How do you winterize a boat?

01Why it matters: what happens if you skip it

Freezing water is the headline risk, but not the only one. Untreated fuel oxidizes into a gum that clogs carburetors; used oil holds acids and combustion byproducts that corrode bearings and cylinder walls all winter; damp air rusts internal engine surfaces (West Marine, Mercury Marine).

The freeze numbers are sobering. Freshwater expands roughly 9% when it turns to ice, and BoatUS describes the force as enough to crack engine blocks, damage fiberglass, split hoses and waterlines, and destroy refrigeration — with damage often invisible until spring commissioning (BoatUS). A GEICO|BoatUS analysis of ten years of claims came down firmly on the side of winterizing. And the claim maps tell an uncomfortable story: the top states for winterizing-related claims include Texas, Mississippi, and Georgia — warm-weather places caught by a sudden freeze. Boats in the Dallas freeze of 2021 with water left in engines and systems took heavy damage (Chubb).

There’s an insurance wrinkle too. Ice and freeze damage is often excluded from marine policies unless you specifically add it, and some endorsements — including BoatUS/GEICO’s Ice and Freezing Coverage — apply only if the boat was winterized by a marine professional (BoatUS, Waterway Guide). Other carriers’ freeze endorsements often require professional winterization as well. If you plan to DIY, read your policy’s lay-up and warranty terms first. Also note: freeze-coverage endorsements typically stop being sold once temperatures drop, usually around the end of October — sort it out early (BoatUS).

02When to winterize

Before the first hard freeze — full stop. The regional windows, country by country, have their own guide: when should you winterize, by region. If you live anywhere there’s a chance of freezing temperatures, you winterize (Yamaha). For most of the northern US and northern Europe that means September through October, and Texas dealer Bluff Creek Marina recommends booking or doing it in that window; waitlists form as summer ends.

Two footnotes. First, winterization isn’t only about freezing — it also protects against corrosion and storm damage, so even temperate-climate owners benefit, and even a few weeks off the water justifies the basics (Bluff Creek Marina, BoatUS). Second, a properly done winterization lasts about a year; some owners stretch it to two, but then you should check rubber parts — hoses, impellers, primer bulbs, belts — for dry rot before relaunch (Bluff Creek Marina).

03The full lay-up, step by step

This is the canonical sequence for a single-engine sterndrive or inboard. Outboard owners: the core is the same, and the series’ outboard winterization article covers leg-specific detail step by step.

1. Fuel: fresh, stabilized, and mostly full

Fuel is where arguments start, so here’s the honest version. What everyone agrees on: untreated fuel degrades and forms gum and varnish, especially in carbureted engines, so add marine fuel stabilizer and run the engine 5–10 minutes on flushing muffs to circulate treated fuel through the system, then replace the fuel filter (Mercury Marine, West Marine).

For built-in vented tanks, the consensus is to fill to about 95% — full enough to limit condensation on the tank walls as temperatures swing, short of overflowing as fuel expands. Leave the vent open; never plug it, which can rupture the fuel system. One nuance: Mercury’s dealer team lead prefers running the tank nearly empty and refilling with fresh fuel, and newer non-vented tanks don’t need filling at all. Fresh fuel matters more than fill level — if your tank fuel is more than a month old, replace it (Mercury Marine, BoatUS, Chubb).

Ethanol caveat: E-10 can phase-separate — water and ethanol settle to the tank bottom — and once that happens, no additive fixes it; the fuel has to be professionally removed. Ethanol-free fuel such as REC-90 is preferred where you can get it (Mercury Marine, BoatUS). Portable tanks are simplest: empty them entirely — unmixed gas into the car, mixed gas into other two-stroke engines — and check your marina’s fuel-storage policy (BoatUS).

2. Oil and filter, on a warm engine

This is standard winterization, not an optional extra. Used oil carries acids, moisture, and combustion byproducts that eat at engine internals over months of sitting (West Marine). Run the engine a few minutes on muffs to warm the oil and suspend contaminants, drain, change the filter, then restart briefly to circulate fresh oil — and check for leaks. A stuck old-filter gasket doubling up under the new filter can dump your oil in minutes; Mercury’s techs have seen engines ruined exactly this way.

Match the owner’s-manual viscosity, quantity, and API service level, and never use automotive oil in a marine engine (Mercury Marine, West Marine). Yamaha even sells prepackaged DIY oil-change kits with the right fluids and filters.

3. Flush the raw-water circuit

Flush the cooling system with fresh water per your manufacturer’s instructions, via muffs or hose, before anything else touches it (Mercury Marine, West Marine).

4. Then fill it with antifreeze

West Marine recommends flushing the raw-water side with antifreeze rather than just draining it — draining leaves residual water in places you can’t see. Use an engine winterizing kit on the muffs: fill the kit’s tank with non-toxic propylene glycol antifreeze, open the valve, restart, and watch the level fall until antifreeze exits the telltale. For an outboard, plan on about a gallon; bigger raw-water circuits with manifolds and heaters take more, so have extra on hand (West Marine). Your engine manual is the authority on volume for your specific engine.

