What a sterndrive actually is
Three layouts move a recreational boat. An outboard is a complete engine and gearbox hanging off the transom. A straight inboard sits inside the hull, with a shaft through the bottom to a fixed prop and a separate rudder to steer. A sterndrive's the hybrid: an automotive-derived engine mounted inside against the transom, driving through it to an outdrive that steers and trims like an outboard leg. Inboard weight distribution, outboard-style handling.
The price of that setup is the hole in the transom. Power, exhaust, cooling water, steering and shift all have to cross it, through a transom assembly sealed against the hull with flexible rubber boots. An outboard has nothing like it. Most sterndrive-only maintenance, and every sterndrive-only catastrophe, comes back to that assembly, not the engine. The engine's usually the healthiest part of the boat.
Bellows and the gimbal bearing
Normally there are three bellows: a big one around the u-joints, one for the exhaust, a small one for the shift cable. The u-joint bellows is the one that sinks boats. It's rubber. It lives folded and flexing in the water, and it perishes from age and ozone, not use, so a boat that barely moves doesn't have healthy bellows just because it barely moves. When it splits, the opening's below the waterline, and the water goes straight into the bilge.
Behind it sits the gimbal bearing, holding up the drive's input shaft where it passes through the transom. It's greased, and it hates two things: water from a failed bellows, and running the engine with the drive trimmed hard over. A dry or rusted gimbal bearing tells on itself with a rumble or a squeal that changes with steering angle or with the drive in gear. Ignore it and it takes the input shaft and the coupler with it. Bellows and bearing get inspected together, since one kills the other.
So here's the honest version for an owner. Bellows get replaced on age, not on failure, and the way they fail is a boat filling with water on its mooring with nobody aboard. If nobody can tell you when the bellows were last done, assume they're due, and get them looked at before the boat spends another season in the water.
U-joints, the coupler, and what comes to light with the drive off
Bellows work means the drive comes off, and that's your chance to look at everything the drive usually hides. The u-joints get checked for notchy or dry movement and rust inside the caps. The engine coupler splines get inspected and greased. Dry splines chew themselves up, and that repair means pulling the engine, so a little of the right grease at the right time is worth a lot.
What else you can see with the drive off: the transom shield and its seals, the shift cable and its bellows, the gimbal ring and steering pivots, the water pickup passage, the trim cylinders and their pins, and the exhaust bellows, checked for soot marks that show it's been leaking. On the drive itself, the gear lube comes out and gets read, the prop shaft seal and the drive's own water pump get attention, and the upper shaft seal gets checked. With the drive already off it's all easy to inspect. A separate visit makes none of it easy.
Anodes, closed cooling and raw water
A sterndrive carries anodes in several spots at once: on the drive, often on the trim cylinders and the gimbal ring, sometimes inside the engine's cooling system. They're a matched set. Replace one and ignore the rest and you're not protected. Drives also call for different anode material in fresh water versus salt, one of the few places where a freshwater lake really changes the part that goes on, not just how fast it wears.
Cooling comes two ways, and they lay up differently. A raw-water-cooled engine pumps lake water straight through the block and the exhaust manifolds. A closed-cooling engine runs coolant through the engine and only uses lake water on the far side of a heat exchanger. That keeps the block clean, but it adds a coolant circuit, a heat exchanger and its own anode to look after. Which one's in the boat? That's the first question before any winter work.
Winterizing a sterndrive properly
A sterndrive holds far more water than an outboard, and it won't all fall out by itself. On a raw-water engine, the block, the manifolds, any heat exchanger or oil coolers, the hoses and the drive's own passages all have to be drained or filled with a suitable antifreeze, and every drain point gets opened and probed. Never assumed clear. One blocked drain plug can crack a manifold in a single cold night.
The rest of the layup is the same discipline as any engine. Fuel stabilized. Oil changed before storage, not after. Gear lube drained and read, battery out and on a maintainer, drive left down so the bellows aren't held folded all winter. On this lake that layup has to carry the boat from an autumn haul-out, whenever the marina sets it, through to an ice-out that in 2026 wasn't declared until April 13. That's a long time for water to sit in a manifold, waiting on a hard freeze.