Are You Damaging Your Gearbox when Changing Gear?
Clutch Dog Gearboxes
Outboard motors and most stern drive gearboxes (like Mercruiser’s Alpha drives) use a Dog Clutch set up for selecting and shifting gear.
If you select gear slowly and carefully with these gearboxes you will actually be damaging the gear box.
Do you hear a grinding or chattering noise when selecting either forward or reverse gear?
If you do, something needs to change or one day your lower unit will not have any drive and it will constantly jump out of gear.
Firstly I will explain how the lower unit/gearbox works.
They have an input shaft that is driven from the engine or ‘power head’. (This input shaft is called the driveshaft. The driveshaft has a gear connected to the bottom of it called the Pinion gear. The pinion gear is always in constant mesh with the forward and reverse gears. When ever the motor is running, the pinion gear is rotating the forward and reverse gears. Between the forward and reverse gear there is the mechanism that selects gear. This is called the Clutch Dog.
The Clutch Dog sits on the propeller shaft that protrudes out the back of the gearbox and drives the propeller. The Clutch Dog has an internal spline that is in constant mesh with the propeller shaft.
Picture the Clutch Dog having a front and a back. At each end it has large, square teeth. When you select gear on the motor you move the Clutch Dog forwards and backwards along the propeller shaft.
When the Clutch Dog is moved to select gear it engages with the same sized square teeth on the particular gear that is being engaged.
Remember that both of the output gears (forward and reverse) are rotating as they are in constant mesh with the pinion gear (input gear).
If you select gear slowly, the Clutch Dog will slowly move into mesh with the gear. This slow movement causes the teeth of the gear to ‘skip’ over the teeth on the Clutch Dog. This continuous skipping wears away the nice sharp edges of the teeth and will eventually cause the motor to jump out of gear.
How to Select Gear
· The way to avoid doing damage to your gearbox is to positively select gear. This means to literally bang the engine into gear so you hear one clean engagement of the gearbox, not chatter.
· You should also pause in neutral when shifting from forwards to reverse or vice versa. This allows the propeller time to stop spinning.
· Do not shift into reverse when you are moving forwards quickly. It creates a huge amount of unnecessary force on the gearbox (and engine). Slow the boat down of the plane, come back to an idle and pause in neutral to let the boat slow right down. Reverse is purely a reverse, not a break…
Engine Idle Speed Too High
· Another cause for damage being done to the Clutch Dog is your idle speed might be too high. If you motor is idling higher than around 900rpm in neutral you will be doing damage every time you are selecting gear.
· Make sure your idle speed is set to what the Manufacturer recommends!
· Positively select gear
· Pause in neutral between gear changes.
· Let the boat slow down before changing into reverse
· Have the gear oil changed every 100 hours or annually, which ever comes first.