How to Winterize a Gas Inboard Ski or Wake Boat (Direct Drive and V-Drive)

Find the winterizing section of your engine manual and your boat owner's manual. Mark every drain on the engine (block, manifolds, risers, water pump, cooler hoses), every heater and shower hose, the transmission cooler, and each ballast tank and pump. Take photos now so you can put hoses back where they came from.

MasterCraft says to do the fuel before any water comes out of the engine, because you need to run the engine to circulate the additives. Its steps: fill the tank to about 90 to 95 percent with fresh gas, add a marine or ethanol-grade stabilizer (and a biocide if you choose to use one) as the label directs, then run the engine for at least 15 minutes in the water. It warns that stabilizer only works in fresh fuel and won't fix gas that has already gone bad. BoatUS adds that a nearly full tank breathes in less moist air over the winter (see the ethanol fuel guide).

Bring the engine to full operating temperature, on the lake or on the trailer with a hose flusher. BoatUS points out that a thermostat blocks some cooling passages until the engine is hot, so antifreeze added to a cold engine may never reach them. Never run it on the trailer without the flusher turned on and connected.

BoatUS explains that acids and moisture in used oil can pit bearings over the winter. Change the oil and filter while the engine is warm, using the grade your engine manual names. On many inboards the oil pan drain is hard to reach in the boat, so owners use an oil extractor or the drain hose some engines come with (see the oil extractor guide). Briefly run the engine (with water) to fill the new filter and check it doesn't leak. Change the fuel filter now too if it's due.

With the engine off, open each drain plug or petcock and pull each drain hose your manual lists. BoatUS warns that small pockets of water can crack expensive parts and that a petcock that won't drain may be clogged with rust; clear it with a wire. Remember the hoses to the water pump, manifolds and the raw-water strainer. If your boat has closed cooling, only the raw-water side is drained; the coolant side stays full, at the mix your manual specifies (see the closed cooling guide).

Most V-drive and direct-drive transmissions are cooled by a small raw-water cooler. Water left in it can freeze and crack it, and a cracked cooler can let water into the transmission. Drain it at the point your manual shows, and check the transmission fluid level and color while you're there (see the V-drive transmission guide for what the fluid tells you).

To fill, reinstall the drains, then feed nontoxic propylene glycol antifreeze into the raw-water intake from a bucket (or the method your manual names) while the engine idles, until antifreeze comes out the exhaust. BoatUS says to keep it running until antifreeze has flowed out of the exhaust for 30 seconds or more, and to make sure it reaches everything downstream of where you add it. MasterCraft uses a -50°F nontoxic propylene glycol RV-type antifreeze for engine, ballast and freshwater systems, and says colder climates may need a -100°F rating. Have more than you think you'll need. Never let the bucket run dry while the engine is running.

BoatUS describes fogging gas engines at the end of the antifreeze run. Many modern fuel-injected inboards have their own procedure or don't call for fogging at all, so follow your engine manual (see the fogging guide).

The cabin heater is easy to forget. It is fed by engine water through two hoses, and its core sits away from the engine, often under the dash. Drain or fill it exactly as your engine or boat manual says. Label the two heater hoses before you pull them, so they go back on the same fittings in spring; reversed hoses are a common spring mistake owners report. The same goes for a transom shower, washdown pump and any freshwater tank (see the freshwater system guide). If you blow lines out with air, use low pressure and only where your manual allows it.

Empty every ballast tank and bag. MasterCraft's method for its boats is to remove and dry soft bags, reconnect the bridge fittings, pour antifreeze into each ballast vent, then use the manual ballast switches to pump it out through the pumps, hoses and valves; some models with check valves need a little antifreeze left in the line. Your boat will differ, so use your owner's manual (see the wake boat ballast guide).

Boating Magazine's general method for rigid tanks is similar: with the boat level on the trailer, open the ballast drain valves in the bilge and pump each tank empty, pour an antifreeze solution into each tank through its hull-side vent fitting with a bucket under the drain fitting, then run that tank's pump in drain mode for a few seconds until antifreeze comes out, and close the valve. For a portable add-on bag that fills through a separate hose, it says to disconnect the fill hose and shake the bag out so no water is left in the pump. Follow your owner's manual where it differs.

MasterCraft's power package checklist includes greasing the throttle and shift linkages and cables, and removing the batteries for winter: fully charged, stored cool and dry, away from heat or sparks (see the marine battery guide). Its spring checklist mentions removing duct tape from the exhaust flaps, so many owners tape the exhaust outlets for the winter to keep animals and debris out (see the exhaust flapper guide).

Leave a tag at the helm listing which plugs are out and which hoses are off. On the trailer, pull the bilge and transom drain plugs so rain can't collect, and store the plugs where you'll find them. Cover the boat so water and snow can't pool on it.

MasterCraft's reactivation list: reinstall every drain plug, reconnect the heater, head, washdown and wet bar hoses and check them for leaks, look for nests, check fluids, look for freeze cracks and loose clamps, grease the prop spline and install the prop, and start with the boat in the water. Note what you did now in BoatMend so the spring list is ready (see the de-winterizing guide).