How to Dewinterize a Boat: Spring Recommissioning in the Right Order

Why the Order Matters

Autumn lay-up has a logic to it: drain, protect, disconnect, cover. Spring has the same logic in reverse, and the reason to respect it is that each step tests the one before. You cannot test a bilge pump without a charged battery. You cannot run the engine without fuel you trust and a raw-water system that is connected. You cannot check the stern gland for drips until the engine has turned the shaft. Start in the middle, with the key, and every fault presents as an engine problem when most of them are not.

The other reason is more uncomfortable. Most boats that sink in the spring do so at their berth, in the first days after launch, from a hose that was taken off in October and never put back. The insurer BoatUS has reported for years that far more boats sink at the dock than underway, and that below-waterline fittings and hoses are the leading cause. Nobody forgets a hose they wrote down. They forget the one they took off in a hurry on the last cold weekend, meaning to note it later. That is why the order starts with the lay-up notes, and why, if you do not have them, the first job of spring is to work out what the previous you did. Our winterization checklist is the list you are now running backwards.

Before the Engine: Batteries

Batteries come first because everything else needs them. A battery that has sat for five months, even disconnected, has self-discharged; a flooded lead-acid battery loses a few per cent a month in the cold and much more in a warm garage. Charge each bank fully on a multi-stage charger rather than the engine alternator, which is a poor charger for a deeply discharged battery and a good way to cook a diode. Let the battery rest for a few hours off charge and read it: 12.6 V or better is a healthy flooded or AGM battery, 12.4 V is about 75 per cent, and anything that will not hold 12.4 V after a full charge is telling you how its season will go.

Clean the terminals to bright metal, tighten them, and smear a little petroleum jelly or terminal grease on top. On flooded cells, top up with distilled water only, to just above the plates. Then follow the cables: the negative to the engine block, the positive to the battery switch and the starter. A green powder on any lug is corrosion that will fail under starter load. If a battery is in its fourth or fifth season, read our guide to how long marine batteries last before you trust it with a night on a mooring.

Before the Engine: Fuel

If you added stabiliser and filled the tank in autumn, the fuel itself is probably fine. What stabiliser cannot stop is condensation, and in petrol with ethanol the water pulls the ethanol out of solution and drops it to the bottom of the tank where the pickup is. So the check is the separator bowl: open the drain and look for a clear layer under the fuel. Water sits at the bottom. Drain until it runs clean fuel, then fit new primary and secondary filters regardless of what they look like, because a filter that spent the winter wet with fuel has done its service.

Squeeze the primer bulb on an outboard and flex every fuel hose you can reach: cracking at the bends, a hose gone hard, a whiff of fuel that was not there before. Confirm the tank vent is clear, because a wasp nest in a vent is a real thing and it presents as a boat that starts, runs for ten minutes and stalls. Diesel owners have one extra job. If the tank was left part-full, look for the black slime of diesel bug at the filter and treat the tank before it blocks a filter mid-passage.

Before the Engine: Antifreeze Out, Water In

Two separate systems had antifreeze poured into them and they come back differently. The raw-water side, meaning the engine cooling circuit that draws from the sea, was filled with pink propylene glycol either by pulling the intake hose off its seacock and running the engine on a bucket, or by pouring it into the strainer. That antifreeze simply flushes out the exhaust on first start; it is non-toxic and a few litres going overboard is normal. The job here is not the flushing, it is the reconnecting: the intake hose back on the seacock, both clips tightened, the strainer lid seated on its gasket. Then open the strainer, lift the basket and clean out whatever settled in it.

