What to do before the winter cover goes on

October 1, 2026

By early December most boats in the North are on the hard, and boats in the South are still in the water, one cold snap from trouble. Either way, the cover is the last step, not the first. Here's what has to be done before it goes on, drawn from the BoatUS guide built on a decade of freeze claims.

Why it matters

Water expands by about 9% when it freezes. BoatUS reviewed ten years of freeze claims and found more than three-quarters were cracked engine blocks or exhaust manifolds, caused by water left in the engine or cooling system. Its short list of essentials would have prevented more than 95% of those claims.

Southern boaters aren't immune. BoatUS says its southern claims include many boats that weren't winterized at all, or that relied on a heater that failed when the power went out. Unless you're in Hawaii or the Florida Keys, BoatUS recommends winterizing the engine if the boat will sit for even a few weeks.

The three engine essentials

1. Change the oil and filter. Run the engine briefly to warm the oil, change oil and filter, then run it for about a minute and check for leaks. Old oil holds acids and moisture that pit bearings over the winter.

2. Change the fuel filter and stabilize the fuel. With ethanol gas, moisture can cause phase separation over the winter. Emptying the tank is the only complete cure. Otherwise, add stabilizer and fill to about 95% to leave room for expansion. A near-full tank breathes in less moist air.

3. Get the water out of the engine. This is where most mistakes happen. For inboards, BoatUS prefers circulating non-toxic (propylene glycol) antifreeze through the raw-water side:

Close the intake seacock if the boat is in the water.
Put the raw-water intake hose in a five-gallon bucket of undiluted non-toxic antifreeze.
Run the engine at idle until antifreeze comes out of the exhaust for 30 seconds or more.
On gasoline engines, fog the cylinders, which will stall the engine.

The engine must reach operating temperature first so the thermostat opens. If it hasn't, antifreeze bypasses parts of the block. On freshwater-cooled engines, check the closed-loop coolant with a hydrometer. BoatUS warns against running more than a 50/50 mix.

Outboards and outdrives

Outboards: stabilize the fuel, flush with fresh water using muffs, then fog while running. Some newer outboards have a built-in winterizing mode. Drain the gear case and refill with fresh lube, store the motor in the down position, and check the anodes.
Outdrives: if the water intake is in the lower unit, run antifreeze through it on muffs. Change the gear oil and check the anodes.

Below decks

Drain the freshwater tank and water heater, bypass the heater, and run antifreeze through the lines.
Pump out the holding tank and put antifreeze in the head.
Put antifreeze through live wells and washdown pumps, per the owner's manual.
Clean and dry the bilge. Check that the bilge pump and float switch work.
Top up battery electrolyte (if your batteries take it) and put the batteries on a marine charger.
Take home cushions, electronics and portable propane.

If it's staying in the water

Close every seacock except the cockpit drains. Double-clamp hoses where you can. Plug the exhaust ports. BoatUS has claims where muskrats chewed through exhaust parts and sank the boat. Center the boat in the slip with chafe guards on the lines.

Then the cover

A cover has to be supported and sloped so snow slides off, and ventilated so moisture can escape. Shrinkwrap without vents traps condensation and breeds mildew, and it can lift the paint where it touches two-part polyurethane. BoatUS's advice on shrinkwrap is blunt: leave it to professionals. It takes one moment of inattention to set the plastic alight, and a fire inside the cover may not be visible at first.

If the yard winterized the boat for you, ask for the work order and confirm the engine was run up to temperature on antifreeze.

Sources

boatus.com — claims analysis (>3/4 of freeze claims = cracked blocks/manifolds; list prevents >95%), 95% fuel fill, oil/fuel filter, antifreeze method, fogging, outboard/outdrive steps, in-water seacocks, shrinkwrap ventilation and fire risk, full checklist
boatus.com — BoatUS checklist release (page body did not render; used only as a pointer to the guide)

From The Logbook, Waterline’s weekly email for people who love their boat. Send us a photo of yours and see it drawn before you pay.

Back to blog