Symptoms of a leaking stern drive on a boat.

The first sign my stern drive was leaking was the appearance of a milky substance in the small fill tank for the gear lube that sits on the top of the engine. At first it was a small amount at the bottom of the fill tank, mixed with the Mercruiser gear lube. But it was obvious. There were no other symptoms at this time, so I elected to monitor the issue and have it addressed during the next boating season's service.

A leak on a boat - even a small leak, can lead to a very big expensive repair bill. Although I was lucky this instance, I don't believe I was wise in waiting as long as I did. For what happened next illustrates that the condition of the stern drive gradually deteriorates at an accelerating rate as the leak becomes worse.

In the next couple of months and with continued use of the boat, I began to notice a more-than-normal amount of water building up in the bilge. As my boat has full bimini and rear sloping covers, minimal rain water can get into the bilge, especially as the drainage from the engine compartment is very good. My concern over the suspected leak increased, but I was not alarmed and continued using the boat.

Having drained and cleaned the bilge a few times, I began to monitor the engine bay after each use for signs of the leak. Immediately after I used the boat, I would venture into the confines of the engine bay and listen and watch for obvious signs of water intrusion into the hull. The drip-drip-dripping sound was evident although not immediately visible. I had to almost crawl under the engines to figure out which one was leaking and from where. 

When I did actually see the leak, I could see it was from the bottom of the main seal around the stern drive.  This is the very confirmation upon which I should have acted and that you need to know to act! The next symptom was a knocking sound which initially was not overly loud. However, with continued use of the boat the knocking sound intensifies. This is most likely the sound of your gimbal bearing under stress and working whilst full of sea water! There is also a strained 'growl' when the stern drive is steered hard to port or starboard.

At this stage I stopped using the boat and quite wisely, too. Further use could have caused immense damage that would have put me on hard stand for quite some time! I pulled the boat for its annual service and found that the u-joint bellows and drive seals had to be replaced. The gimbal bearing also needed replacement. There was no damage done to the gimbal housing or outdrive unit and I was lucky that this was the case.

These seals come in a kit and like most boat parts, seems excessively expensive. However, this kind of maintenance is not normally required annually, so you need to be aware of when the seals were last replaced. It's cheaper and easier to have them replaced prior to suffering a leak and risking damaging your engines. Peace of mind to have them done and keeping your stern drives happy for prolonged use of your baby!