Fishing Line Behind the Prop: How It Ruins Gearcase Seals and How to Catch It Early

Tilt the outboard or sterndrive up (engine off, key out) and shine a light into the gap between the prop hub and the gearcase. You're looking for a strand of line sticking out, a few loops around the hub, or a shiny ring of melted plastic. BoatUS shows exactly this: a strand poking out from behind the thrust washer means line has wrapped the shaft and will cause a leak if it stays. Yamaha goes further and says to look over the lower unit before every outing.

Check under the prop and along the bottom of the gearcase for an oily film or drips. Sport Fishing describes milky-brown fluid coming out from under the prop as a sign to get the boat to a shop. In the BoatTEST owner's account, a thick white "milkshake" was dripping from the gearcase a week after line was pulled off the shaft. BoatUS warns that water often washes leaking lube away before you see it, so a clean gearcase does not prove the seals are fine.

If you saw any line, or it's been a while, pull the prop. BoatUS suggests at least once a season; Sport Fishing suggests pulling it after a trip and then every other trip during the fishing season. Have the boat on the trailer or out of the water if you can. On the water, only a small prop that can be reached safely from the boat, never by leaning out over a running engine. Remove the key and kill switch clip first.

Straighten and pull the cotter pin or bend back the tab washer, then remove the prop nut with the wood block holding the blades. Slide off the spacer or rear washer, the prop, and the forward thrust washer. Lay the parts out in the order they came off, or take a phone photo, so they go back the same way (see the propeller guide for details on your type of hub).

Look closely at the shaft right in front of the seal and behind where the thrust washer sat. Line is often caked in grease and hard to see. Pick and cut it away with a hook-blade knife, seam ripper or pick. BoatUS says you may need a sharp knife. Cut along the shaft, not into it, and keep the tip away from the seal lips. A hard, cloudy plastic disc is melted line; peel it off in pieces. Also check inside the prop hub and the thrust washer, where loops hide.

With the line gone, look at the seal where the shaft leaves the gearcase. BoatUS suggests checking for cuts or ruptures with a magnifying glass. A torn, nicked or lumpy lip, lube weeping around it, or a deep groove worn in the shaft are all warning signs. A seal can also look fine and still leak.

With the engine upright and cool, crack the bottom drain screw, then loosen the top vent screw and let a little lube run into a clean clear jar. Yamaha's guidance: milky means water; burnt-looking or smelling means it is due for a change. BoatUS says clean lube is clear amber, blue or red depending on the brand, and milky, bubbly or coffee-colored lube means water. Yamaha also warns that running the engine just before checking can whip air into the lube and make it look milky, so check it cold. Retighten the screws with new gaskets and top up from the bottom hole with the lube your manual names (see the lower unit oil change guide).

• Clean lube, no seal damage seen: reinstall the prop and keep checking after trips.
• Line found but lube clean: good catch. Check the lube again after the next couple of outings, since a nicked seal can start leaking later.
• Milky or watery lube: the seal is likely leaking. Yamaha says to have the seals inspected with a pressure test and replaced if needed. Don't just change the lube and keep running.
• Black, burnt lube or larger metal chips on the drain magnet: BoatUS says the gearcase has an internal problem and needs service.

Wipe the shaft splines clean, coat them with the marine grease your manual specifies, and reinstall the thrust washer, prop, spacer and nut in the original order. Tighten the nut to your manual's spec and fit a new cotter pin or tab washer. Remove the wood block before you reconnect anything.

Get the gearcase pressure and vacuum tested (see the gearcase seal pressure test guide), and fix the seal. If it's the end of the season, BoatUS says never to store a gearcase with old or contaminated lube: drain and refill so water isn't sitting on the steel parts all winter, even if the seal job has to wait until spring.

On inboard boats, line and rope wrap the shaft between the prop and the strut, where they can damage the cutless bearing, and a heavy wrap can pull on the shaft seal. Look at the shaft when the boat is hauled or when a diver cleans the bottom. Sterndrive props come off much like outboard props; saildrives have their own seals (see the saildrive guide). A new vibration after running over line is a reason to check right away.

Keep cut line in a bag or a bin on board, never over the side, and drop it in a monofilament recycling tube at the ramp or pier. BoatUS points out that line in the water also kills fish, turtles and birds. Some owners of inboard boats fit a shaft-mounted line cutter; ask your yard whether one suits your shaft and strut.

Note in BoatMend each time you pull the prop, what you found, and the lube color. A pattern of line every few trips tells you to check more often.