How To Winterize A Boat

If you live in the southernmost regions of the United States, you can probably stop reading right now and chuckle with a smug sense of superiority that you can go boating all year round. If you live in places where it gets cold enough that you can’t go boating in the winter, this boat winterizing guide is for you.

Yes, you must winterize your boat, and that’s regardless of whether it’s powered by a two-stroke, four-stroke, inboard, sterndrive or outboard. It’s a necessary part of owning a boat. Depending on the size of your boat and the number of engines, it can cost between $500 and $1,000 to have your boat winterized for you, which is why a lot of people do it themselves. Generally speaking, there are several considerations when winterizing your boat:

  • Preparing the engine
  • Preparing the boat
  • Determining storage options
  • Considering springtime in advance
Winterizing Your Boat
Boats in Winter. Photo: Doug Logan.

Preparing the Engine

Probably all OEM marine propulsion manufacturers would prefer that a dealer technician perform your winterization procedure, but if you’re going to do it yourself because of the cost of professional services — and a lot of people do — here are some tips to help you do it right.

We all know that we have to stabilize the fuel, fog the cylinders and change the oil and filter then drain the cooling system of our engine(s) before we tuck our boats away for the winter, but there is an order of operations you will need to follow:

  • Fuel system
  • Cooling system
  • Engine oil

Why? Because if you change the oil and filter, then run the engine or engines to winterize the fuel system, you’ll be unduly exposing the new oil to contaminants and byproducts of combustion, which can lead to increased breakdown over the winter months. It would be worse, however, to leave the engine oil in that’s been run all season.

Your first step, preparing the fuel system for winter storage, has become far easier with fuel injection, but it was never too difficult with carburetors, so we’ll cover both. Experts have gone back and forth on whether you should fill or drain your tanks for winter storage, but the logic now is that it’s better that tanks be empty because of the methanol content in modern fuels.

Alcohol is hydroscopic, so it attracts and absorbs water. If there is no fuel in the tank, it can’t absorb water. Come springtime, you can put fresh gas in there, treat it with marine grade Sta-Bil or Sea Foam and go boating. Be sure to leave just a little bit in the tanks to complete the winterization process.

Now that you have the fuel system done, hook up a garden hose to your boat’s cooling system and run the engine at a fast idle so the treated fuel can get up into the carburetor’s circuits. About five minutes will do. After you’ve done that, it’s time to fog the engine so that the cylinders can better resist corrosion over the winter. It’s vital because when you shut off the engine, some of the valves — or ports in a two-stroke engine — will be open to the intake and exhaust manifolds, which can invite moisture from the atmosphere or any humidity that remains in the exhaust system.

The proper procedure for fogging a carbureted engine is to remove the flame arrestor and run it at fast idle at 1,500 rpm, squirt 8 ounces of something like Mercury Marine’s Quicksilver Storage Seal or a 20W engine oil into the primary venturis of the carburetor, then stall the engine by squirting the last 2 ounces of the oil into the carburetor. That’s it for a carbureted engine.

For fuel-injected engines, get a regular 6-gallon outboard fuel tank and put 5 gallons of regular 87-octane gas in it, put some two-cycle outboard oil in that tank and 5 ounces of fuel stabilizer.

In this procedure, you disconnect the fuel line from the water separating fuel filter inlet and connect the remote outboard tank with that fogging mixture, and if the boat’s out of the water, you want to hook up your water muffs to the sides of the lower unit, run the engine at 1,300 rpm for five minutes. Slowly reduce rpm to idle and shut off engine. On engines without the cool fuel (system), run them for 10 minutes at 1,300 rpm.

Now you can address the cooling system. The idea is to get all remaining water out of the system so you can avoid damage caused by freezing, which is not covered by any warranties. On older engines, there are pipe plugs in the bottom of the exhaust manifolds and other coolant passages, but it’s still nearly impossible to get all the water out of a block regardless of its age, which is why companies like MerCruiser recommend filling the cooling system with propylene glycol coolant. Here’s why.

Earlier model engine blocks had brass plugs and you can’t just twist them off with your fingers. On modern engines, most manufacturers are using plastic drain plugs with a wing-nut you can remove easily. You can see them easier, for one thing, so it’s easier to see where the points are to drain your engine. In the past, you may not even have known where to look, for one thing, and if you didn’t know where to look, you’d never find the plugs, so the OEMs have tried to simplify that. And, of course, now there are systems that are a little more user friendly, like the quick drains and the air-actuated drains that didn’t come on earlier engines.

You can be sure to fill the engine completely by removing the water outlet, or thermostat housing, then pouring coolant in until it overflows. For big blocks, you’ll need between one and two gallons per engine, less for smaller engines.

When spring comes, drain the coolant into a collection pan, then dispose of it in an environmentally responsible way. Propylene glycol is nontoxic in and of itself, but after it’s been in your engine, it may become contaminated with petrochemicals. Many auto parts stores accept refuse coolant. If not, a local radiator shop will have collection tanks.

