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Winterize Your Boat

Updated: Jun. 09, 2022

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Step 1: Prep – Gather the key tools and materials

If you’ve been paying the marina $150 or more to winterize your boat every fall, it’s time to learn how to winterize a boat yourself and save some dough. We brought in marine mechanic Sam Kelley to guide you through the process and share his expert tips for winterizing an inboard/outboard (I/O) drive boat. The steps are much simpler for an outboard, and we’ll cover those in another issue.

An I/O is full of water. So if you don’t drain the water properly, it can freeze and cause major damage. However, if your boat will be in heated storage, you can skip the draining part.

How to winterize a boat isn’t hard, but we have to warn you: If you skip a step or miss a drain plug, you could wind up with a repair bill of at least $3,000 for a replacement engine. (And none of that cost will be covered by insurance.) So if you’re at all nervous, invest in a factory service manual to locate all the drain ports and cooling units to begin learning how to winterize a boat.

Before you start the winterizing procedure build a boat winterizing kit, pick up a few gallons of RV antifreeze, a can of fogging oil, motor oil, an oil filter, lower-unit lube and drain plug gaskets, an oil suction pump and a lower-unit lube injection pump. Then gather up your screwdrivers, sockets and combination wrenches.

Step 2: Change the oil and clean the carb

Photo 1: Change your oil

Yank the dipstick. Then shove the suction hose down the dipstick tube and suck out all the old oil. Change out the oil filter and then refill the engine with fresh oil.

Photo 2: Spray fogging oil down the carb

Hold the carburetor choke open (if closed), start the engine and spray fogging oil directly down the carb until the engine chokes out.

Start by sucking out the oil and changing the filter (Photo 1). (Oil suction pumps can be found at most auto parts stores.) Then run the boat over to the marina and top off the gas tanks with non-oxygenated fuel (if available). Add a marine fuel stabilizer to the tank and run the boat to a landing. With the boat still in the water, remove the spark arrester from the carburetor and fog the engine (Photo 2). Then trailer it and perform the rest of the procedure on land.

Step 3: Drain the coolant

Photo 3: Yank the hull plug

Remove the hull plug with an open or box-end wrench. Don’t use an adjustable wrench—it will round off the shoulders on the brass plug.

Photo 4: Drain the power steering cooler

Loosen the hose clamp and remove the drain hose from the power steering cooler (if it doesn’t have a drain plug).

Photo 5: Drain the block

Locate and remove the block drain plug(s) at the bottom of the block. Cooling systems always suck in sand, so as soon as you remove a drain plug, shove a bent coat hanger inside the drain port and wiggle it around to loosen any sand or rust dams.

Photo 6: Flush the engine with RV antifreeze

Pour RV antifreeze directly into the thermostat opening or into the large hose attached to it. Keep pouring until you see a steady stream of the pink stuff running out of the block drain plug.

Tip the trailer up (so it drains well) and place a bucket under the hull drain plug. Then remove the plug (Photo 3).

Hop in the boat and start draining near the top of the engine. Remove the drain plugs (or the hoses) for the exhaust manifolds, power steering cooler (Photo 4), oil cooler (if equipped) and block drain plug(s); Photo 5. See “Draining Tip.”

Leave the block drain plugs out, but reinstall all the other drain plugs and hoses. Then remove the thermostat or the hose attached to the thermostat housing and pour in RV antifreeze until it drains out the block drains (Photo 6). Once the antifreeze stops draining, reinstall the block drain plugs and the hull drain plug.

Note: Sam likes to leave the engine block flushed but empty for the winter. However, some marine engine manufacturers recommend filling the entire engine with RV antifreeze. Always follow your engine manufacturer’s advice.

Step 4: Lube the lower unit

Photo 7: Refill the lower unit

Screw in the pump nozzle and pump in the new lube until it comes out the top vent. Insert new gaskets in the drain and vent holes and install the plugs.

Next, unscrew the bottom drain plug on the lower unit. Allow the lube to drain a few minutes before removing the upper vent plug. Once the old lube is out, refill with fresh lube (Photo 7). Sam recommends refilling with high-performance lube. Finally, remove the battery terminals and give it a full charge. Sam has had better luck leaving the fully charged battery in the boat. But you can also bring it inside and attach a battery maintainer. Cover the boat and dream about spring. Brush up on these boat maintenance tips during the winter, too.

Required Tools for this How to Winterize a Boat Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

• 4-in-1 screwdriver
• Bucket
• Nut driver
• Socket/ratchet set
• Wrench set

You’ll also need an oil suction pump, a lower-unit injection pump and plastic gloves.

Required Materials for this How to Winterize a Boat Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.

• Drain plug gaskets
• Fogging oil
• Motor oil
• Oil filter
• RV antifreeze

Up next, check out how to store a boat for Winter.