How to Winterize Your Boat Properly

Winterizing a boat has gotten complicated with all the conflicting advice flying around. As someone who’s learned the hard way what happens when you skip steps, I learned everything there is to know about putting your boat to bed for the season. Today, I will share it all with you.

Quick Answer: A solid winterization job covers draining or treating every water system, fogging your engines, swapping fluids, adding fuel stabilizer, and getting your boat properly covered or stored. If you’re paying someone else to do it, expect somewhere between $300-800 depending on how big your boat is and how many systems it has.

Engine Winterization

Outboard Engines

First thing’s first — flush with fresh water. You’d be amazed how much salt and gunk builds up in those cooling passages over a season. Hook up a flush attachment and let it run until the engine hits operating temp. That way, the thermostat opens and water actually gets everywhere it needs to go.

Now here’s a step I once skipped and paid dearly for: draining the lower unit. You’ve got to pull both the top vent plug and the bottom drain plug. Why? Water left sitting in the gear case expands when it freezes. I’ve seen cracked housings and destroyed gears from this alone. Once it’s bone dry, refill with fresh gear oil so those bearings don’t corrode while your boat sits idle.

Fogging the engine is one of those things that feels like overkill until you see what corroded cylinder walls look like in spring. Spray fogging oil into the carb or air intake while the engine’s running — it coats everything inside. Or, if you want to be thorough, pull the spark plugs and spray directly into each cylinder. I do both, honestly.

Don’t forget fuel stabilizer. Add it to the tank and run the engine long enough so the treated fuel reaches the carburetor or injection system. Untreated gas goes bad faster than you’d think — within weeks it starts forming varnish that’ll clog everything up.

Inboard Engines

If you’re running a raw water-cooled engine, you need to drain every single cooling passage, hose, and heat exchanger. Open all those petcocks, pull hoses from low points, and don’t leave anything to chance. Even a tiny pocket of trapped water can freeze and crack something expensive. Ask me how I know.

Closed cooling systems are a bit more forgiving, but you still need antifreeze rated for whatever temps your area gets. Grab a hydrometer or refractometer and check your coolant concentration — adjust if the numbers aren’t where they should be for your climate.

Here’s a pro tip that a lot of people overlook: change your engine oil and filters before storage, not after. Used oil is full of acids and contaminants that’ll eat away at your engine internals all winter long. Fresh oil acts like a protective layer through those cold months.

Fog the cylinders just like you would on an outboard. Running diesel? Change those fuel filters while you’re at it and add biocide to keep algae from setting up camp in your fuel.

Stern Drive Systems

Stern drives need a bit of extra love. Pay close attention to the universal joints, gimbal bearing, and bellows. Check all the rubber components for any signs of cracking or wear — if something looks sketchy, replace it now rather than discovering it in April. Grease every fitting per whatever your manufacturer recommends.

Drain the drive unit and refill with fresh gear oil. Here’s an important one: look at the old oil when it comes out. If it’s milky, that’s water contamination, and it means your seals have a problem. Don’t put that off — it’ll only get worse over winter.

Fuel System Preparation

Treating Fuel

Probably should have led with this section, honestly. Add stabilizer to the tank before your last fall outing. The dosage matters — usually it’s about one ounce per gallon, but check the label on your particular brand. Run the engine afterward so the treated fuel circulates through every line, filter, and injector.

Fill your tanks to about 90-95% capacity. This cuts down on condensation inside the tank while still leaving a little breathing room for expansion. An empty tank is basically an invitation for moisture, corrosion, and contaminated fuel come spring.

Ethanol Considerations

Can we talk about ethanol for a second? Modern E10 gas is kind of a nightmare for winter storage. Ethanol is hygroscopic — meaning it pulls water right out of the air — and during long storage periods it can actually separate from the gasoline. That phase separation can do real damage to your fuel system. If you can find non-ethanol fuel in your area, it’s worth the extra cost for winter storage. Otherwise, at minimum use an ethanol-specific fuel treatment.

Fresh Water System Winterization

Complete Drainage

Open every single faucet and drain valve on the boat, and yes, that includes the water heater. Run the freshwater pump until pressure drops, then open the pressure switch access to get the remaining water out of the pump housing. It’s tedious, I know. But a cracked pump or burst line is way more tedious to deal with.

Pull your inline water filters and drain them completely. Frozen filters crack like nobody’s business, and they’re not cheap to replace.

Antifreeze Protection

This is important: only use non-toxic propylene glycol antifreeze — the pink stuff that’s rated for potable water systems. Never, ever use automotive antifreeze. It’s poisonous, and once it’s in your water system you’ll spend ages trying to flush it all out. That’s what makes winterizing your freshwater system endearing to us boaters — there’s a real satisfaction in knowing you’ve done it right and your drinking water lines will be safe come spring.

Pump that pink antifreeze through every line until you see it coming out each faucet. And don’t forget the heads, showers, and any other fixture tied into the water system. Every single one.

Sanitation System Winterization

Holding Tanks

Pump out your holding tanks completely before you button things up. Nobody wants to deal with a frozen holding tank situation — trust me on that. Add your preferred tank treatment product, then pour in non-toxic antifreeze to guard against freezing.

Marine Heads

Pump the heads dry, then add pink antifreeze to the bowl. Work the pump a few times to push antifreeze through the valves and hoses. If you’ve got manual heads, give the handle a good workout to coat all the internal parts. It takes maybe five minutes and saves you from a really unpleasant surprise in spring.

Battery and Electrical Systems

Battery Storage

Disconnect your batteries so they don’t slowly drain from phantom loads — every boat’s got electronics and switches quietly sipping power even when everything’s “off.” Clean up the terminals with a baking soda and water solution, then hit them with some corrosion-preventive grease.

Store batteries somewhere cool and dry, and hook them up to a quality battery tender. Here’s something most people don’t realize: a fully charged battery won’t freeze until something like -75 degrees, but a discharged battery? It can freeze at a relatively mild temperature and get permanently damaged. Keep them topped off.

Electronics

Anything portable should come off the boat and go into climate-controlled storage. For the stuff that stays, consider running a small heater or dehumidifier if your boat’s covered — moisture is the silent killer of marine electronics.

Hull and Cover Protection

Canvas and Covers

A good cover does more than keep snow off — it prevents ice and debris from piling up while still letting air circulate underneath. A bad cover, though? It traps moisture and basically creates a mold and mildew factory that’ll destroy your upholstery and headliners. So don’t cheap out here.

Use a frame system to support your cover so water doesn’t pool on top. And check on it through the winter — clear off heavy snow loads and make adjustments as needed. I try to peek at mine at least once a month.

Shrink Wrapping

If your boat lives outdoors all winter, professional shrink wrap is hard to beat. Just make sure whoever does it includes vents for airflow and an access door so you can pop in for inspections. You’re looking at roughly $12-20 per linear foot for a decent job.

Spring Commissioning Preview

Here’s the payoff for all this effort: when spring rolls around, commissioning your boat becomes almost enjoyable instead of a nightmare. My best advice? Write down every step you took during winterization so you (or your yard) can reverse the whole process without guessing. I also like to snap photos of drain plug locations and jot notes about anything I noticed that might need attention. Future you will be grateful, believe me.