Basically you have to remove the top so that you can withdraw the bottom.
Circlip on top gear and the flat washer underneath to be removed first, then remove the top gear wheel. This gives access to the sintered bronze bush in the lower gear wheel. Remove the bush carefully, usually need to use pliers to grip it
. This allows removal of the lower gear wheel. The support tube for the drive shaft can now be removed
. The support tube outer housing can now be mounted in the hull from the inside on a bed of sealant and bolted firmly to the hull. I have used aluminium doubling plates inside the hull on both my builds as the forces on the hull can be quite severe if a working tug.
Before refitting the support tube for the drive shaft is a good time to dismantle the lower gear box and grease it well. Take note of the means of adjusting the clearance will make reassembling and setting it up easier.
Caution, if you have two units do one at a time as the washer/s under the circlip are matched to the unit and not a standard fitting.If they are new units run them in slowly and note that after several hours running the units will swivel easier, though not on full power, always ease off the power before rotating them