MARINE TRANSMISSIONS

Outboard gear box
This system runs from a pinion wheel directly from the engine which turns 2 gears at the same time, however only one is ever engaged and driving (under power) by way of a dog clutch.
The gears are selected by a selector rod which has a slot on a slant that moves the dog (that is fixed to the prop shaft) into the desired forward or reverse gear.
the gears are helical tapered gears which run directly onto the prop shaft.

Stern leg system
Unlike the outboard gearbox the stern leg transmission is vertically positioned on the shaft that drives the prop shaft. The clutch system unlike the outboard dog with teeth the stern leg clutch is a tapered system that will engage either forward or reverse depending on the position of the selector shaft.
At the end of the drive shaft where the prop shaft joins is a pinion wheel and one or two permanently engaged gears - if the engine is duo prop or single props.

Inboard gearbox
An inboard gearbox uses hydraulic fluid to compress a clutch pack which therefor engages the prop to move forwards or backwards. The concept of this system is much like a motorbikes.

What type of gears are used in stern legs and outboards?
tapered helical cut gears are used in outboards and stern legs because the offer the best contact between teeth all the time especially on the pinion - this type of gear also reduces noise.

The duo prop system
This works by having a pinion drive and two constant mesh gears rotating in opposite directions and moving one shaft inside the other. a propeller is fixed to each of the shafts which are rotating in different directions, this means that one of the propels has to be cut counter rotational drive so the engines props are always moving the boat the same way whilst the props are not.

The "V" drive system.
this gear box is to change the direction of the drive shaft, it also enables you to focus the majority of the engines weight further back in the boat. it does this through 2 stages in the gear box
1. the drive shaft enters the gear box and drives the first stage which is 2 shafts the first one is the one that the drive shaft is turning, it has straight cut teeth that drive a chain that turns the next shaft which also as well as having the straight cut teeth and chain linkage, has a tapered helical cut gear on the end of it so it acts like a pinion.
2. the second shaft drives the output or third shaft on the helical cut tapered gear - allowing for the change in direction.

Jet unit and Surface drive.
A jet unit has a water intake on the bottom of the boat, the water is then pushed through a prop in the tube causing the water to exit under very high pressure (enough to push a boat). this type of drive is very effective if you are going over shallow sand bars or shallow rivers etc.
A jet unit is especially good at turning this is achieved by moving a cap over the water out put and directing it  to where ever you want without loosing power, this is why a jet boat can turn almost 180 degrees without moving.
The downside to a jet boat is that you have to be under power to maneuver the boat otherwise you will just drift.
A surface drive works on the theory that the more you have in the water the more drag you have and the slower you will go. a surface drive always has one blade out of the water at any time, this is so you accelerate faster, there is less drag and you use less fuel.