Winterizing Your Sterndrive Boat: A Step-by-Step Guide

STEP 1 - Stabilize the fuel

Gas left sitting for months breaks down and turns gummy, and ethanol blends pull in moisture.

• Fill the tank to about 95% (not empty - an empty tank collects condensation; not 100% - fuel needs room to expand).
• Add fuel stabilizer at the dose on the bottle.
• Run the engine for several minutes on a water supply (muffs on the drive) so treated fuel reaches the lines, injectors or carburetor.
• Replace the fuel filter and fuel/water separator.

Pro tip: Never run the engine without water flowing to it - the water pump impeller can be destroyed in seconds.

STEP 2 - Change the oil and filter

Used oil holds acids and moisture that sit against bearings all winter. Fresh oil stores clean.

• Run the engine briefly first so the oil is warm, drains easily and carries contaminants out.
• Change the oil and filter, then check the new filter for leaks.

STEP 3 - Drain the engine block and exhaust manifolds (the most important step)

Most sterndrive engines are cooled by the lake or sea water they float in. Whatever is left in the block and manifolds expands as it freezes and can split the casting.

(raw-water cooled engines):

• Open every drain plug or petcock on the engine block and exhaust manifolds, and disconnect the cooling hoses your manual lists.
• Push a piece of stiff wire into each drain hole - rust flakes and sediment clog them and trap water behind.
• Let everything drain completely, then reinstall plugs per your manual (some owners leave them out and tag the engine).
• Alternatively, as your manual allows, circulate non-toxic propylene glycol antifreeze through the raw-water system until it comes out the exhaust. Make sure the thermostat is open so it reaches the whole system.

(freshwater/closed-cooled engines):

• Check the coolant with a hydrometer and keep it at a 50/50 mix. Replace rusty or dirty coolant. (Mixes stronger than 50% can actually gel in the cold.)
• The raw-water side of the system (heat exchanger, manifolds, pump) still needs to be drained or filled with antifreeze.

STEP 4 - Fog the engine

A thin coat of fogging oil protects cylinders from rust while the engine sits.

• After the cooling system is protected, spray fogging oil into the cylinders following your manual.

STEP 5 - Change the outdrive gear lube and check for water

The outdrive's gearcase sits at the lowest point. Water that slips past a worn seal settles there, freezes and can crack the housing.

• Remove the drain screw and let the gear lube drain.
• Look at it: milky or creamy lube means water got in - have the seals repaired before next season.
• Refill with fresh gear lube, pumping it in from the bottom until it comes out the top vent. Reinstall both screws.

STEP 6 - Inspect, clean and grease the drive

Small problems found now are cheap fixes over winter instead of launch-week emergencies.

• Scrape off any plant growth or barnacles.
• Inspect the rubber bellows/boots for cracks or pinholes.
• Wash the drive with soap and water and grease all fittings.
• Remove the propeller, check for wrapped fishing line, grease the shaft and reinstall.
• Check fluid levels in the power trim / hydraulic steering pump.

STEP 7 - Store the drive DOWN

Stored in the down position, water drains out of the drive instead of freezing inside and cracking the housing.

• Lower the drive fully for storage.

Pro tip: Outdrives are a frequent theft target. For long storage, consider having the outdrive removed and stored indoors.

STEP 8 - Drain the boat's water systems

Livewells, sinks, the head, showers and freshwater tanks freeze just like the engine.

• Drain tanks and the water heater. Run each pump dry for a few seconds.
• Run non-toxic (pink, propylene glycol) antifreeze through freshwater lines by opening each faucet and shower until antifreeze comes out, then add it to the water heater. Never use automotive antifreeze.
• Head: pump out the holding tank at an approved facility, flush several times with fresh water, and add antifreeze to the hoses, tank, macerator and Y-valve per the manufacturer.
• Empty, clean and dry the bilge. If you can't get every drop out, add a small amount of non-toxic antifreeze.

STEP 9 - Pull the hull drain plug

Rain and snowmelt drain away instead of pooling and freezing in the bilge.

• Remove the drain plug for storage on land.

Pro tip: Clip the plug to your ignition key or steering wheel so it goes back in before launch.

STEP 10 - Charge and disconnect the battery

Batteries slowly drain even with everything off, and a discharged battery can freeze; a fully charged one holds up far better in the cold.

• Fully charge the battery.
• Disconnect the negative cable first, then the positive.
• Ideally, take the battery home and keep it on a smart maintainer somewhere dry.

STEP 11 - Clean and dry out the interior

Damp gear under a cover turns into mold and mildew.

• Take life jackets, cushions, towels and lines home, or stand cushions on their sides to air out.
• Inspect and test safety gear (life jackets, fire extinguisher, flares) and electronics; replace anything that isn't working.
• Clean lockers and any fridge or cooler.
• Leave a moisture absorber aboard and add rodent deterrent - mice chew wiring and upholstery.
• Clean the hull, especially the bottom, and wax it. Growth wipes off easily in the fall but dries hard over winter.

STEP 12 - Cover the boat

Snow is heavy. A flat cover collects water, sags and tears.

• Use a fitted storage cover or professional shrink-wrap (better protection than a tarp).
• Prop the cover with a pole or frame so it peaks and sheds snow.
• Make sure it's vented so trapped moisture can escape.