How to Winterize a Boat: Outboard, Sterndrive and Inboard

If your boat will sit through freezing weather, winterizing is the most important maintenance job of the year. Water left in the wrong place can freeze, expand and crack an engine block, a pump or a fitting. Over a long lay-up, moisture also invites mold, mildew and corrosion. Discover Boating, the industry's consumer program, puts the main goal simply: keep water and moisture out as much as possible.

This guide explains what winterizing involves for each engine type and the systems that are often forgotten. It also helps you decide whether to do it yourself or hire a pro.

Always follow your engine and boat manufacturers' instructions. Procedures differ between makes and models. This guide explains the general approach; your owner's manual has the specific steps.

Why it matters, including for insurance

The Maryland Insurance Administration's boat insurance guide lists freezing among common policy exclusions, unless reasonable care has been used to winterize the watercraft. In other words, skipping winterization can leave you paying for freeze damage yourself. Check your own policy's wording. See Boat Insurance Explained.

The overall checklist

Discover Boating's step-by-step outline:

  1. Prep the engine. Make sure all water is drained and removed.
  2. Protect the engine against corrosion, including fogging if your manual calls for it.
  3. Add fuel stabilizer to the fuel, then run the engine so treated fuel reaches the whole system.
  4. Change fuel filters and any fuel/water separators.
  5. Drain freshwater plumbing: sinks, tanks and heads.
  6. Add non-toxic antifreeze to the plumbing.
  7. Remove water from every other system: raw-water washdowns, livewells, the bilge and pumps.
  8. Remove the drain plugs (on land).
  9. Cover the boat or put it into storage.

The sections below explain how this differs by engine type.

Outboard engines

Outboards are generally the simplest to winterize.

  • Let the cooling water drain. Discover Boating notes outboards are designed to drain their cooling systems completely when tilted all the way down. Leave the engine down for storage. If you cover it, allow enough ventilation that condensation doesn't form.
  • Protect the internals. Older carbureted and two-stroke outboards were traditionally "fogged" by running fogging oil through them. Many modern outboards use an easier method: some makers offer an additive you run through from a portable tank, and some engines have an electronic "winterize" mode. Follow your manual.
  • Change the lower unit (gearcase) oil. Fall is a good time. Look at the old oil for signs of water getting in. Change the powerhead (engine) oil too if your engine uses it, so it sits with fresh fluids.
  • If you boat year-round: Discover Boating notes that long periods of not running cause more problems for outboards than cold itself. If you can run the engine to full operating temperature every few weeks, extra winterization steps usually aren't needed.

Sterndrives

Sterndrives take more work because water has to come out of both the engine block and the drive unit, and antifreeze may need to go through both.

  • Warm the engine to operating temperature first, to drive out moisture before fogging.
  • Drain the block by removing the block drain plugs, as your manual directs. Discover Boating notes this also helps clear sediment and scale. Alternatively, run antifreeze through the cooling system.
  • Fog the engine.
  • Change the oil and filter and the drive's gear lube.
  • Inspect hoses, clamps, the bellows and other wear parts after the season.

Inboard engines

Inboards are treated much like sterndrives, minus the drive unit.

  • Drain the engine by opening petcocks or removing plugs, or run antifreeze through the cooling system. Discover Boating says running antifreeze is usually the preferred method.
  • Warm the engine first. Until it reaches operating temperature, the thermostat can block antifreeze from reaching some cooling passages.
  • If the boat stays in the water: Discover Boating describes closing the intake seacock, taking the intake hose off it, putting the hose end into a bucket of environmentally safe antifreeze, and running the engine until antifreeze comes out of the exhaust.
  • Fog the engine, change the oil and filter, and inspect for parts that need replacing.

The fuel system (all gasoline boats)

  • Add a good fuel stabilizer and run the engine so the treated fuel reaches the whole system. Discover Boating warns that ethanol-blended fuel left untreated can separate over the winter, leaving water-laden fuel in the spring.
  • Replace, don't just empty, fuel filters and fuel/water separators. Discover Boating warns that tipping a separator out and refitting it can let water and debris get to the wrong side of the filter.
  • Separately, Discover Boating's engine-care advice notes that no marine engine is certified for fuel with more than 10% ethanol (E10).

Freshwater plumbing, heads and holding tanks

  1. Open all faucets and run them until the freshwater tank is empty, then close them.
  2. Add non-toxic (potable) antifreeze to the tank.
  3. Open the faucet farthest from the tank until antifreeze comes out, then close it. Repeat with each faucet in turn, working back toward the tank.
  4. If you have a water heater, run both hot and cold taps. Some systems let you bypass the heater and drain it separately.
  5. Pump out the holding tank and pump antifreeze through the heads.
  6. Empty and remove any portable toilet.

Everything else that holds water

  • Run raw-water washdowns and livewell pumps until they're dry. If water is still trapped, add antifreeze.
  • With the boat on land, remove the drain plugs and make sure the boat sits so the bilge drains completely.

If the boat comes out of the water

Discover Boating suggests:

  • pressure-washing the hull;
  • cleaning growth off the propellers, shafts, rudders, struts and trim tabs;
  • cleaning the through-hulls and strainers;
  • opening the seacocks so they can drain;
  • checking the hull for blisters and noting anything for your service shop.

Batteries

Discover Boating's spring guide notes many owners take batteries out for the winter and keep them on a maintenance charger. If you leave them aboard, keep them charged.

DIY or professional?

Discover Boating's cost pages give these national ranges:

  • Professional winterization: roughly $150-$600 per season
  • DIY supplies: roughly $50-$100
  • Shrink-wrap storage for a 21-foot boat: roughly $200-$300 per winter
  • Indoor rack storage for a 21-foot boat: around $1,000 per season

DIY makes sense for a simple outboard boat if you're comfortable following the manual. For sterndrives, inboards and boats with plumbing, many owners use a professional, often combined with the annual engine service. Add your choice to the True Cost tool to see how it affects your yearly budget.

When spring comes

De-winterizing ("spring commissioning") reverses much of this:

  • put the plugs back in;
  • flush the antifreeze out of the plumbing;
  • reconnect and charge the batteries;
  • inspect fuel lines;
  • run the engine on a flusher before launch.

See First-Season Checklist and Boat Maintenance Calendar by Season.

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