Accidentally Put Boat In Reverse While Driving?
When running a boat. Things can pop up in the water at any moment! And our first reaction is to pull out of gear. But.
What if you accidentally go too far and put the boat in reverse? If you have an inboard with specific transmissions, you will be ok. If you have a sterndrive or outboard, excessive stress on the lower unit can happen, even to the point of destroying the lower unit, as well as potential engine damage.
Here are the main considerations and things to inspect as well as what to look for if this has just happened to you!
What Makes Your Boat Go Forward & Reverse?
The actual drive system of the boat plays a huge part, because different boats use different transmission systems to transfer power from the engine to the propeller.
Here is a YouTube video from the Born Again Boater Channel that goes over the top types of drive systems that you can find on a boat.
Since many inboard ski and wakeboard boats utilize transmissions instead of lower units found in outboards and I/Os, accidentally shifting into reverse while turning to retrieve a rider, especially from a higher forward speed (like 30 mph attempting to shift to 10 mph reverse versus a gentler shift from 5 mph forward), will generally cause the transmission to bear the brunt of the prop's continued spin in the water.
The force of the water resisting the change in direction will typically lead to a momentary hydraulic lock-up of the transmission, often stalling the engine. While slower speed shifts might only result in a loud thud, high-speed occurrences, though potentially damaging over time, often result in a lucky escape with no immediate serious issues; however, lower units and older transmissions would likely respond quite differently to such a scenario.
Understanding The Insides Of A Lower Unit
The functionality of a lower unit, found on outboard and inboard/outboard (I/O) engines, allows for gear shifting through a clever arrangement of four main gears. A constantly spinning pinion gear, connected to the vertical driveshaft from the engine, engages with a forward and reverse gear that freely rotate on the horizontal propeller shaft.
The crucial component enabling gear selection is the clutch dog, which is fixed to the propeller shaft but can slide back and forth. When the operator shifts the control lever, this movement engages the clutch dog with either the forward or reverse gear, depending on the desired direction and the lower unit's rotation (standard or counter). Disengaging from one gear and connecting to the other provides the forward and reverse motion of the propeller.
Inspecting For Damage
Understanding the mechanics of shifting offers a stark visualization of the potential consequences of an accidental high-speed shift from forward to reverse. Depending on the boat's speed, the immense force of water against the propeller might resist the lower unit's attempt to change gears due to the pressure exerted on the clutch dog and forward gear. While the feasibility of such a shift hinges on operator strength and the drive system, panic can indeed lead to this scenario, and it's a surprisingly frequent occurrence.
The confluence of the boat's weight, engine power, and hydrodynamic forces generates significant momentum. Consequently, when the small clutch dog abruptly halts its rotation in one direction and instantly reverses at speeds ranging from 600 to 6,000 RPM, the resulting stress can cause it to shear or even explode, leaving the operator with complete loss of shifting capability.
Damage inspection can sometimes reveal a straightforward breach in the lower unit casing. However, if the casing remains intact, the color of the gear lubricant becomes a crucial indicator of internal damage (further details on lubricant color meaning can be found in another article). The presence of metal shavings necessitates removing the bearing carrier for a thorough assessment of the internal components.
Here is a video of Born Again Boater rebuilding a lower for some reference to the parts inside the lower unit.
The Potential Engine Damage.
While damage to the lower unit and transmission are common concerns during sudden stops, other serious issues can arise. In a worst-case scenario, the abrupt deceleration from speed, like 40 mph forward to a halt, causes the boat's stern to squat. If reverse is then immediately engaged, this can force a wall of water up through the engine's exhaust. When the engine ingests this water, it can lead to hydrolock, resulting in bent valves, broken rods, and potentially catastrophic engine failure.
Although this sequence of events – involving speed, boat dynamics, weight distribution, and water conditions – needs to align for such severe damage to occur, making it relatively rare, the possibility of completely destroying the engine exists. Hopefully, this is an issue you'll never have to experience.