Ultimate Guide to Winterizing Your Boat's Engine and Internal Systems

Preparing your boat for the winter prevents damage from freezing temperatures, but it does so much more when you follow a full boat winterization checklist. Don’t let winter storage lead to mold growth, corrosion on metal components, or other forms of costly damage.

Unless you live in a warm climate where you can boat year-round, you’ll need this boat winterization guide to use as a checklist, whether you’re planning on tackling the tasks yourself as a DIYer or you want to hire a professional service and need to make sure you’re getting the right package. Tackle those boat maintenance chores with our help.

The Engines

The most critical step in winterizing a boat is preparing the engine or engines for cold temperatures.

Outboard motors are the easiest to winterize because they’re used on smaller boats that are easily lifted or tilted out of the water to keep the motor from freezing.

For inboard and stern drive engines that are located inside the boat instead, marine engine winterization requires more care and experience. You must drain these engines manually and fill them with antifreeze designed for marine use. We'll go over how to do this below, but if you’re not confident in handling this step yourself, make sure you have a professional do it instead.

1. Fuel System

All fuel contains at least a small amount of water from the atmosphere, and it tends to condense on the upper part of the tank if there’s an air gap left due to partial filling. You can drain the fuel out of most outboard engines with ease, and it’s recommended that you at least clear fuel from the carburetor to protect it.

For inboard and stern drive engines, keeping your fuel tank full over the winter is the best way to prevent condensation that occurs in cold air. A full tank is less likely to experience corrosion from the condensation during storage, and it’s easier to achieve than a fully drained fuel tank.

After filling the tank, make sure to add an appropriate amount of fuel stabilizer to prevent impurities from affecting the engine when the boat is started next spring. The stabilizer stops the breakdown of the fuel and reduces condensation that leads to corrosion.

2. Oil & Fluids

Change the engine oil to protect it over the winter. Inboard and stern drive engines especially need the new oil, but outboard motors benefit from gear oil changes as well, since fresh oil is free from acids that can damage the engines over the winter.

To change the oil, warm up the engine for a few minutes to get the oil moving, then shut it off and let it sit for about 10 minutes before draining it. Be careful not to touch the warm oil to avoid burns.

While you're changing the oil, change the filter as well and consider adding an oil stabilizer if you’ll have the boat in storage for more than three months.

Lastly, check transmission fluid levels, and look for signs of moisture — like separation in the fluid. If the fuel has a significant amount of water mixed in, you should change it. Then use the gear and lower unit lubes that are recommended by your boat engine manufacturer to ensure you’re matching the right GL rating to fight corrosion.

3. Cooling System

To prevent major freezing damage to your boat, you have to drain all the remaining water from the engine cooling system and bilge. Each boat has a different draining procedure. Most require only gravity to drain, but other boats may need you to tilt or lift them, and a few require an air compressor to blow the drainage system out. Your boat's manual will help you decide which procedure is right for your engine. Start with the main drain plug and run all pumps with the outlets open until they run dry.

Once you have the water out of the boat’s plumbing system, the blocks and manifolds of the engines, and the bilge, you’re ready to add antifreeze. First, flush the engine’s water compartments with fresh water to remove salt residue if necessary, then add the antifreeze according to the manufacturer’s directions. Missing even a small amount of water can have big consequences, so double-check your work before storing your boat in a cold climate.

4. Internal Components

Fogging the engine cylinders is the best way to protect them against rust and corrosion during winter storage. If your boat features a carbureted engine, start the process with the boat running, and spray the protective oil directly into the carburetor. The engine will stop on its own. Then remove the spark plugs and spray oil down onto the cylinders to coat them.

For boats featuring fuel-injected engines, you’ll mix a fogging oil product into your fuel. Run your engine until you see white smoke from the exhaust, then turn it off.

Either way, it’s a simple step that can provide a lot of protection against moisture damage.

The Hull and Exterior

Taking care of the hull before sending your vessel off for boat storage not only prevents further damage but speeds up recommissioning in the spring. Stick to the following checklist for preparing the hull, and make plans for more extensive refinishing or repairs as needed after giving your hull a winterizing inspection.

1. Thorough Cleaning

The decks, hull, and cockpit all need a thorough scrubbing to remove all deposits of dirt, grime, algae, marine growth, and salt. Use boat-safe cleaning products to treat stains. Once the exterior is clean, apply a protective coating of marine wax.

2. Damage Inspection

Winter cleaning prep is the perfect time to inspect the hull up close for any signs of damage. If you notice extensive damage like cracks and blisters, consider getting them repaired before long-term storage. Minor issues like scuffs, thinning coatings, and fading paint can wait until the spring if that’s more convenient for your schedule.

3. Choose a Storage Option

The condition of the hull affects which winter storage option is right for your boat. In-water storage tends to provide the most affordable protection from the elements. But if that’s not an option and your boat is smaller, bringing it home on a trailer gives you a chance to keep a close eye on it and keep it wrapped any way you prefer.

If the hull needs work, or you’re concerned about your in-water storage facilities, a dry stack covered facility may work best since it’s easier to ship your boat out for repair work once it’s out of the water.

The Interior and Systems

Skipping out on cleaning steps in the cabin could lead to costly mold issues or damage to soft materials like carpeting and upholstery. This is particularly important for sailboat winterization, especially for live-in vessels with extensive accommodations.

1. Electronics & Valuables

Remove any electronics, personal items, personal flotation devices (PFDs), and valuables like decorations. You should also pack up flares and other safety devices that can expire, such as fire extinguishers. Not only can some of these items go off and cause damage if left in place but they also expire faster than expected if left in the boat while it’s stored.

2. Freshwater Systems

To protect your drinking water tanks, you'll need to pour a non-toxic marine-grade antifreeze down each drain and use the intake hose on the seacock to pump it through the water systems. Don’t stop pumping antifreeze from your bucket until it runs through all the faucets, shower heads, and other outlets in the boat. Then shut the faucets and turn off the pump.

Last, drain the freshwater tanks and add a small amount of antifreeze to each one to prevent damage from any moisture that remains.

3. Sanitation (Head)

Empty any waste holding tanks, rinse them out, and add antifreeze. Fill the discharge and intake hoses with antifreeze as well. Then pump the antifreeze through the system until it fills the bowls of the heads to prevent damage from freezing. Pour enough into the intake lines that it flushes out of the hull.

4. Ventilation

Wipe down all surfaces to remove oils and dirt that can feed mold. Maintain good air circulation in the boat by opening windows and portholes before storing, even if you plan to wrap the boat. Use passive dehumidifiers or moisture-absorbing products to prevent mold and corrosion during storage.

5. The Battery

The battery can’t stay connected and in the boat over the winter because it will slowly lose charge and could corrode. Disconnect each battery in your system and clean any white residues from its case. Store the battery in a cool, dry place that doesn’t freeze, and use a trickle charger to maintain its charge over the winter so it’s ready to go when spring rolls around.

6. The Cover

Using tarps may protect your boat from residue and UV damage while it’s in storage, but inexpensive plastic often leaves gaps for insects to sneak in. Using shrink wrapping kits can provide a tighter fit. However, leaving shrink wrapping a boat up to the pros is recommended because it involves building a wood and webbing frame — which can damage the hull if not built correctly — to protect the deck from damage before applying the plastic.