By Sam Halberstadt · Reviewed by Marina Chen
Editor · USCG-licensed Master 50 GT · Updated May 6, 2026
Why winterization matters
Water expands ~9% when it freezes. Trapped water in a powerhead, sterndrive block, or cooling passage can crack cast iron or aluminum housings. The damage often isn't visible — you start the engine in spring and watch coolant pour from a hairline crack.
Fuel left untreated for 4+ months degrades, gums up carburetor jets and injectors, and (with ethanol) phase-separates into a corrosive layer at the bottom of the tank.
Batteries left discharged in cold weather sulfate and lose capacity permanently. A 5-year battery left flat all winter may have 50% capacity by spring.
Plumbing (fresh water, raw water washdown, head, A/C) can freeze and crack the same way the engine can.
Fuel system: stabilizer and fogging
Add fuel stabilizer (Sta-Bil Marine, Star Tron) to a near-full tank — full tank reduces condensation, treated fuel resists phase separation. Run the engine for 10 minutes to circulate stabilized fuel through the carb or injectors.
Fog the cylinders: while the engine is running, spray fogging oil into the air intake until the engine stalls or runs rough on the oil. This coats the cylinder walls with a corrosion-preventive film.
Some four-stroke manufacturers have a specific procedure (no fogging, just stabilizer). Check your owner's manual — there's not one universal answer.
Cooling system: drain everything
Outboards: tilt fully down to drain water out of the powerhead, gear case, and tell-tale. Run on flushing muffs with fresh water for 5 minutes to flush salt before the final drain.
Sterndrives: remove drain plugs from the engine block, exhaust manifold, and any thermostat housing. Some require running antifreeze through the raw-water intake — follow the manual.
Inboards (closed cooling): freshwater coolant should be tested with a hydrometer; replace if specific gravity is below freeze rating for your region.
Lower unit and gearcase
Drain lower unit gear oil from the lower screw. Watch the color: clean = green/amber = good; milky white = water intrusion (a leaking seal that needs replacement before next season).
Refill from the bottom screw with 90W gear oil until oil exits the upper vent hole. Replace both gaskets.
Catching water intrusion in the fall vs the spring saves a season of corrosion damage.
Battery, plumbing, and the rest
Batteries: disconnect, clean terminals with baking soda and water, top up electrolyte (flooded lead-acid only), trickle charge or top up monthly. Store at >50% charge in a cool place above freezing.
Fresh water system: drain the tank, blow out lines with low-pressure compressed air, then run RV-grade non-toxic antifreeze (pink) through every faucet, the head, and the shower.
Bilge: pump dry, vacuum any standing water, leave drain plugs out and the boat slightly bow-up so any winter rain runs out the transom.
Cover and elevate: cover with a fitted boat cover or shrink-wrap. Elevate the bow slightly so rainwater runs off the cover. Open vents to prevent mold.
Step by step
- 1
Run the engine and add stabilizer
Add fuel stabilizer to a near-full tank, run engine for 10 minutes on muffs to circulate stabilized fuel through carb or injectors.
- 2
Fog the cylinders
While running, spray fogging oil into the air intake until the engine stalls or runs rough. Coats cylinder walls against corrosion.
- 3
Drain water from cooling system
Tilt outboard fully down. For sterndrives/inboards, pull drain plugs from block, manifold, thermostat housing per manual.
- 4
Change lower unit gear oil
Drain via lower screw, fill from top until oil exits the upper vent hole. Watch for milky color (water intrusion = failed seal).
- 5
Disconnect and store battery
Clean terminals, top up electrolyte, trickle charge monthly. Store at 50%+ charge in a cool, above-freezing location.
- 6
Winterize plumbing
Drain fresh water tank, blow out lines, run RV antifreeze through every faucet, the head, and the shower until pink fluid emerges.
- 7
Cover and elevate
Cover the boat, raise the bow slightly to drain rainwater, open vents to prevent mold.
Frequently asked
Yes for many models, though some manufacturers specify a different procedure (some say stabilizer-only). Check your owner's manual — there's no universal answer.
