Why Boats Crack, Freeze, and Corrode in Winter (and How to Stop It)

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16 Dec 2025 13:00

When the boating season comes to a close with the arrival of colder temperatures, it’s time to prep your boat to face off against Old Man Winter. This job is no small matter. Fail to winterize your boat properly and you could be looking at thousands of dollars in repairs from freeze damage in the spring (same goes for your sprinkler system, hot tub, and outdoor faucets, too). Ahead, we walk you through how to winterize a boat with help from Roland Afzelius, a product manager at Digital Sportsman, a service that matches outdoor enthusiasts with local guides, and the founder of Oyster Bay Water Sports, a charter company on the northern coast of Long Island, N.Y.

Why Winterizing Your Boat Matters

If you fail to winterize your boat properly, very bad things can happen to these parts of the vessel:

Engine: Fail to drain the sea water lines and the components inside the engine start freezing when temperatures drop. “Engine blocks can crack, manifolds and risers can split, plumbing lines can burst, and lower units can be ruined,” Afzelius says.

Fuel system: When temperatures drop to extreme cold, condensation in the fuel system can actually freeze, cracking components and clogging fuel lines.

Plumbing: Water left in a boat’s plumbing system can freeze, cracking plumbing lines and destroying water pumps.

Common Winter-Related Failures (Cracked Blocks, Burst Hoses, Corrosion)

Along with damage to the boat’s engine, fuel system, and plumbing, failing to winterize your boat can also lead to other issues. Ice can accumulate on the hull or deck, causing damage, and cold temperatures can also drain batteries.

Which Boats Are Most at Risk (Inboard, Outboard, Sterndrive, Freshwater vs Saltwater)

The type of boat you have can determine just how susceptible it is to freeze damage:

Stern Drive: “Stern drives are usually at the highest risk for freeze damage because their cooling systems, engine blocks and manifolds hold more water,” says Afzelius. The engine block, heat exchangers, manifolds, and cooling lines are all susceptible to freezing if not properly winterized.

Outboard: Outboard motors are designed to self drain before you shut them down, so they aren’t as susceptible to freeze damage. Afzelius does advise storing outboard motors tilted fully down to ensure water isn’t trapped inside parts of the motor.

Freshwater vs Saltwater: Boats used in freshwater are at a greater risk for freezing than those in saltwater. That's because freshwater freezes at 32 degrees Fahrenheit while saltwater has a lower freezing point of 28.6 degrees Fahrenheit.

Tools & Supplies You Need to Winterize a Boat

Fuel stabilizer
Fogging oil
Marine antifreeze
Oil and filter
Gear lube
Drain plugs
Cleaning supplies
Moisture absorbers

Make sure each fluid is rated for marine use. Use oil, oil filters, gear lube, and drain plugs that are all rated to work with the boat and engine. And don’t use just any cleaning supplies. Make sure to use those that are rated to work with boat surfaces without degrading their finish and that are biodegradable.

How to Winterize a Boat Engine

Afzelius recommends the following steps when winterizing a boat engine:

Step 1: Treat the fuel system with stabilizer.

Fuel stabilizer prevents fuel in the gas tank from going bad while the boat’s in storage, preventing it from gumming up the carburetor and lines. Add the correct amount of stabilizer for the amount of fuel in the engine.

Step 2: Run engine to circulate stabilizer.

Run the engine for about 10 minutes to circulate the stabilizer through the carburetor or fuel injectors.

Step 3: Change engine oil and filters.

“Change the oil and filters so dirty oil isn’t sitting in there all winter,” Afzelius says. Drain the old oil by removing the oil plug. Replace the old oil filter with a new one, then add new oil based on the engine’s specifications. Run the engine briefly to circulate the new oil through the motor.

Step 4: Flush cooling system.

Flushing the cooling system removes salt, sand, and other impurities that can corrode and clog parts. To flush the engine, attach a garden hose to the engine’s flush port, then run the water for about 10 minutes to flush the system. Run marine antifreeze through it until you see it come out the exhaust.

Step 5: Fog the engine to prevent corrosion.