5. Fog the cylinders — the right way for YOUR engine

Fogging oil coats pistons, rings, and cylinder walls against storage corrosion. But the method splits three ways, and using the wrong one can cause real damage:

6. Gear lube (outboards and sterndrives)

Drain through the bottom plug — the magnetic tip collects fine metal particles, which are normal wear; coarse chips mean excessive wear and warrant a professional look. Pull the top vent plug, then fill from the bottom up until oil overflows the vent; replace the vent plug first, then the bottom plug, with new gaskets. Milky oil means water has gotten into the gearbox — stop and take it to a service shop before laying it up (West Marine).

7. Every other water system on the boat

This is where first-timers lose boats: the engine survives but a freshwater line or holding tank splits. Drain and fill the potable-water system, water heater, head, holding tank, washdowns, and air conditioning with propylene glycol as the manufacturer specifies (West Marine, Yamaha). A water heater bypass kit isolates the heater so it drains separately and cuts the antifreeze you need. In saltwater service, running antifreeze through washdown and bilge pumps also protects them against corrosion (Yamaha).

8. Batteries and electronics

Charge batteries fully and disconnect them at the terminals. Cold can freeze a lead-acid or AGM battery or let it drain completely, so removal to a warm, dry place is best; a regulated onboard charger left plugged in is an acceptable alternative (Yamaha). Take removable electronics home — moisture, mildew, and theft are all reasons — and keep a written list of what went where (Chubb).

9. Hull, drain plug, and the inspection pass

Pressure-wash the hull and follow your region’s invasive-species rules (Chubb). Pull the drain plug and stow it somewhere impossible to miss — a mesh bag on a stern cleat works — because the single most common spring failure is launching with the plug still in a cupholder. While you’re at it: check zincs/anodes, grease the lubrication points, inspect belts, hoses, hose clamps, through-hull fittings, and hydraulic steering, and exercise every seacock. Service anything questionable now, not in April (Mercury Marine, Chubb).

10. Cover it

The final step — and a whole decision of its own, handled just below.

04Printable checklist

Engine & fuel

• Built-in tank filled to ~95% with fresh, stabilized fuel; circulated 5–10 min
• Fuel filter replaced
• Warm-engine oil and filter change; restart, check for leaks
• Gear lube changed, filled bottom-up; milky oil or coarse chips checked
• Cylinders fogged by the correct method for your engine type
• Non-toxic PG antifreeze run through the raw-water circuit

Systems

• Freshwater tank, water heater (bypass), head, holding tank, and A/C drained and antifreeze-filled
• Livewells and washdowns drained; antifreeze in bilge/washdown pumps

Boat

• Batteries charged, disconnected, removed indoors if possible
• Electronics removed and itemized
• Drain plug removed and stowed visibly
• Zincs, grease points, bellows, hoses, seacocks inspected
• Hull washed; canvas dry and stored; cover on

Sailboat additions

• If unstepping the mast: mark all turnbuckles before loosening; keep cotter pins and parts labeled for spring (Chubb)
• Holding tank fully pumped out and filled with propylene glycol antifreeze

One honest note for saildrive owners: the engine core of this list applies to you as it does to a sterndrive, but saildrive legs have manufacturer-specific lay-up procedures — consult your Yanmar or Volvo Penta manual before running antifreeze through the leg, and follow it exactly.

05If you own a bass boat

The engine work follows the outboard procedure (see the series’ dedicated article). The bass-boat-specific stakes are the livewells: pumps, recirculation plumbing, and hoses must be emptied and in most cases treated with marine antifreeze, or freeze damage to pumps and lines gets expensive fast (Yamaha). Drain the trolling motor’s connections from the story of neglect too — inspect connectors and stow the battery indoors with the house batteries. Saltwater anglers: antifreeze through the bilge and washdown pumps as well.

06A word on jet boats and PWC

Personal watercraft are a standard winterization service category at shops, with published prices typically $135–$329 at the marinas we checked (Timber Marine, Sportsman’s Outfitter, Bluff Creek Marina). For jet boats, the drive system’s flush and lay-up differs enough from sterndrives that your manufacturer’s manual should lead — follow its flush-port and drain procedure rather than improvising from sterndrive advice.

07Antifreeze: the pink stuff vs. the green stuff

This is the most safety-critical choice in the whole job, so plainly:

Use non-toxic propylene glycol (PG) marine/RV antifreeze for every system that carries water you might touch or that pumps overboard — potable water, head, holding tank, raw-water circuit, bilge, livewells. It’s relatively low in toxicity, biodegradable, and the base for potable-water formulations (West Marine).