The fresh-water system takes longer. In autumn the water heater was bypassed and its two hoses probably joined with a short link, the tank was drained and antifreeze was pumped through every tap. Reverse it: heater hoses back on, bypass link out, tank filled. Run every tap, hot and cold, shower and deck wash, until the water runs clear and tastes of nothing. Then sanitise. A dilute solution of unscented household bleach in the full tank, left for several hours, then pumped through and the tank flushed twice with clean water. If the boat has a watermaker, it has its own pickling routine and its own manual. And if the head was winterised with antifreeze through the intake, that flushes through on first use; check the intake seacock is open and the joker valve is not stuck before you pump. Our guide to how a marine toilet works covers the valve.

Before the Engine: Impeller, Belts, Hoses, Oil

The raw-water pump impeller is a rubber wheel with flexible vanes that bend against a cam every revolution. Left compressed against that cam all winter, the vanes take a set, and the rubber hardens with age. A vane that cracks at the root does not just reduce cooling; it travels downstream and lodges in the heat exchanger, where it is found months later after a summer of unexplained running hot. If the impeller went in last spring, change it now and keep the old one as a spare if every vane is perfect. If it is newer, pull the cover, look at each vane and feel whether the rubber is still soft. Never run the pump dry to test it. Thirty seconds without water is enough to destroy a new impeller.

Belts glaze and crack on the inside face; twist one and look. Set the tension so the belt deflects about 12 mm (1/2 in) under firm thumb pressure at the mid-point, or to the figure in the engine manual. Flex every coolant and exhaust hose and look at the bends for cracks. Any hose clamp with rust on the band or the screw gets replaced with a 316 stainless one, and every hose below the waterline should carry two.

If you changed the oil in autumn, as you should have, check the level and move on. If you did not, change it now, before the first start, because used oil is acidic and has spent the winter sitting on bearing surfaces. Then the gearbox, or the lower unit on an outboard or sterndrive. Pull the lower drain screw and look at the first oil out. Clear amber or grey is fine. Milky or creamy means water has been getting past a shaft seal, and no amount of topping up fixes that; the seal gets replaced or the gears are next. Check the inboard engine's closed-circuit coolant level and its concentration while you are there.

Below the Waterline, Before Launch

This is the one week of the year when the bottom of the boat is in the air, so do everything here now. The hull gets its antifoul, and while the sander is out, look at every fitting that passes through it.

Zincs and other anodes are replaced when about half wasted, not when they are gone. A shaft anode that has vanished entirely means the prop and shaft have been the anode since it disappeared. Check the fit too: an anode on a painted surface is decoration. Work every seacock through its full travel several times. A seacock that will not move gets freed and greased now, because a seacock that will not shut is a hole in the boat you cannot close, and the day you need to close it will not be a calm one. Look at the hose on each one from the inside, confirm it is on and double-clipped, and confirm that the ones you closed in autumn are set the way you want them for launch.

Run a hand around the propeller. Dings on the leading edges cost speed and vibrate; fishing line behind the hub cuts the shaft seal. Check the cutless bearing for play by trying to move the shaft up and down at the prop. Then the stern gland or shaft seal. A traditional packed gland should drip a few drops a minute when running and stay nearly dry at rest; if it was tightened up hard in autumn, slacken it back. A dripless seal wants its bellows checked for cracks and its vent line connected. On a sterndrive, look at the bellows behind the transom for cracks; a split bellows is the sterndrive version of a hose off a seacock.

Systems and Safety Kit

With the batteries charged, test what they run. Lift the bilge pump float switch by hand and hear the pump run, then run it on manual and, if you can, pour a bucket in and watch it pump out through the correct outlet. A pump that hums but does not pump has a blocked strainer or a split hose. Switch on every navigation light and walk round the boat; a green corrosion bloom on a bulb contact shows as a dark light at dusk, not as a warning. Our navigation lights guide covers which ones your boat must show. Switch on the VHF and ask for a radio check on a working channel, never on channel 16. Confirm the depth sounder reads and the plotter finds satellites.

