How an Outboard's Lower Unit Works

The lower unit (or gearcase) is the part of the outboard that takes the power and rotation from the engine and transfers them to the prop shaft and the propeller. 

This causes the propeller to rotate, which pushes your boat through the water. If a part or component within the lower unit isn't working or fails completely, you'll find yourself stranded on the water. Understanding how your lower unit works and how to service it is essential.

Outboard Lower Unit: Driveshaft

How It Works: The driveshaft takes the rotational power from the engine (or powerhead) and transmits it down to the water pump and gearcase.

Outboard engine driveshaft diagram

Maintenance and Repair: There's little to no maintenance needed for the driveshaft, and provided the lower unit is well maintained, it should last for the lifetime of the outboard.

Outboard Lower Unit: Water Pump

How It Works: The water pump pushes water up through the engine to keep it cool. The water pump assembly is attached to the drive shaft, and uses the rotation of the driveshaft to power the water pump. As the driveshaft rotates, it turns an impeller inside the water pump housing.

Outboard motor water pump housing

The movement of the impeller's blades within the housing sucks water from outside the lower housing through a channel, and pushes the water up through the engine. After passing through the engine, the water is jettisoned out of the outboard and back to where it came from.

If the water pump fails, the engine will quickly overheat, which leads to badly worn pistons and cylinders, and can cause the engine to seize.

Yamaha outboard water pump impeller

Maintenance and Repair: Yamaha recommends its outboard water pumps are disassembled and inspected, and the impeller replaced at least once every 100-hours or once per year. The water pump's cartridge, bottom plate, gasket and O-rings will also need to be replaced from time to time. 

Outboard Lower Unit: Shift Shaft

How It Works: The shift shaft engages with the transmission to put the outboard into or out of gear. It also interacts with the transmission to dictate whether the prop shaft should rotate in one direction or the other, making the propeller move the boat forward or in reverse.

Outboard motor lower unit shift shaft

Maintenance and Repair: There's little to no maintenance required for the shift shaft, and provided the lower unit is well maintained, it should last for the lifetime of the outboard.

Outboard Lower Unit: Gearbox

How It Works: The gearcase takes the vertical rotational power from the driveshaft and converts it to horizontal rotational power along the prop shaft and to the propeller. The gearbox also dictates which direction the prop shaft and propeller rotate in.

Yamaha outboard engine lower unit parts

The drive shaft enters the gearbox and ends at a pinion gear. The pinion gear is permanently engaged with the forward gear and the reverse gear, so when the driveshaft is rotating, the pinion gear is rotating, and the forward and reverse gears are rotating. Shims behind the pinion, forward and reverse gears keep the gears properly engaged with one another, with no play between them.

Between the forward and reverse gears sits a dog clutch that is attached to the shift shaft, with the prop shaft being attached to the dog clutch. When the shift shaft is in neutral, the clutch isn't engaged with either the forward or reverse gears, so it remains motionless, as does the prop shaft.

Yamaha lower unit prop shaft rotation

When the shift shaft is put into its "forward" position, it engages the dog clutch with the forward gear, which causes the dog clutch (and the prop shaft) to rotate in one direction.

Yamaha outboard lower unit dog clutch

When the shift shaft is put into its "reverse" position, it engages the dog clutch with the reverse gear, which causes the dog clutch (and the prop shaft) to rotate in the opposite direction.

The forward gear, reverse gear and prop shaft are mounted on bearings to keep everything supported and rotating freely within the gearcase.

Outboard lower unit dog clutch

Maintenance and Repair: Over time, the teeth on the dog clutch will wear and it may need replacing. If you hear grinding as you shift the outboard into gear, it's usually a worn dog clutch. To prevent premature dog clutch wear, always shift in and out of gear at the lowest possible RPM. Also, make sure your rigging (throttle cables, shift cables, etc.) is well maintained so they aren't stretched, which gives your engine incorrect inputs.

Yamaha outboard engine lower unit parts

The pinion, forward and reverse gear shims also wear over time. When this happens, the transfer of power between driveshaft and prop shaft is slightly reduced, and your outboard will feel like it's losing performance. The gearbox will also be noisier than usual, as the loose gears "chatter" together. If you think your outboard is down on power or is excessively noisy, it might be time to inspect and replace the gearbox shims.

Outboard Lower Unit: Oil Seals

How It Works: To keep the lubricating oil within the gearbox and to prevent water from entering the gearcase, there are a number of oil seals and O-rings in the lower unit. These seals are:

  • Driveshaft oil seal
  • Prop shaft oil seal
  • Shift shaft oil seal
  • Shift shaft plate O-ring
  • Gearbox bearing housing O-rings

Yamaha outboard lower unit prop shaft oil seal

Maintenance and Repair: Because the prop shaft oil seal is exposed and prone to damage, you should inspect it regularly and replace it if it is worn or leaking.

Outboard motor lower unit parts seals

The other seals and O-rings in the gearcase are not readily visible, but you'll know if an internal seal or O-ring has failed because water will start mixing with the gear lube to create a gray sludge. This shows when you change the gear lube in the lower unit. 

Yamaha recommends changing the gear lube every 100-hours on its outboards: an ideal time to inspect the old gear lube for signs of water. If you find water has mixed with the gear oil, replace the faulty seal or O-ring immediately.

Yamaha outboard engine lower unit

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