Outboard > Lower unit
The lower unit contain the gearbox, gear selector, drive shaft, water pump, propshaft, bearing carrier, oil seals, skeg, speedometer inlet and trim tab anode.
The gearbox doesn't go into gear when shifting the lever
The water pump is not working or needs servicing
The water pump draws water in from the surrounding river, lake, ocean or sea and pumps it up through the mid-section to the engine to cool it.
The trim tab anode is worn or misaligned
The trim anode is the small metal fin just above the propeller. It serves a few different purposes which this option will explain.
There is water in the gearbox oil
If you go to change the oil in your gearbox and the oil that comes out is emulsified (milky coloured), water has got into the gearbox and been mixed in with the oil by the gears. Sometimes if an outboard has been sitting still for a long time the oil and water will separated and the water will drain out before the oil.