risers — rather than pooling away from them. Check your engine manual for any positioning guidance specific to your model.
The same consideration applies after the antifreeze procedure is complete. If the hull is stored bow-down, residual water that was not fully displaced may migrate toward the bow and concentrate in passages that were not fully protected. A slight bow-up tilt on adjustable stands encourages drainage toward the engine's low-point drain locations.
Working with a boatyard that uses properly adjustable boat stands allows the vessel to be positioned precisely for this procedure. CE-documented support equipment with adjustable saddles and locking mechanisms can hold a specific hull angle throughout the winter storage season, giving you confidence that the tilt set during winterization is maintained until spring. KIPAC supplies CE-documented boat stands and keel supports used by professional boatyards for exactly this kind of controlled storage setup.
Guide
Disposing of Used Antifreeze Responsibly
When you drain and replace coolant from a closed engine cooling loop, you are left with used ethylene glycol antifreeze that must be disposed of properly. Used antifreeze can contain heavy metals and combustion byproducts picked up from the engine, making it a regulated waste in many jurisdictions even if the base product is not acutely toxic.
Propylene glycol from raw water systems and freshwater lines is generally considered lower-risk due to its non-toxic base chemistry, but it should still not be poured down storm drains or discharged overboard. Many marine service facilities, auto parts stores, and municipal hazardous waste collection programs accept used antifreeze for recycling.
Check your local regulations before disposing of any antifreeze product. In some areas, even dilute propylene glycol discharges to water are regulated during the winter season. Responsible disposal is straightforward — collect used antifreeze in sealed containers and take it to an appropriate collection point.
Equipment
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View equipment →FAQ
FAQ
RV antifreeze is typically propylene glycol-based and non-toxic, which makes it chemically similar to non-toxic marine antifreeze. Many boaters use it successfully in raw water passages and freshwater systems. However, verify the product's freeze protection rating is appropriate for your storage climate, and confirm it meets the specifications for any system in your boat that has manufacturer requirements. For the closed engine cooling loop, always follow the engine manufacturer's coolant specification rather than substituting RV antifreeze.
The quantity depends on the size of your cooling system and the method used. A typical outboard or small inboard raw water system generally requires one to two gallons of propylene glycol antifreeze to ensure thorough coverage. Larger engines with extensive cooling passages and exhaust systems may need more. For freshwater onboard systems, the volume depends on tank and hose capacity. It is generally better to purchase slightly more than you estimate and have leftover product than to run short mid-procedure.
Non-toxic propylene glycol marine antifreeze is most commonly pink. Some brands produce a blue version. The color is added by the manufacturer to make it easy to see when the antifreeze has fully flushed through a system — when pink or blue liquid exits the exhaust outlet or faucet, you know the passage is covered. Ethylene glycol-based closed-loop coolants are typically green or orange, depending on the formulation. Never identify an antifreeze product by color alone — always read the label to confirm the base chemistry.
Non-toxic propylene glycol marine antifreeze has low acute toxicity to aquatic life and is biodegradable, which is why it is considered acceptable for raw water systems that discharge overboard. However, concentrated discharge in enclosed waterways can still deplete oxygen and affect local water quality. Ethylene glycol antifreeze from closed cooling systems is significantly more toxic to aquatic organisms and should never be discharged overboard or into storm drains. Collect all used antifreeze and dispose of it at an appropriate facility.
Replacement intervals for closed-loop coolant vary by engine manufacturer, coolant product type, and operating hours. Conventional green ethylene glycol coolant is typically replaced every two years or according to the engine manufacturer's hour-based service interval. Extended-life OAT or HOAT coolants may have longer service intervals — sometimes up to five years in low-hour marine applications. Check your engine service manual for the recommended interval. Running degraded coolant can accelerate corrosion in aluminum components and reduce freeze protection over time.
Mixing antifreeze types is generally not recommended. Different coolant formulations use different corrosion inhibitor chemistries that can react when combined, forming deposits or reducing effectiveness. In the closed engine cooling loop, mixing conventional green coolant with an OAT or HOAT product can cause inhibitor dropout, which appears as a gel or sludge that clogs passages. If you are unsure what coolant is currently in a system, the safest approach is to flush the system completely with clean water before refilling with the correct product. For raw water systems, propylene glycol products from different brands are generally compatible, but verify with the product label.
