Your Boat Engine's Chillin'

By Beth A. Leonard

It's that time of year again. Before the mercury dips below freezing, you need to winterize your engine or risk missing next year's boating season.

Photo of sunken boat in snow

Fresh water expands in volume by about nine percent when it freezes and can push outward with a force of tens of thousands of pounds per square inch. An analysis of 10 years of freeze claims from the BoatUS Marine Insurance claim files found that more than three-quarters involved cracks in the engine block or the exhaust manifolds because water remained in the engine or cooling system during a hard freeze. Those claims come from all over the country, not just from the states that get snow every year. Unless you're located in Hawaii or the Florida Keys, we recommend winterizing your engine if you'll be laying up the boat for even a few weeks. It will minimize the chances that a sudden freeze will put you out of commission next season.

Engine Winterizing 101

At its most basic, winterizing means draining any water aboard or replacing it with enough of the right kind of antifreeze to protect against the lowest temperatures your boat might experience. No matter what kind of engine you have, there are three essential winterizing steps:

  • Change the oil and oil filter. Residual acids and moisture left in the crankcase over the winter can pit bearings and other vital engine parts, so it's important to change the oil just before laying up the boat. For best results, run the engine for a few minutes to lower the oil's viscosity and pick up impurities. Shut off the engine, change the oil, and replace the filter. Restart the engine and run for about a minute to circulate the fresh oil to internal parts. While it's running, check to make sure the oil filter isn't leaking.
  • Change the fuel filter and stabilize the fuel. Running the engine after this step ensures stabilized fuel reaches critical fuel system components. Replace the primary and secondary fuel filter(s) and/or change the fuel filter canisters. Like the oil filter, the fuel filter should be checked for leaks while the engine is still running. If your boat runs on gasoline and your gasoline has ethanol in it (E10), the ethanol will absorb moisture over the winter. If it absorbs enough water, the ethanol can separate from the gasoline, creating a caustic mixture at the bottom of your tank that can damage the engine. This process, called phase separation, can only be completely prevented by emptying the tank. If that isn't practical, add stabilizer to keep the gas from oxidizing over the winter, then fill the tank to 95-percent full to allow for expansion. A tank that's almost full has less "lung capacity" to breathe in moist air, so the ethanol should not be able to absorb enough moisture to separate. Diesel tanks should also be left full over the winter to minimize condensation and algae growth.
  • Drain the water from the engine or use antifreeze. This is the part that seems most often to confuse owners, and mistakes in this step lead to the vast majority of the freeze claims in the BoatUS Marine Insurance files. Exactly how this is done varies from one type of engine to another.

Winterizing An Inboard

Most boat engines these days are fresh water-cooled. That means there are actually two systems — a raw-water side and a closed-loop system that contains a coolant/water mixture. The raw-water side cools the coolant/water mixture, which in turn cools the engine.

Winterizing the fresh-water side simply involves making sure its year-round permanent coolant is an adequate mixture to meet the manufacturer's recommendations for your locality, typically a 50/50 mix with water. The coolant you use in the fresh-water system is not the same as what you'll use on the raw-water side of the engine — ethylene glycol is effective but highly toxic. With the engine at room temperature, slowly remove the cap on the expansion tank and check the mixture with a coolant hydrometer. If the coolant is rusty or dirty, it needs to be drained and replaced with a fresh mixture. (Resist the temptation to use more than 50-percent coolant. An overly rich mixture will congeal, with 100-percent coolant turning to a solid gel at -8 degrees F.)

The preferred technique for winterizing the raw-water cooling side of a fresh water-cooled engine, circulating nontoxic antifreeze (propylene glycol) throughout the cooling system, protects the engine better than draining it and, assuming that the antifreeze you use also contains rust inhibitors, can actually extend engine life. It also is far less likely to result in freeze damage, as long as the engine is run long enough to circulate the antifreeze to every part of the cooling system. On some boats, it can be difficult to reach the intake hose for the engine, or to get it off the seacock. A variety of products have been designed to make getting antifreeze into the engine possible without undoing that hose. These include three-way valves that attach to the seacock or the sea strainer, funnels that fit inside the sea strainer, and reservoirs with hoses that can be run to the raw-water intake. Google "boat winterizing products" and you'll find a selection. If you choose to use one of these, just be certain that the antifreeze reaches everything downstream of where you're adding it, not just upstream.

Photo of a burned boatHeaters are not a good alternative to winterizing. If the electricity goes out, your engine is left unprotected, and a fault in the heater, cord, or electrical system could cause a fire.

