Ski boats, some cruisers, and high-performance boats that don’t have shiftable stern drives use transmissions instead, and that particular piece of the drivetrain is often ignored, which means used boat shoppers should pay special attention to it.
One of the biggest causes of transmission failure is too much throttle in reverse gear. It can happen when trying to keep the bow from hitting a piling, or when you’re trying to power the boat off the trailer at a shallow ramp, or stopping before you run over a downed skier.
To keep reverse gear from failing, the rules are quite simple. No more than 2,500 rpm in reverse gear. No shifting at more than 1,300 rpm, either. Two other culprits are low fluid levels and moisture intrusion.
While you’re checking the transmission fluid level, take a moment to look closely at the fluid and smell it. It should be a translucent reddish pink, and should just smell like plain old automatic transmission fluid, which doesn’t have much of an odor. If it smells burnt—and if you’ve ever smelled burnt transmission fluid before, your mind is probably recreating it for you right now—caveat emptor.
Visually inspect the fluid to see if it looks murky or emulsified or shows other signs of moisture intrusion. Even trace amounts of water inside a transmission can wreak havoc at the temperatures at which transmissions operate—roughly 130 to 150 degrees Fahrenheit.
Typically, fluid should be changed annually, and good old Ford Type F or FA works best.
Low fluid is a leading cause of clutch failure, so keeping fluid at optimal levels is critical. To check the fluid level, start the engine, shift it a couple of times into forward and reverse, turn it off and check it quickly, within a minute. If the oil cooler is mounted above the transmission, when you turn it off the oil will drain back into the transmission. So if you check it after it’s been sitting too long you get a false fluid reading. It will look as though it’s overfilled, which also is a no-no.
In general, one-speed boat transmissions don’t produce any debris, so there’s no filter to change; there’s nothing for the filter to catch until something major fails, at which point you need to remove and rebuild it.
If you can get a sea trial, do it, and run the transmission lever through several gear changes to be sure the transmission is functioning properly. And remember, don’t use too much throttle in reverse.