Ethylene glycol — car coolant, the green stuff — is a different chemical entirely. The EPA documents that ingestion causes central nervous system depression, then cardiopulmonary effects, then kidney damage. It is highly toxic to pets and wildlife and its disposal is genuinely problematic (West Marine, EPA). It has a place only in a sealed engine coolant loop, and only at the specification your manual calls for. Engine coolant never goes into potable or raw-water systems. For potable systems, buy virgin PG formulated and labeled for that use — it costs a bit more and is worth it (West Marine).

On the label ratings: compare products on their stated freeze- or burst-protection rating and whether they’re ready-to-use or need dilution, and buy the formulation your engine manual or the antifreeze maker calls for (West Marine).

08Building your own kit

A “winterization kit” isn’t mysterious — it’s just the hardware that lets you pull antifreeze into the raw-water circuit instead of lake water: a tank-and-valve assembly plumbed to your flushing muffs (West Marine sells its own engine winterizing kit, plus Camco’s inboard/outboard version).

The full DIY shopping list, per Mercury Marine and West Marine:

• Non-toxic propylene glycol antifreeze (a gallon covers a typical outboard flush; have extra for systems and larger engines)
• Engine winterizing kit and flushing muffs
• Fuel stabilizer
• Four-stroke oil and filter; oil filter wrench; drain pan
• Gear lube with a bottom-fill pump
• Fogging oil — the correct formula for carbureted or EFI
• Spark plugs, gap tool, prop wrench/socket
• Marine grease and gun; corrosion inhibitor
• Funnel, rags, waste-oil container

Buy early — Yamaha notes dealers sell out of winterization supplies “more frequently than you might think.” You’ll see improvised gravity-fed bucket rigs on YouTube; this guide sticks to the purpose-built kits that marine suppliers actually document (West Marine). Skip the car antifreeze in the garage, whatever else you do.

09What professional winterization costs

Published 2025–2027 marina price lists tell a clear story: the engine type sets the floor, and the service tier sets everything else.

Basic engine-only winterization in the US spans roughly $135 to $1,000+ depending on engine and, crucially, scope. Bluff Creek prices per engine, so a twin sterndrive doubles the line. Bluff Creek quotes pickup/delivery at $165/hour and Sportsman’s charges $399–595 extra for a “lake call” — winterizing in your slip.

In Sweden and northern Europe the structure differs: the norm is indoor storage in a heated, dehumidified hall (varmhall), with haul-out, launch, hull wash, and stands bundled into a calculator-priced base rate, and engine conservation (motorkonservering) plus freshwater/toilet winterization sold as separately priced add-ons (Mälarbåtar). Even heated-hall storage doesn’t include engine lay-up by default. Booking matters on both continents: Swedish yards want reservations by October 1 and fill by August; US dealers recommend booking September–October (Mälarbåtar, Bluff Creek Marina).

10Hiring it out: how to vet the service

Marina service departments, authorized dealers (Mercury and Yamaha both route you there), and mobile techs all offer winterization — several shops we found even do pickup, delivery, or in-slip service. Before booking, get the itemized checklist in writing. A quote worth accepting states:

• Exactly which tier you’re buying: fuel treatment? oil and filter? gear lube? fogging? antifreeze fill of the raw-water side?
• Whether freshwater, head, holding tank, ballast, A/C, and generator systems are included — some marinas price these only on request (Bluff Creek Marina)
• Which antifreeze and how much — you want to hear “non-toxic propylene glycol” and a real quantity (West Marine)
• Whether batteries, drive inspection, and cover are in or extra

One more reason the professional route earns its price: technicians find things you’d miss — throttle and shift cables needing adjustment, loose transom bolts, a bent propshaft — and marinas often discount winterization when bundled with storage (Mercury Marine).

And one reason it can matter legally: as noted above, freeze-damage endorsements from BoatUS/GEICO and several other carriers are conditioned on professional winterization. If your policy has that wording, DIY saves a service fee and risks a denied claim.

11Cover, shrink wrap, and the heater question

Covering is the last lay-up step, but the choice among covers is its own topic — we compare fitted covers, tarps, and shrink wrap in the storage-options article. The short version here: a good fitted canvas can suffice for smaller boats if it can carry a snow load without tearing; shrink wrap is extremely popular in heavy-snow regions; and if you wrap, drop a jar or two of dehumidifier inside first (Yamaha). Dry canvas completely and store it indoors. Pricing runs $23–$26 per foot for shrink wrap at the US shops we checked.

The heater question deserves a blunt answer, because it’s a documented claims category: a plug-in space heater is not winterization. GEICO marine claims files show owners substituting a heater for lay-up and filing claims for catastrophic engine damage and fire — heaters tip over, short out, ignite combustibles (BoatUS). Heated and dehumidified storage — the Swedish varmhall model — reduces mildew and frost risk, but even there, engine conservation is sold separately, because warm air doesn’t empty a water heater. Chubb’s rule is the one to keep: get the water out, and where you can’t drain it, use environmentally safe antifreeze — even if you “never freeze.”

When the ice melts, the reverse procedure awaits — see our spring commissioning guide (in research), and the outboard winterization article for the leg-by-leg detail this overview pointed to.

12FAQ