For engines with closed cooling systems, which is most of them if they were built in the last 10 years, drain water from raw water side of cooling system, and fill it with propylene glycol. As for the closed system, the engine coolant, you shouldn’t need to mess with it. The coolant is designed to last five years between change intervals.

At this point, your oil should still be warm from running the engine during the fogging process, so it should be easy to drain and change. On stern drives, be sure to store the boat with the drive in neutral trim position so the bellows boot isn’t stretched on one side all winter.

That’s the basic winterizing procedure, but here are a few more helpful hints you might consider:

  • Lubricate cables and linkages. You can use 30W oil or 25W-40 from or a non-flammable spray lubricant.
  • Drain the sterndrive or outboard lower gear case and transmission, if your boat has one.
  • Lubricate the engine coupler, gimbal bearing and U-joints. You also might consider having the engine aligned with the drive, which is something a MerCruiser dealer should do periodically anyway.
  • Change your spark plugs and the water-separating filter on EFI engines.
  • Remove the boat’s batteries and keep them in the garage on an automatic trickle charger for an extra measure of protection.
  • It’s a good time to “nut and bolt” the whole craft. That means putting a wrench on every nut and bolt to ensure they’re tight. The side benefit of that practice is that you can potentially spot other problems, such as wiring and plumbing problems or a worn cable.

Preparing the Boat

If your boat has creature comforts such as a freshwater system, air conditioning or a gray water system, winterizing is more involved. You can’t have water freezing in the hoses and plumbing, because a broken hose or pipe can be difficult to access and repair, particularly if it’s belowdecks.

If you have a water heater, disconnect it from the power supply, then disconnect it from the onboard water system and drain it. Be sure to refill the heater first in the spring.

Drain the freshwater system and refill with four to six gallons of marine antifreeze, again, propylene glycol. Turn on the system’s water pump and then open the faucet farthest from the pump. When the antifreeze begins to come out, turn off the tap. Repeat the same procedure on the cold tap. Be sure to dispose of the antifreeze in the spring in an environmentally responsible way and flush the system.

Some other items to consider for preparing the boat for winter:

  • If storing indoors, lay the boat covers over the boat but don’t snap them on.
  • Remove moisture wicking items from the stowage compartments.
  • Put the cushions back in place, but leave gaps so the stowage compartments underneath can breathe.
  • Lay Isinglass side curtains flat, and don’t put anything on top of them.
  • Use a product such as Damp Rid inside the cockpit and interior if you’re storing the boat outside.

Determining Storage Options

In terms of storing the boat, you have a few choices: outdoor, indoor cold, indoor heated, and you will have a commensurate price escalation in the order these options are presented.

If you can afford it, heated indoor storage is the kindest to your boat, for all the obvious reasons. Indoor cold storage comes next. Ventilated buildings work best, but it’s still important to have all the onboard fluid systems ready for freezing temperatures.

If you’re storing a boat outside, consider shrink-wrapping it. Essentially, professional installers will tape some plastic wrap to the gunwales of your boat, then heat it with a large torch to make the film contract and form a tight seal around the boat. There are do-it-yourself kits available through marine retailers, but it’s probably best just to have someone else perform this service — what with the open flame near your boat’s gelcoat and upholstery and all. Also, if you use a professional installer, you don’t have to store big rolls of film and all the tools needed to do it.

Done professionally, shrink wrapping will keep snow, water and ice from accumulating in your boat or on the plastic film itself. Professional installers can add straps and “tent poles” so that water, snow and ice just slide right off. They also can add vents and even zippered doors so you can still access the inside of the boat during winter.

Most storage yards and marinas offer the service, and mobile services can come right to your home or storage lot to wrap your boat. Prices range from roughly $10 to $50 a foot. Be sure they are bonded and insured — what with the open flame near your boat’s gelcoat and upholstery and all.

Considering Springtime in Advance

When you are performing your winterization ritual, think about your future self. Don’t be in a hurry. Do things right. Do things you’ll be able to understand six months from now.

One form of “cheap insurance” is replacing the water pump impeller during spring fit out. Mercury specifies that it be serviced every other year, or 200 hours, whichever comes first. However, lots of owners replace them as part of their annual spring fitting out. In northern climates, it is best to install them before the boating season begins rather than as part of winterization because the rubber impeller blades tend to “take a set” when stored for long periods, especially at plunging temperatures.


You can look at winterization either as post-season drudgery, or as a means to get back out on the water as quickly as possible when springtime comes. You’d be well served to take the latter perspective.

Winterization is important, and done correctly it makes spring fit-out easier and quicker, and it will help give your boat the stamina to help it last a lifetime.

Editor’s Note: This article was first published in 2018 and last updated in October 2020.

Written by: Brett Becker

Brett Becker is a freelance writer and photographer who has covered
the marine industry for 15 years. In addition to covering the ski boat
and runabout markets for, he regularly writes and shoots for Based in Ventura, Calif., Becker holds a bachelor’s
degree in journalism and a master’s in mass communication from the
University of Central Florida in Orlando.


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