Boat winterization checklist

For owners and service companies · 4 October 2026

Winterizing means different things to different people, so the first step is to write down what the job covers. The BoatUS winterizing guide makes the point plainly: telling someone to winterize the engines and the fresh water system does not mean they will also close the seacocks and winterize the head.

This boat winterization checklist pulls its steps from the engine makers and BoatUS and says who recommends each one. Where the sources disagree, it says so. Your engine's owner's manual governs. BoatUS tells boaters to read the manual before they begin. Mercury, Yamaha and Suzuki also point back to their manuals for model-specific details.

When to winterize

BoatUS says to winterize if the boat will sit for a month or more, and to do it before the first hard freeze. A boat stored ashore freezes sooner, so it needs the work earlier. Mercury's version is a rule of thumb: once the temperature consistently dips below 50F, it is time.

Outboards

• Stabilize the fuel and run the engine so it circulates. Mercury gives 10 to 15 minutes (7 mistakes). Suzuki's published procedure, quoted on a dealer parts page, runs the engine at about 1500 rpm in neutral for five minutes.
• Flush with fresh water, then leave the engine trimmed fully down to drain. Mercury and Yamaha both say so, and the BoatUS checklist lists flushing with muffs and fresh water.
• Fog the engine. Methods differ by engine, see the fuel section below. Mercury's how-to adds one ounce of DFI outboard oil to each cylinder on direct injection two-strokes. BoatUS notes that some newer outboards have an automatic winterizing feature.
• Change the engine oil and filter on four-strokes. Mercury, Yamaha, Suzuki and BoatUS all list it. Mercury says every 100 hours or once a season.
• Change the lower unit gear lube and look at what drains out. Mercury says white or creamy lube can point to water getting in. Yamaha says discoloration or a foamy residue can do the same.
• Pull the propeller. Check the shaft for fishing line, regrease the splines and torque the nut (Mercury how-to, Boating Magazine).
• Grease the fittings, check the power trim fluid, replace the fuel filter. Yamaha and Mercury list these. BoatUS says to inspect or replace anodes, and Mercury puts the replacement point at about 50 percent wasted.
• Protect the powerhead. Mercury says to coat it. Suzuki says to spray the whole powerhead with a rust preventative, and adds that it should not be a penetrating oil product.
• Store it trimmed fully down, dry and ventilated. Mercury and Suzuki describe the position as upright or vertical, while BoatUS says tilted down. Boating Magazine also says not to tilt the drive back up and to guard against mice.

Inboards and sterndrives

Raw water system. BoatUS accepts two methods and calls circulating antifreeze the preferred technique for inboard engines. It also accepts draining and leaving the system dry, but says the draining has to be thorough because small pockets of water can crack expensive parts.

• Drain the block, manifolds and risers through the petcocks. Clear any that are clogged (Boating Magazine suggests a pipe cleaner). Drain the sea strainer, the seacock body and the water-lift muffler too.
• Circulate non-toxic propylene glycol antifreeze through the raw water side until it discharges at the exhaust for 30 seconds or more, with the engine warm and the thermostat open (BoatUS; Boating Magazine). Boating Magazine also says engine makers such as MerCruiser require propylene glycol with rust inhibitor where it freezes. We did not read the MerCruiser manual, so check it for your drive.
• Closed cooling: check the coolant mix on the fresh water side, typically 50/50, and still protect the raw water side (BoatUS).
• Change the oil and filter, and the fuel filters and water separators. Fog gasoline engines. Store outdrives in the down position. BoatUS lists all of these, and says to follow the manual for drive muffs and antifreeze.

Fresh water system

BoatUS says simply draining the fresh water system is not enough. Its procedure replaces the water with antifreeze.

• Drain the tank and the water heater, and bypass the heater so it does not fill with antifreeze.
• Run non-toxic propylene glycol antifreeze to every hot and cold outlet.
• Drain the shower sump. On the hard, open the seacocks so they drain.

Heads and holding tank

BoatUS says to empty the holding tank, then pump disinfectant and antifreeze through the bowl, tank and Y valve, and close the seacocks. The same guide cautions that some head makers advise against non-toxic antifreeze and specifically names Raritan's PH II. Check the head's manual before adding anything.

Bilge

• Clean and dry the bilge. Test each pump and its float switch.
• BoatUS says drain plugs must be out for the winter. In hurricane zones it mentions a one-way plug for a boat that is shrink-wrapped.
• If the boat stays in the water, do not winterize the bilge pumps. Test them instead (BoatUS).

Batteries

BoatUS says to take the batteries home or keep them on a marine smart charger, top up wet cells and protect the terminals. It warns against leaving an automotive trickle charger on long term. Mercury's advice is simpler: disconnect the batteries and store them somewhere cool and dry.

Fuel: where the sources disagree

Tank full or nearly empty. The sources disagree, and two Mercury pages give different advice.

