Your boat’s lower unit is important to service on a regular basis. Whether you have an I/O or an outboard, there are several items to check regularly to make sure your boat keeps running smoothly.
Gear Oil
One of the items that can be serviced by you is the lower unit gear oil. This should be changed at least once a year at the end of the season. Why at the end? Because you don’t want any metal shavings or foreign substances such as water in the oil sitting in the gear case all winter. This is not a place to go cheap; always use quality oil, not whatever is least expensive.
Changing it is easy. Tilt the outdrive or outboard up until the lower drain screw is at the lowest point, and then put a bucket underneath. Remove the upper vent screw, then the drain screw, and as it drains inspect the flow for metal particles. Drain screws are usually magnetized to attract metal shavings from the gears to help keep them out of circulation. A few small particles are all right, but a lot of them may mean you have a problem that should be checked out by your mechanic. Also, if the oil looks like coffee with cream, there was water intrusion, which needs to be checked. Very dark oil that smells burned is a warning sign of excess friction somewhere.
After making sure the oil’s fully drained, replace the screws’ O-ring seals, and after tilting the outdrive down, fill from the bottom hole until the gear oil comes out of the top hole. Put the upper screw in first and then the lower.
Propellor
Periodically inspect your prop for dings and gouges. Prop repair is pretty technical and should be left to a professional, but tiny nicks can be filed off. Every couple of trips, remove the prop from the shaft and check things out. Often, you’ll find fishing line wrapped around the shaft. If your lower unit is dripping oil, this line is the likely culprit. Remove the line and see if this fixes the problem. Many people will assume the seal is bad and replace it, but hold off until you see if it’s still leaking. Once a year, wipe off the old grease and inspect the splines and thrust washer for signs of wear, then liberally re-lube the shaft.
Most props have a rubber hub meant to absorb an accidental hit and prevent the shaft from snapping, but these can dry out and slip. An easy way to check for slippage is to take a silver Sharpie, mark the hub and the case at the same spot, and then run it for a while. If the line on the hub no longer matches the one on the case, you’ve got slippage, and it needs a trip to the service shop for re-hubbing.
Skeg
The skeg is designed to protect the prop by being the first point of contact when you hit an obstruction. If it’s bent, it can cause your boat to track incorrectly. If it’s broken, it won’t protect your prop the next time you hit something. You have a couple of options: Take it to your marine mechanic, who will fabricate and weld on a new section, or buy an aftermarket replacement.