Marine Transmission

Marine Transmissions

Gear selection systems

Outboard gear cases - An out board gearcase uses a 'Dog clutch' system. this is a double sided engaging gear. The dog clutch is mounted to the propellor shaft with the ability to slide. The selector rod pushes the dog clutch fowards or backwards to engage the propellor shaft to the driven gears.

Stern drive - Unlike an outboard the gear selector system is in the top of a stern leg, and sits vertical instead of horizontal. In this system a selector arm pushes a tapered cone, often made of brass, up or down the driveshaft to engage the foward or reverse gear. This system works similar to a baulk ring in an automotive gearbox and uses friction to engage the gear.

Inboard gearbox - Inboard gearboxes use a hydrolic clutch type engagemet system similar to a motorbike clutch. The selector changes the flow of fluid to each clutch unit within the gearbox as pressure builds the clutch plates engage the desired gear.

What type of gears are used in Outboards & Sternlegs?

Helical cut, constant mesh, gears are used in both outboard gearcases and sternlegs. This type of gear is used because having and helical cut gear provides more contact area between gears and reduces noise. Having a constant mesh gear system also reduces moving parts reducing possible problems. These gears are then engaged to the propellor shaft via an engagement system such as a dog clutch.

Duo Propellor systems

Invented by Volvo Penta this system consists of two propellors rotating in opposite directions. The main propellor is mounted to the main shaft with a second shaft mounted through its center. A second propellor is mounted to the second shaft and spins in the oposite direction. This system in designed to reduce cavitation, vibrations and increase power and handling.

'V' drive systems
A v drive system is used instead of a standard shaft drive system when there is little space to work with. This system allows the drive line to move around tight angles or change direction as in this diagram.

 

Jet Unit or Surface drive?

Jet unit - A jet unit is like an internal propellor. Water floods through the intake as the propellor turns which inturn blows out the rear of the boat under pressure. This system is good for conditions such as shallow waterways.

Surface drive -  A surface drive unit operates with the propellors half inn half out of the water. This type of system is used because it reduces drag therfore allowing for increased speed and lower fuel comsuption.

Both systems have their own advantages however I would lean more the Jet unit as this would be suitable for both shallow and deep water. This unit also has the added safety of not having exposed moving parts, but retains power and control.