Boat Antifreeze: Which Kind and Where It Goes

Boat antifreeze for winterizing is propylene glycol, sold as “RV/marine” or “non-toxic” antifreeze and usually pink. Marine antifreeze of this kind goes into every system that holds water, empties overboard or feeds a tap, and is not drained completely: the engine’s raw-water cooling side, the fresh-water system, the head, livewells and washdown pumps. Automotive antifreeze, usually green ethylene glycol, is much more toxic and does not belong in any of those. The number on the jug, often -50°F, is a burst rating, not the temperature at which it starts to freeze. A closed engine cooling system is a separate case: it takes the coolant its engine maker specifies. In spring, the antifreeze is flushed into a container, not into the lake.

Propylene glycol vs ethylene glycol

New York Sea Grant and the EPA give the same rule: use propylene glycol antifreeze (usually pink), which is less toxic than ethylene glycol (usually green), to winterize all systems except “closed” or fresh-water cooling systems. Discover Boating’s winterizing products page calls ethylene glycol “the green stuff”, used in car cooling systems and some sterndrive and inboard engines with closed cooling, and toxic; propylene glycol, “the pink stuff”, is the one for a boat’s drinking-water system.

Both sources say “usually”, and people also search for blue and green boat antifreeze: the color is a hint, not a guarantee. Read the label for the glycol type.

Why ethylene glycol is kept off the raw-water side

The federal Agency for Toxic Substances and Disease Registry (ATSDR) says ethylene glycol is odorless but sweet-tasting, that accidental swallowing happens because antifreeze is “a sweet tasting, brightly colored liquid”, and that exposure to large amounts can damage the kidneys, nervous system, lungs and heart. It advises against spilling or draining antifreeze on the ground “to prevent children from playing in a puddle of ethylene glycol”. Connecticut’s guidebook adds that it poses a serious health hazard to humans and animals if swallowed. Whatever goes through a raw-water engine, a head or a washdown is headed overboard, so those systems get the less toxic glycol.

Less toxic is not harmless. ATSDR notes that frequent skin exposure to propylene glycol can sometimes irritate the skin, and Connecticut’s Department of Environmental Protection says antifreeze in general “is harmful to marine and aquatic life” and should be handled with eye protection, clothing that covers the skin, and rubber gloves.

Burst point vs freeze point: reading the label

An antifreeze maker’s product data sheet defines the two numbers. The freeze point is “the temperature at which ice crystals begin to form”. The burst point is the temperature at which the fluid “overcomes the hydrostatic pressure of the pipe and bursts”. In its words, burst protection is needed when equipment or plumbing “remains idle”, freeze protection when the fluid and plumbing must stay flowable and pumpable. A boat laid up for winter is the first case.

Its own table shows how far apart the numbers are:

So every -50°F product on that sheet starts forming ice crystals above 0°F, at 2°F to 20°F depending on the blend. That is what burst protection allows for in idle, drained plumbing, but water left in the system dilutes the fluid. The data sheet says that if you suspect a significant volume of water in cavities or plumbing, you should consider a lower burst-protection fluid; in practice, drain first, then fill. The same figures mean a jug stored somewhere colder than its freeze point will start to form ice crystals.

Where it goes on a boat

Every system that holds water needs one of two treatments: drained completely, or filled with propylene glycol. Discover Boating’s winterizing guide covers each.

The engine’s raw-water side

Raw-water-cooled engines pump lake or sea water through the block. Discover Boating’s products page, which names outboards, stern drives and many inboards, says running a few gallons of propylene glycol through such an engine before storage displaces the water left in the cooling passages, and its guide calls antifreeze “usually preferred” for inboards, after warming the engine so the thermostat does not block it. But one inboard engine maker’s storage sheets drain the raw-water side instead and leave the plugs and the pump impeller out until spring, and two outboard makers’ manuals drain the cooling water instead, leaving the motor upright until it has drained. Which method your engine uses, and where it drains, is in its owner’s manual; the steps are in winterizing an inboard or sterndrive and winterizing an outboard motor.

The fresh-water (drinking water) system

1. Drain the tank by running the faucets until they run dry.
2. Bypass the water heater if the boat has one, as Discover Boating’s guide allows, so it does not have to fill with antifreeze. A bypassed heater still holds water, so drain it as its manual directs.
3. Add non-toxic antifreeze to the system.
4. Open each faucet in turn, starting with the one farthest from the pump, hot and cold, and close it once antifreeze comes out. Look for undiluted antifreeze, not water mixed with it.
5. Do the shower and any other outlets the same way.

The head

Pump out the holding tank, then pump antifreeze through the toilet, per Discover Boating.

Livewells and raw-water washdowns

Run the pumps dry; if they still gurgle, there is water left, and Discover Boating says to add antifreeze.

The closed cooling system: not winterizing antifreeze

A closed-cooled engine holds a sealed coolant loop, like a car’s, and that coolant stays in all year. It is chosen by the engine maker, not by the jug on the shelf. One inboard maker fills its engines at the factory with propylene glycol coolant, says not to add ethylene glycol unless the system is drained completely and refilled, and allows a specific ethylene glycol coolant mixed 50:50 with water as the alternative; it tests the coolant with litmus paper and replaces it if the paper turns pink or red. Your manual decides the coolant, the mix and when to change it. New York Sea Grant’s propylene glycol rule explicitly leaves closed systems out.

Handling and disposal

Used antifreeze cannot simply be poured out.

• Never into the water or a drain. Connecticut’s environmental agency says antifreeze “may not be discharged to storm drains, septic systems or sanitary sewers or to surface waters” without its authorization. New Hampshire law, quoted in that state’s fact sheet, makes it unlawful to discharge antifreeze to surface water or groundwater without a written permit.
• Capture it. New Hampshire’s fact sheet says not to drain any engine fluids into the water or on the ground, and to capture them “in a container large enough to hold the anticipated volume of waste”.
• Keep it separate and labeled. Connecticut’s best practices: segregate used antifreeze from other wastes, label the container “Waste Antifreeze”, and never mix antifreeze with other chemicals.
• Treat engine antifreeze with care. The same guidebook warns that antifreeze that has been in an engine can become contaminated with lead or fuel to the point where it must be managed as hazardous waste. Ask your marina or town where used antifreeze is accepted.
• Use as little as the job needs. New York Sea Grant advises using “the minimum amount of antifreeze necessary for the job”; draining first is how.

In spring: flush it into a container

New Hampshire’s environmental agency puts it in one line: when winterizing, use propylene glycol rather than ethylene glycol, “and be sure to flush these fluids into a container prior to launching in the spring.” Discover Boating’s spring checklist says to flush the fresh-water system and test faucets, showers and sinks: run each one until no antifreeze comes out before anyone drinks the water, and do it before launch, catching what you can in a container. For the engine, flushing the raw-water side happens on a water supply before launch; see getting a boat ready in spring.