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Inboard Transmission Maintenance


When planning to change the fluids on your boat, don’t forget the transmission.

Photo: Mark Corke

Yes, it’s a messy job, but someone’s got to do it, and there’s no reason it can’t be you. In fact, you could argue that it’s better if you do this annual task yourself. That way you’ll recognize any changes in your transmission fluid that might indicate trouble. A little prep and know-how will make short work of this job.


The trick here is to do the research once, then keep a record of what you needed to purchase to complete the task. Start by reading the owner’s manual. In addition to telling you the recommended type and quantity of transmission fluid you’ll need, it also provides tips on how best to complete the task. Once you have those in hand, gather the necessary tools, then place catch pans and oil-absorbent pads beneath the transmission before beginning. This adds an extra layer of protection to keep accidental leaks contained.

Check your transmission fluids at the same time you check your engine oil. A vacuum pump is the easy way to remove transmission oil. (Photo: Mark Corke)

Out With The Old

Marine transmissions typically have no drain plug, which means you’ll have to use a portable vacuum pump to remove the fluid via the dipstick/fill port. If you have one on hand to change the engine oil, the same unit can be used to change the transmission fluid as well. Just don’t cross-contaminate the two in whatever container you use to catch the old fluid. Take a good look at the used fluid once it’s removed. Like new fluid, it should be a translucent, reddish pink with little or no odor. Fluid that’s thick or milky in appearance (a sign of water contamination), has a burnt smell, or has particulates which you can feel between your fingers is your boat’s way of telling you that all isn’t well and that inspection by a professional is necessary.

If annual transmission-fluid changes are recommended but boat-use time is heavy, change the fluid every six months.

In With The New

Once the old fluid is removed, add the correct type and amount of fluid. A funnel is your friend, and having an appropriately sized one on hand makes adding fluid easier and minimizes spills. After changing your transmission fluid, it’s always a good idea to verify that the level is correct. Some transmissions require that fluid levels be checked at operating temperature. To do this, follow the directions provided in your engine manual, but in general, you’ll run the engine for five minutes or so with the boat in the water, shift into forward and reverse a few times, shut the engine down, then immediately pull the dipstick and check the fluid level. This is particularly important for transmissions having an external cooler. If you wait too long after shutting down the engine, fluid in the cooler can drain back into the transmission, giving an erroneous “over full” reading.


Always contain and dispose of waste fluids properly. Store waste fluids separately. Mixing fluids can make recycling impossible and create a veritable Hell’s Broth that’s even more toxic (and difficult) to dispose of. Your marina likely has a disposal or recycling program available for waste oil but may not have such a program for transmission fluid. There are other options; for instance, many automotive-parts stores maintain separate stations for recycling transmission fluid.

Frank Lanier

Contributing Editor, BoatUS Magazine