Boat Engines

Shaft-drive diesels, sterndrives and outboards:

the drive comes before the engine, because each one is built into a different kind of hull. A shaft-drive inboard sits low in the middle of the boat and turns the propeller through a marine gearbox, usually a ZF or a Twin Disc. Heavy sportfishers and cruisers carry them, mostly diesels from Cummins, Caterpillar, Yanmar or Volvo Penta, and a well-kept one is judged on hours run rather than age. A sterndrive puts a petrol V6 or V8 inside the hull against the transom and drives through a leg that tilts like an outboard. MerCruiser and Volvo Penta built most of the ones found in bowriders and cabin runabouts. Outboards hang off the transom, tilt clear for beaching and shallow river mouths, and can be lifted off and taken to a workshop, which is why they power most pangas and centre consoles; they have their own listings under outboard engines. Impellers, filters, exhaust elbows and gaskets for all three are listed separately in boat engine parts.

Repowering a hull with a different engine:

the power has to suit what the hull was designed for, and the builder’s capacity plate, where there is one, sets the ceiling. Weight matters as much as horsepower. A four-stroke outboard is heavier than a two-stroke of the same rating, and those extra kilos sit right on the transom, so a boat that floated level with the old engine can sit low at the stern with the new one. Changing brand rarely stops at the engine: controls, gauges, wiring harness and often the propeller change with it. A twin installation wants a matched pair, with one propeller turning the opposite way. Going from a sterndrive to an outboard means reworking the transom, which is boatyard work.

Smoke, exhaust risers and bellows on a used inboard:

ask to see a diesel started from cold, because a warm engine hides a hard start. A puff of smoke that clears is normal. Black smoke under load points to fuel or air supply, blue smoke to oil being burnt, and white smoke that keeps coming once the engine is warm can mean water or coolant getting where it should not. Look at the exhaust risers at the back of the engine for rust weeping at the joints: when one corrodes through, seawater can drain into the turbo and the cylinders every time the engine is shut down. Ask when the heat exchanger and aftercooler were last cleaned, and whether there is a record of oil analysis, which shows wear long before you can hear it. On a sterndrive, the rubber bellows around the drive are what keep the sea out of the hull. Cracked or hardened bellows let water in, a drive left tilted up for months can make them split once it goes back down, and a gimbal bearing that rumbles under way usually means water got past them some time ago.

Listing an engine still in the boat, or already out of it:

say which in the first line, because it decides who can buy it. An installed engine can be shown working, so film it from a cold start with the gauges and the exhaust outlet in shot. Once it is out, list what came with it: gearbox, harness, panel, exhaust elbows, mounts and controls. Give make, model, serial number and hours, the date of the last overhaul with the invoice if you kept it, and whether it came off a charter boat or a private one, since a charter engine piles up hours a weekend owner never reaches. A buyer driving from Tamarindo with a pickup and a hoist needs to know whether they are collecting a power package ready to bolt into a hull or a core for a rebuild, and your photos should settle that before anyone sets off.