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Winterize Your Boat

Winterize your inboard/outboard motorboat and save $150. Our instructions include an oil change, draining the engine block, lubing the lower unit and more.

By the DIY experts of The Family Handyman Magazine

Winterize Your Boat

Winterize your inboard/outboard motorboat and save $150. Our instructions include an oil change, draining the engine block, lubing the lower unit and more.

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 do the job 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.

The job 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.

Before you start the winterizing procedure, 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.

Meet the Pro

Sam Kelley owns Sam’s Marine and Performance in Stillwater, MN. He’s been maintaining, repairing and installing performance upgrades for almost 25 years. We asked Sam to help with this story because he winterizes about 150 boats a year and hasn’t cracked a block yet.

Step 2: Change the oil and clean the carb

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

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.

Close-up of petcock

Close-up of petcock

Draining Tip

Don’t even bother trying to screw open drain petcocks. Just remove the entire valve. It’ll drain much better and the “wings” won’t break off.

Step 4: Lube the lower unit

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.

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Required Tools for this Project

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

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

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

Required Materials for this Project

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

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

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