While some people skip fogging the engine, Afzelius says it’s worth the extra effort. “(Fogging) puts a protective film on the cylinder walls and valves, and for an engine that’s going to sit for a while, it’s worth the few extra minutes of work,” he says. Choose a fogging oil that suits your engine type (2-stroke or 4-stroke). Warm up the engine for about 5 to 10 minutes. Then, with the engine still running, spray fogging oil directly into the carburetor or air intake. Continue to spray until you see white smoke coming out of the engine’s exhaust. For extra protection, you can also add fogging oil directly to the engine by spraying it into each of its spark plug ports.

Step 6: Drain and replace lower-unit gear lube.

Start by trimming the engine to its vertical position. Place a drain pan beneath the drain plug and then remove the drain plug with a screwdriver. Remove the top vent plug and allow the oil to drain into the pan. Attach the gear lube pump to the lower drain hole and to the new bottle of lube. Slowly pump the lube into the engine until oil starts coming out the upper vent hole in a steady stream. Close the top vent then remove the pump and attach the lower drain plug.

How to Winterize an Outboard vs Inboard/Sterndrive Engine

While the process for changing the oil and fogging the engines are very similar for outboard engines versus stern drives, winterizing the cooling system is quite different. Outboard motors are self-draining, so adding anti-freeze isn’t required. That’s not the case with sterndrive engines, which hold water in the block, riser and hoses, all of which must be drained and flushed with marine anti-freeze.

How to Prevent Rust and Corrosion During Long-Term Storage

Fogging the engine is the best step against corrosion on a boat's internal parts for both outboard and sterndrive engines. Draining the water cooling system and filling it with non-toxic anti-freeze is also key for sterndrive boats. To prevent corrosion on external parts, rinse with freshwater then wipe down with a corrosion inhibitor.

How to Winterize Boat Plumbing Systems

Draining Freshwater Tanks, Lines, and Pumps

Draining all of a boat's fresh water system is key to preventing the lines from freezing and bursting during the winter. Open all faucets, shower heads and freshwater receptacles, then turn on the freshwater pump and allow it to run until the boat's potable water tank is dry. Add five gallons of non-toxic antifreeze to the boat’s main potable water tank. Turn on the pump then turn on each fixture’s hot and cold tap until it runs pink, beginning with the faucet that’s furthest from the pump.

Using Non-Toxic Marine Antifreeze

Remember, only use non-toxic anti-freeze rated for boats and RVs as car antifreeze is highly toxic and will contaminate the system.

Protecting Showers, Heads, and Washdown Systems

Showers, heads, and washdown systems have pumps and valves that will freeze if not properly winterized. Drain shower and sink drains, toilets, and any deck washdown systems of water. Add antifreeze to each sink and shower drain, then run the pump until pink liquid runs out of the hull.

For toilets, disconnect the intake hose and place it in a bucket of non-toxic antifreeze. Flush or pump the toilet until pink antifreeze enters the bowl then add a few gallons of antifreeze into the holding tank.

For deck washdown systems, disconnect the intake hose that leads to the pump and put it in a buck of non-toxic antifreeze. Run the pump until a stream of anti-freeze comes out of the washdown nozzle.

Pre-Storage Boat Cleaning and Exterior Prep

Cleaning Hull, Bilge, Decks, and Canvas

Prep your boat for winter storage by rinsing it with fresh water. Clean the hull with a designated hull cleaner that can remove waterline stain and rust. Next, clean the deck using a deck cleaner and brush. Consider adding a marine wax or sealant after cleaning to make cleaning easier on the next go around.

The bilge is an ideal breeding ground for mold and mildew so cleaning it is crucial. Add bilge cleaner then use a brush to distribute it through the bilge. Pump out the cleaner using a wet dry vacuum, then rinse with fresh water.

Finally, clean the upholstery on the boat using a product that cleans and conditions marine vinyl to prevent it from drying out and cracking. Remove any cushions and store them indoors for the winter.

Mold and Mildew Prevention

If you plan on covering your boat when storing it, make sure to treat surfaces with a mold inhibitor after cleaning to prevent mold and mildew growth.