Then the kit with dates on it. Flares carry an expiry date, typically three to four years from manufacture; expired ones are disposed of through the coastguard or a chandler, not kept as spares. Fire extinguishers have an inspection date and a gauge, and the USCG and most national authorities require them to be in date and serviceable. Inflatable lifejackets have a gas cylinder and an automatic firing cartridge with a date, and the bladder wants inflating by mouth and leaving overnight once a season. Check the EPIRB battery date and that the registration is current. Count lifejackets against the largest crew you expect. The full list, with what each country requires, is in our boat safety equipment checklist.

The First Start in 10 Steps

Only now does the key turn. The first start happens either on the hard with a hose on the muffs of an outboard or sterndrive, or at the dock with the boat afloat and the intake seacock open. Either way, it is a two-person job: one at the helm, one at the stern watching the exhaust. The second column is the whole point of each step.

A few notes on the steps. If the engine cranks and does not fire, stop after ten seconds and wait a minute; a starter is not a continuous-duty motor and an impeller is being turned dry while you crank. Outboards show their cooling water at the tell-tale rather than the exhaust, and the same 30-second rule applies: no stream, stop. On an inboard the temperature should climb to its normal running figure and hold; a gauge that keeps climbing past it at idle is a raw-water problem, an air lock in the closed circuit or a thermostat stuck shut. And the sea trial is short and sheltered on purpose. The point is to find out what you missed within sight of the berth, not three miles out.

The Three Spring Mistakes

Launching with a seacock open and a hose off

This is the one that sinks boats at their berths in April. The raw-water intake hose was pulled off to pour antifreeze in, or a water heater hose was disconnected, or the speed-log paddle wheel was pulled and its blanking plug never went in. The boat is launched with the seacock open and the water comes in at a rate the bilge pump cannot match. The defence is procedural: before the travel lift picks the boat up, walk every through-hull with the lay-up notes in hand and put a hand on every hose. Then launch with every seacock closed, and open them one at a time once afloat, looking at each one as you do.

Forgetting the drain plug

The trailer-boat version of the same mistake. The transom drain plug was pulled in autumn so rain and melt water could run out, and in spring the boat goes down the ramp without it. Water comes in at the transom faster than most people expect, and the first sign is the boat sitting low at the stern. Put the plug in at home, before the boat is hitched, and make it the first line of the pre-launch check. It is covered, with the rest of the ramp routine, in our guide to launching a boat from a trailer.

Starting a dry raw-water pump

Muffs not flowing, seacock still shut, strainer lid loose and sucking air: the result is the same. The impeller runs dry, the vanes heat and shear within a minute, and the engine that started perfectly overheats on the way out of the harbour. Water on before the key, every time, and a second person watching the exhaust. If you are alone, prop a phone camera on the exhaust or run the engine from the cockpit where you can see the tell-tale. A new impeller costs less than an hour of an engineer's time; a heat exchanger blocked with fragments of the old one costs the rest of the season.

Why the Autumn Notes Are the Spring Checklist

Every item above is easier if the lay-up was written down. Which hoses came off. Which seacocks were closed and which left open. The date the impeller went in and the engine hours when it did. The oil grade and quantity. The flare and extinguisher dates, so you know in March what expires in July. A ship's log that records maintenance as well as passages turns dewinterizing from a rediscovery of the boat into a list to work through, and the parts, dates and hours in it are what tell you whether that impeller is on its first season or its third. Marine Logbook keeps maintenance, fuel, trip log and inventory in one place, with dates and engine hours against each entry, so the autumn you writes the list that the spring you reads.

The Bottom Line

Dewinterizing is winterizing run backwards. Everything you put in has to come out, everything you took off has to go back on, and the engine is the last thing to touch, not the first. Batteries, fuel, cooling and oil before the key; hull, anodes, seacocks, prop and gland before the launch; someone watching the exhaust for water within 30 seconds; a short sea trial and a dry bilge when you get back. The boats that flood in spring are not the ones with the worst engines. They are the ones whose owners did not write down what they disconnected in October.