• Fill it. Mercury's 7 mistakes says to fill the tank to about 95 percent before long-term storage. BoatUS says the same, to allow for expansion.
• Run it nearly empty. Mercury's how-to says that when possible you should run the tank almost empty at the end of the season and add fresh fuel before storage. It also says newer non-venting tanks do not need to be filled.
• Where BoatUS lands on ethanol. It says phase separation can only be completely prevented by emptying the tank. Its checklist says to empty portable tanks, and it notes that some dry racks require empty tanks.

The sources do not give one rule for every tank. BoatUS says portable tanks should be emptied, while other guidance above recommends filling some built-in tanks. Follow the boat and engine manuals and the yard's rules.

Does stabilizer stop ethanol separation? Mercury's 7 mistakes page says a stabilizer prevents it. BoatUS's E-10 guidance, as summarized in search results, says stabilizers do not prevent phase separation. We could not read that BoatUS page in full, so do not promise an owner that stabilizer solves it.

Fogging. Mercury squirts oil into the cylinders. Yamaha runs a treated fuel mix for 10 minutes at fast idle. BoatUS describes injecting fogging oil at the intake until the engine smokes and stalls, or pulling the spark plugs. Suzuki's published steps list a rust preventative and do not mention fogging oil. Engine design decides which is right, so follow the manual. Either way, replace the fuel filter.

Cover and ventilation

• Use a supported, vented cover. Do not tie it to the jackstands (BoatUS).
• If you shrink-wrap, vent it along its length, and leave the wrapping to professionals because of the fire risk (BoatUS). Mercury also says to use a breathable cover or shrink-wrap.
• BoatUS says to open the lockers for ventilation, remove food, take cushions home or stand them on edge, secure the hatches, and switch the breakers off except the bilge pump and the smoke and vapor alarms.

Before haul-out and storage ashore

• Check the anodes, propeller, shaft seal and shaft for fishing line, and the gearcase lube (Mercury).
• Check the through-hulls and seacocks. BoatUS says to open all of them while the boat is on the hard to make sure they have drained completely, then close them.
• Use a custom cradle or jackstands. BoatUS says at least three per side on boats over 26 feet. On outboard and sterndrive boats, unload the transom by lowering the drive onto a block, and level the boat.
• Recheck the stands after about two weeks (BoatUS).
• If the boat stays afloat, BoatUS says to visit every few weeks to check the lines, drains and bilges. It also says to confirm what your insurance policy says about ice and freeze damage.

For service companies: winterizing a fleet

The checklist above changes with every boat. A twin diesel with a head and a water heater needs steps that an outboard center console does not, and the owner's manual differs engine to engine. Doing a whole fleet from one generic sheet is how a seacock gets left open or a block left undrained.

In YachtGrade, each boat gets a checklist built from what is aboard, taken from its equipment profile, so the outboard boat never shows raw water steps and the twin diesel does. The tech takes photos of each step, and the owner sees the record through a report link with no account. See how the checklists are built and what the owner receives. It records what the tech checked and photographed. It does not replace the manual, and it is not a survey.

Common questions

When should I winterize my boat?
BoatUS says to winterize if the boat will sit for a month or more and before the first hard freeze. It says a boat ashore needs the work earlier. Mercury's separate rule of thumb is when temperatures stay below 50F.

Should I fill the fuel tank or leave it nearly empty?
The sources disagree. Mercury's 7 Mistakes page and BoatUS say to fill to about 95 percent, while Mercury's How to Winterize Your Outboard says to run the tank almost empty and add fresh fuel before storage when possible. BoatUS says portable tanks should be emptied. Check the manual and the yard's rules.

Do I have to use antifreeze in an inboard engine?
BoatUS calls circulating non-toxic propylene glycol antifreeze the preferred technique and also lists draining as an accepted method. Draining has to be thorough.

How do I winterize an outboard?
Stabilize the fuel and run the engine, flush with fresh water, fog or protect the engine as its manual specifies, change the oil and gear lube where the cited procedures call for it, and store the outboard trimmed fully down.

Is draining the water system enough?
No. BoatUS says simply draining the fresh water system is not enough, and says to run non-toxic antifreeze to every outlet.

Which instructions win if they conflict?
The engine's owner's manual. Procedures differ by model, and the sources here point back to the manual for the specific engine.

More for boat owners

• How many hours is a lot on a boat engine
• How to write a boat hurricane plan
• Boat and yacht maintenance cost per year
• Boat renewals: registration, flares, life rafts
• Outboard maintenance schedules
• What to check when buying a used boat

Sources

BoatUS, The Boater's Guide to Winterizing (PDF) and its winterization hub; E-10 In Winter (summary only); Mercury Marine, 7 Mistakes to Avoid When Winterizing Your Mercury Outboard and How to Winterize Your Outboard; Yamaha Outboards, How to Winterize Your Yamaha Four-Cylinder Outboard; Suzuki's storage procedure on a dealer parts page; Boating Magazine, Marine Engine Winterizing Tips (10 October 2023). Retrieved 4 October 2026. Publication dates for the BoatUS PDF and the Mercury pages were not shown.

YachtGrade was designed by a USCG 100-ton captain who spent five years managing, detailing, running and selling boats in South Florida, after four years building IT systems for businesses. Who built it →