Lubricating Fittings, Cables, and Hinges

Many boats have grease fittings where you need to add lubrication periodically. You’ll find these fittings on steering systems, drive components, and trailer hubs. Locate the fittings and use a grease gun to apply waterproof marine lubricant.

Next, apply a thin layer of grease to any throttle or shift cables using silicone spray at any points where the cable is exposed from the protective sheath. Finally, apply marine grade lubricant on all joints and hinge pins and locks.

How to Store Your Boat for Winter (Indoor, Outdoor, or Trailer)

Shrink-Wrapping vs Full Cover Storage

Both storage options for your boat have advantages and disadvantages, Afzelius says. “Shrink wrap is the gold standard if you’re dealing with heavy snow or freezing winds,” he says. “It creates a tight, waterproof barrier that sheds snow and ice and keeps rain out.”

Canvas covers don’t offer the same level of protection but do allow for air flow, preventing mold and mildew growth. “Canvas covers are great since they’re reusable and more breathable,” Afzelius says. “They cost more up front, but if you take care of them, they last for years.”

If heavy snow and freezing winds are a concern where you live and store your boat, shrink wrapping it for the winter is the way to go.

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Ventilation To Prevent Condensation

Whether shrink wrapping or using a cover to store your boat, making sure there is good ventilation is key to preventing mold growth. If using a cover, make sure there is enough open space for ventilation. “I like to use support poles to keep the cover lifted for better ventilation,” Afzelius says. “I also put DampRid in the cabin and storage areas and make sure hatches are slightly open so air can circulate.” For shrink-wrapped boats, make sure the professional shrink wrapper installs vents near the top of the wrap and low on the sides of the wrap near the bow and stern.

Battery Removal and Electrical System Prep

Extreme cold is the enemy of any battery. Fully charge the battery then disconnect it, black cable first. Remove it from the boat and store it in a cool, dry location where the temperature won’t drop below freezing. Connect the battery to a trickle charger, to maintain the charge through the winter. Remove any electronics from the boat that might be damaged during the winter, including radios, fish finders, and GPS devices.

Common Mistakes to Avoid When Winterizing a Boat

Forgetting Drain Plugs

Boat owners often replace the boat's drain plugs when storing it. This is a mistake, because rain and snow can accumulate inside the boat if the drain plugs are left in.

Using Automotive Antifreeze

Automotive antifreeze is highly toxic and can corrode the internal components of boat engines and plumbing. “If you use automotive stuff, it will ruin your systems,” Afzelius says. “So you want propylene glycol designed for boats.”

Not Treating the Fuel System

Either leaving the fuel tank empty or with untreated fuel is a mistake. An empty take can collect condensation, causing the tank to rust. Untreated fuel will go stale, gumming up fuel lines and the carburetor. Fill the tank to 95 percent full, then add a marine-grade fuel stabilizer.

Leaving Moisture Inside the Boat

Make sure to remove any items that can hold moisture, including towels, ropes, buckets, cushions, and lines. “I learned the hard way when I accidentally left a robe in a compartment under the seat,” Afzelius said. “When I unwrapped my boat in the spring, the entire interior was covered with mold.”

FAQs About Winterizing a Boat

When should you winterize your boat?

You generally want to winterize a boat in mid fall, which is typically October to early November, but can vary depending on the climate. Keep an eye on weather forecasts and make sure to winterize your boat before temperatures drop below 32 degrees.

Can you skip fogging the engine?

You can forgo the fogging if you’re planning to store the boat in a climate-controlled space for a month or so. But if you’re putting the boat into storage for longer, then fogging is highly recommended to coat the engine’s internal parts and prevent corrosion.

Do electric boats need winterization?

Electric boats require less maintenance than gas-powered boats, but they still need winterization. Focus on properly winterizing the battery by charging it and storing it in a climate-controlled area, if possible. You should also clean the boat's exterior, drain and winterize any plumbing, and securely cover the boat.

How long does winterization take?

The amount of time it takes to winterize the boat depends on the boat’s size. That said, for most boats it takes a DIYer between two and four hours to properly winterize it.