To winterize the engine with antifreeze, you'll need a five-gallon bucket and enough antifreeze for your engine and related plumbing (at least two gallons). If your engine takes a lot of antifreeze, you might also need a helper to add antifreeze to the bucket as needed. Note that in raw water-cooled engines a thermostat blocks water flow in some cooling passages until the engine heats up. To be thoroughly protected, raw water-cooled engines must reach operating temperature and the thermostat must be open before adding the antifreeze. One method is to remove the thermostat, a simple job on most engines. It can be left out for the winter, but don't forget to reinstall it in the spring along with a new gasket.

Here's how to circulate antifreeze through the engine:

  1. If the boat is in the water, close the intake seacock.
  2. Fill a five-gallon bucket with antifreeze. Have additional antifreeze handy if you know your boat takes more than that, or if you've never winterized before. You can use any leftovers in the boat's plumbing systems.
  3. Remove the raw-water intake hose from the seacock (this is sometimes difficult, as hose clamps and hoses may be mechanically frozen in place) and insert the end in the bucket.
  4. Start the engine and run at idle until antifreeze discharges from the exhaust outlet for 30 seconds or more.
  5. For gasoline engines, fog the cylinders.
  6. Shut off the engine and secure the intake hose back on the seacock.

In addition to the bucket and the antifreeze, for gasoline engines you'll also need a can of engine fogging fluid or light motor oil, some plastic sheeting, and duct tape.

If a fuel valve is installed, turn it off about halfway through the process to run the carburetor dry. For the last 10 seconds or so of engine operation, spray the fogging fluid into the flame arrester on the carburetor. Light oil can be trickled down the carburetor if fogging fluid isn't available. This may stall the engine, which is OK. Finally, wrap duct tape around the flame arrester and cover the top of the engine with plastic after it has cooled. This helps prevent corrosion from forming on internal engine parts. Attach a tag somewhere as a reminder about the duct tape, which will prevent the engine from starting next spring. Note that diesel engines don't need to be fogged.

Winterizing Sterndrives

There are different types of sterndrives, so check your manual for winterizing specifics. Some inboard/outboard (I/O) engines take in cooling water from a thru-hull in the engine compartment, and some take it in through the outdrive. With I/Os that use a thru-hull, the outdrive will be protected once the engine has had antifreeze run through it because the antifreeze also runs through the outdrive. Other models may require the use of "muffs" to first flush, then run antifreeze through the drive. For those drives, warm up the engine with the muffs, then use a five-gallon bucket of antifreeze plumbed to the muffs and run the engine until it's all been run through. Afterward, all outdrives should be stored in the down position to prevent water from entering the exhaust hub of the prop. In cold weather, any accumulated water in the outdrive can freeze; ice may expand and crack the housing.

While not technically necessary for winterizing, it makes sense to drain and fill the outdrive oil now. It's not difficult and will protect your outdrive from the effects of worn-out lube over the winter while letting you hit the water faster in the spring. Lower the outdrive as far as possible, and drain the oil by removing the lube drain/fill screw at the bottom and the vent plug screw at the top. Keep in mind that refilling your outdrive oil is counterintuitive: New oil must be pumped UP into the outdrive; never refill from the top port or vent hole. Pump fresh oil into the drive from the drain hole at the bottom until fluid appears out the vent hole at the top. Install the vent screw, which slows the lube from coming back out the drain, and then replace the drain/fill screw quickly. It's a good idea to put new gaskets on the plugs. If your boat has a reservoir in the engine compartment, fill it also.

Outdrives are expensive and have become a frequent target for thieves. Even if the boat is kept in your driveway, consider taking the outdrive off and storing it in your garage or basement for the winter. 

— Published: October/November 2014

Engine Room Checklist

Engine and Fuel

  • Fill fuel tanks and add stabilizer to gasoline
  • Change oil and filter
  • Change fuel filters/separators in engine(s)
  • Check coolant level in freshwater-cooling system and add coolant if necessary
  • Run antifreeze through raw-water-cooling system
  • Make sure water strainers are filled with antifreeze
  • Fog cylinders in gasoline engines
  • Top up battery electrolyte level and put batteries on marine charger

Installed Generator

  • Change oil and filter
  • Change fuel filters/separators
  • Check coolant level in freshwater-cooling system and add coolant if necessary
  • Run antifreeze through raw-water-cooling system
  • Make sure water strainers are filled with antifreeze

Other Systems

  • Flush and fill air conditioning system with antifreeze
  • Run antifreeze through water maker and pickle as per owner's manual


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