How a Lower Units Works

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 How Gear Box Works

 The input shaft 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.

Whenever the motor is running, the pinion gear is rotating the forward and reverse gears.

 

 

The Clutch Dog

 

Between the forward and reverse gear there is the mechanism that selects gear.

This is called the Clutch Dog. 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.

 


 

How Gear Box Works

When the Clutch Dog is moved to select gear, it engages with the same sized square teeth on the gear that is being engaged.

Remember that both 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.

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.

 

Get here the Marine Transmissions Book where you can find all the information on this and many other topics.
 

Get the book
 
 

Propeller Diameter and Pitch

 

A lower blade pitch propeller will push less water in one revolution than one with high pitch, so it is easier for the engine to get it going.

Every boat and engine combination has different propeller needs, but in general I have observed that a higher pitch prop will give better fuel economy at cruising speeds and a faster top speed, so long as the engine can achieve the rated RPM range for wide open throttle

 

Pitch Vs RPM

 

The pitch is the most important number because different pitches will make the engine R.P.M.'s change at full speed.

The theory is for every one inch in pitch the RPM's will change 150 to 200. The higher the number pitch means lower R.P.M.'s, and the lower the number pitch means higher RPM's

So, the quest is to get the right pitched prop that will get you in your recommended W.O.T. R.P.M. operating range.

 
 
 
 
 

Schedule a Video Call with Mr. Lopez for remote troubleshooting



Learn more about Marine Technical Training at www.mrlopezclasses.com

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