Winter Layup Master Checklist (Step-by-Step, With Links to Each Job)

Don't wait for the first snow. Mercury's rule of thumb is to start winterizing once temperatures stay below about 50°F, and to finish before the first hard freeze. Where the boat spends the winter decides several later steps:

• Indoors or under a roof: the easiest on the boat; still winterize fully.
• Outdoors on its trailer: see the trailer tires and wheel bearing guides before storage.
• Ashore on stands or a cradle at a yard: see boat stands and blocking and the haul-out checklist.
• On a lift: see boat lift care.
• In the water: see winter in the water and de-icers. Bilge pumps and batteries must stay working all winter.

Use the boat one more time if you can. Note anything that needs fixing (a slow-to-start engine, a weak bilge pump, a leaking hose) so you can order parts over the winter instead of discovering it in spring. A warm engine also drains its oil better for the next steps.

Add a fuel stabilizer at the dose on the label and run the engine long enough for treated fuel to reach the engine (Mercury suggests about 10 minutes). Run it only with cooling water: in the water, in a test tank or on external flush muffs. An outboard's built-in flush port is for flushing with the engine off, never for running it; inboards, sterndrives and PWCs follow the method in their manual. Discover Boating stresses stabilizing ethanol fuel, which can phase-separate during a long layup. Then replace (do not just empty and reuse) the fuel filters and water-separating filter.

How full to leave the tank depends on your engine maker, tank and fuel, so follow your manual. Mercury's advice: stale fuel causes most of the spring problems its dealers see, so if the fuel is more than about a month old, use it up or have it pumped out and refill with fresh, stabilized fuel (ethanol-free if you can get it). Older tanks that vent freely to the air are best filled to about 95% to limit condensation while leaving room for expansion; newer sealed tanks don't collect moisture the same way and don't need to be filled.

• Ethanol fuel care
• Fuel additives explained
• Fuel-water separator
• Diesel boats: diesel fuel polishing

Fresh oil keeps acidic, contaminated oil from sitting in the engine all winter. Change the lower unit or drive oil too, and look at the old oil: milky or gray oil means water is getting in, which can freeze and crack the gearcase. Fix that before spring.

• Outboard oil change
• Lower unit oil change
• Diesel oil and filters
• PWC oil change
• Diesel transmission oil

This is the core job. Pick your guide:

• Outboard: winterize an outboard. Discover Boating notes outboards drain their cooling water when tilted fully down, so store them down.
• Sterndrive (I/O): winterize a sterndrive and exhaust manifolds and risers
• Gas or diesel inboard: winterize a diesel inboard and closed cooling coolant
• Personal watercraft: winterize a PWC
• Fogging: fogging an engine, only if your manual calls for it
• Generator: generator maintenance

On raw-water-cooled engines, BoatUS points out the thermostat must be open (engine at operating temperature) when you run antifreeze through, or some passages stay full of water.

• Inspect sterndrive bellows and grease the gimbal: sterndrive bellows and gimbal
• Grease the outboard: outboard grease points
• Pull the prop, check for fishing line and grease the shaft: propeller
• Check anodes: sacrificial anodes
• Check trim fluid: trim and tilt fluid

Drain the tank, bypass or drain the water heater (turn off its electric element first) and pump non-toxic antifreeze through every hot and cold outlet, starting with the one farthest from the tank. BoatUS warns that antifreeze first mixes with the water still in the lines, so keep running each outlet until it comes out at full color.

• Freshwater system winterizing
• Marine water heater
• Marine air conditioning raw water

Pump out the holding tank, flush with clean water and run antifreeze through the toilet, hoses and Y-valve. Portable toilets get emptied and cleaned. Check your toilet maker's instructions: BoatUS notes some heads advise against certain antifreeze because it can soften seals.

• Marine head and holding tank
• Sump and macerator pumps

Run livewell, baitwell and washdown pumps dry, then give them antifreeze if water stays trapped. Pump and sponge the bilge dry: Discover Boating notes bilge pumps never remove the last of the water, and what is left can freeze.

• Livewell and washdown pumps
• Bilge pump
• On the hard, exercise each seacock, then close it: seacocks and through-hulls

A discharged flooded battery can freeze and crack. Charge every battery fully, clean the terminals and either remove them to a cool, dry place or leave them aboard on a charger designed for long-term maintenance. Batteries off a charger need a top-up charge about once a month. Turn the battery switches off.

• Marine battery
• Onboard battery charger
• Parasitic battery drain
• Lithium batteries: follow the battery maker's storage instructions; see lithium battery conversion

Washing off salt, scum and growth now is far easier than in spring, and the haul-out is your best chance to look at the bottom.

• Hull cleaning and waxing
• Hull blisters
• Bottom paint and antifouling
• Aluminum boats: aluminum hull corrosion

Remove food, drinks and anything that can freeze or attract animals. Take cushions, PFDs, linens and electronics home if you can, or stand cushions on edge for airflow. Mercury suggests rolling clear enclosure curtains instead of folding them, so they don't keep creases. Prop the refrigerator, lockers and hatches open. Shut off the propane at the tank.

• Mold and mildew prevention
• Canvas and vinyl care and isinglass enclosure care
• Marine refrigeration
• Inflatable PFD care
• Propane system safety
• Sailboats: sail care and storage and rigging inspection

Mice, rats, wasps and spiders move into stored boats and chew wiring and cushions. Seal openings, remove food and nesting material, and set traps you can check.

• Rodent prevention
• Insects and spiders onboard

• Support: follow boat stands and blocking. BoatUS reports boats falling over when stands sat on soft ground without plywood, were too few or were not chained together.
• Drain: on a trailer, remove the drain plug, raise the bow so rain drains out and tag the plug somewhere obvious (the helm or the key ring) so you put it back in spring.
• Cover: use a fitted cover or shrink wrap, supported so snow and water cannot pool, padded at chafe points and vented. Never tie a cover to the jackstands: BoatUS has claims where wind filled a cover and pulled the stands out from under the boat. Leave scuppers and cockpit drains open.
• Secure: lock the trailer coupler, chain the wheels, remove valuables and never leave keys aboard. See theft prevention and security.

Visit every few weeks, and always after heavy snow, storms or a hard freeze, as BoatUS recommends. Check the cover, the stands, the bilge (and de-icer and bilge pumps if the boat is in the water) and battery charge. Use the repair list from step 2 to order parts. In spring, work through de-winterizing your boat, starting with putting the drain plug back in.