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 passing through the bottom to a fixed prop, steered by a separate rudder. A sterndrive is the hybrid: an automotive-derived engine mounted inside against the transom, driving through the transom to an outdrive that steers and trims like an outboard leg. You get inboard weight distribution with outboard-style handling.
The cost of that arrangement is the hole in the transom. Power, exhaust, cooling water, steering and shift all have to cross it, and they do so through a transom assembly sealed against the hull with flexible rubber boots. Nothing on an outboard corresponds to it. Most sterndrive-specific maintenance, and every sterndrive-specific catastrophe, traces back to that assembly rather than to the engine, which is usually the healthiest part of the boat.
Bellows and the gimbal bearing
There are normally three bellows: a large one around the u-joints, one carrying the exhaust, and a small one for the shift cable. The u-joint bellows is the one that sinks boats. It is rubber, it lives folded and flexing in water, and it perishes with age and ozone rather than with use, so a boat that barely moves is not a boat with healthy bellows. When it splits, the opening is below the waterline and water goes straight into the bilge.
Behind it sits the gimbal bearing, which supports the drive's input shaft where it passes through the transom. It is 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 announces itself as a rumble or a squeal that changes with steering angle or with the drive in gear, and ignoring it takes the input shaft and the coupler with it. Bellows and bearing get inspected together because one destroys the other.
The honest summary for an owner is this. Bellows are a replace-on-age item, not a replace-on-failure item, and the failure mode is a boat filling with water while it sits on a mooring with nobody aboard. If nobody can tell you when the bellows were last replaced, assume they are due and have 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
The drive has to come off for bellows work, and that is the moment to look at everything the drive normally hides. The u-joints get checked for notchy or dry movement and for rust inside the caps. The engine coupler splines get inspected and greased. Dry splines chew themselves out, and that repair means pulling the engine, which is why a small amount of the correct grease at the right time is worth so much.
Also visible 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 for soot marks that show it has been leaking. On the drive itself the gear lube comes out and is read, the prop shaft seal and the drive's own water pump get attention, and the upper shaft seal is checked. A drive already off makes all of this cheap to inspect; a separate visit makes none of it cheap.
Anodes, closed cooling and raw water
A sterndrive carries anodes in several places at once: on the drive itself, often on the trim cylinders and the gimbal ring, and sometimes inside the engine's cooling system. They are a matched set, and replacing one while ignoring the others is not protection. Drives also differ in the anode material specified for fresh water against salt, which is one of the few places where a freshwater lake genuinely changes the part that gets fitted rather than just the rate of wear.
Cooling comes in two arrangements and they lay up differently. A raw-water-cooled engine pumps lake water through the block and the exhaust manifolds directly. A closed-cooling engine circulates coolant through the engine and uses lake water only on the other side of a heat exchanger, which keeps the block clean but adds a coolant circuit, a heat exchanger and its own anode to look after. Knowing which one is in the boat is the first question before any winter work starts.
Winterizing a sterndrive properly
A sterndrive holds far more water than an outboard, and it does not all fall out on its own. 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, with every drain point opened and probed rather than assumed to have run clear. One blocked drain plug is enough to crack a manifold over a single cold night.
The rest of the layup is the same discipline as any engine: fuel stabilised, oil changed before storage rather than after it, gear lube drained and read, battery out and on a maintainer, drive left down so the bellows are not 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 was not declared until 13 April. That is a long time for water to sit in a manifold waiting for a hard freeze.