When running a boat. Things can pop up in the water in the way at any moment! And our first reaction is to pull out of gear. But.
If You Accidentally Go Too Far And Put Boat In Reverse While Driving: 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. In other cases, the lower unit and engine will be fine.
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 is going to play a huge part in this scenario here.
That is because different boats use different transmission systems to transfer power from the engine. Down to the propeller.
Here is a Youtube video from our Channel that goes over the top types of drive systems that you can find on a boat. And this is going to be key to what happens here!
Now, many inboard boats have transmissions. Which are common in ski and wake board boats.
And it’s not uncommon for people to accidentally go back into reverse when they need to turn around to get a rider.
These kinds of boats use transmissions over lower units like outboards and I/Os.
Which plays a huge part in this situation. Because the transmission in most inboard boats will handle this scenario a lot better.
And in most cases with these boats, of course depending on the speed and severity of the shift. Say 30 mph in forward to trying to hit 10 mph in reverse.
Versus 5 mph in forward to just bumping it into reverse. As long as you aren’t doing this on a regular, what will generally happen.
Is that the transmission is going to take the brunt of the prop spinning in the water. The weight of the water forces the prop to continue to spin.
And your transmission is going to take the force of this change from spinning in one direction, (forward) to spinning in the opposite direction, (reverse).
This force will generally just hydraulically lock up the transmission for a moment, which will stop the engine from spinning. Making the engine stall out.
At slower speeds, you probably won’t even stall the engine. Just hear a loud thud.
But, at higher speeds, you can cause serious damage, but one-time hits, usually let you luck out!
Now when it comes to a lower unit though, and or older transmissions. It’s a different story!
Understanding The Insides Of A Lower Unit
This is mainly because of how a lower unit functions. And what makes the unit shift between the two gears.
Lower units are what you will find on outboard engines, as well as inboard outboard or I/O engines.
Inside of the lower unit you have four main gears. The first gear is what is called a pinion. This gear is attached to the driveshaft that comes from the engine.
And this gear spins all the time when the engine is running. It’s important to note that this shaft is vertical, compared to your prop shaft. Which is horizontal.
On the prop shaft you have the other three gears. One forward gear, and one reverse gear. And these two gears basically connect to the pinion gear.
So when the pinion is spinning. These two gears will also be spinning. But they aren’t “fixed” to the prop shaft.
They just basically ride on the prop shaft. Then we have our last gear, being what is called a clutch dog.
This gear is “fixed” to the prop shaft, but has the ability to slide back and forth. When you shift the control lever, it moves this clutch back and forth.
When the clutch dog is moved all the way back, it connects with either the forward or reverse gear. Which is dependent upon whether you have a counter rotation or a standard rotation lower.
Then when you shift the other direction. It disconnects from that other gear and then connects into the opposite gear.
Giving you forward and reverse!
Inspecting For Damage
But now seeing how this mechanical shifting is done. You can start to picture what would happen if you accidentally shift from forward into reverse when running at speed.
Depending on the speed. The power of water on the prop, might not let you shift the lower unit. Just because of the force being put on the clutch dog and the forward gear.
But that depends on your strength and what type of drive system you are operating. It can be done in a panic though. And it happens quite often.
The problem is that with all the force, from the weight of the boat, the power of the engine, the force of the water on the prop.
There is a ton of movement and momentum here in a couple of directions. So when that small little clutch dog goes from spinning one way. To a sudden stop and then immediately in the other direction.
At speeds of 600 RPM to say 6,000 RPM at wide open! It can literally just snap or explode! Leaving you with no shifting ability at all.
The inspection of damage is pretty simple in some cases. You might just find that the lower unit now has a hole in the side of it!
Or if you don’t see a hole in the case. Then the next thing to look at is what color the gear lube is.
(Here is another article that will teach you about what each color means.)
This will tell you what kind of damage you are looking at. If you find a bunch of metal, you will need to pull the bearing carrier to look at what everything looks like.
Here is a video of us rebuilding a lower for some reference to the parts inside the lower.
The Potential Engine Damage.
Now, the lower unit and transmission isn’t the only thing that can be potentially damaged when this happens though. Hopefully you don’t have to deal with any of these last things we’ve talked about.
But, in the worst case scenario, there can be engine damage. And even catastrophic outboard engine failure! Learn more here.
Generally, what ends up causing engine damage here. Is when the engine goes from say 40 mph in forward.
To a sudden stop. The back of the boat usually squats down in the water. Then if you hit reverse, it will pull the back of the boat into a wall of water.
Which can get water up through the exhaust of the engine. And when the engine takes in the water.
It will usually lock it up and then bend valves, break rods, and cause other types of serious damage.
Potentially destroying the engine! Though this does happen.
It’s a fairly rare case, because it does take all the different events to happen at the same time.
Speed, boat, weight, and water conditions. So, hopefully you never have to deal with this!
Check Us Out!
We hope that this has helped you understand more about what can happen if you accidentally put your boat in reverse while driving!
And what you can look for and inspect to make sure that your boat is ok.
If you have more questions or want to learn more about your boat and its different systems. You should consider joining our Boating Academy where we have created HUNDREDS of video courses teaching you basically everything about your boat!
There are also even more helpful videos on our Youtube channel where we create even more boating videos!
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