Page 39Forwards – And Backwards?

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Boats don’t really do backwards.

But reverse gear is needed to be able to slow the boats down.
When there are passengers on board, and you are coming into a mooring, you need brakes.

What sort of reversing gearbox?

You’ve guessed it. One that requires the least friction.

And there is a superb option. The epicyclic or “sun and planets” gear box.

You just push a lever to go forwards, pull it back to go backwards. In the middle is neutral.
No clutch or meshing of gears. It’s a delight.

The real beauty is that when in forward, no gears are meshing.
The mechanism is just a spinning mass. Like a flywheel.
The gearbox does not waste any power.

In reverse – yes – all the gears are spinning – and there is a power loss.
But the boat is being braked.

The gearbox should be sited on the propeller shaft.
The engine shaft is already busy with the toothed belt pulley and probably also
with a starting handle.

The “trick” with the epicyclic gear box is that gear changing is achieved by
applying a brake.
When the planet ring is held, the planet wheels have to spin and rotation is reversed.

It could be that you already know all about the epicyclic gearbox.
Many bicycles use a three speed mechanism called the “Sturmey Archer”

But have a look at Malcolm’s video

There are 3 parts to the mechanism. Two are fixed.
The ring gear is fixed to the propeller shaft. The “sun” gear is fixed to the engine shaft.

For forward gear, the planet carrier ring is clamped to the ring gear and propeller shaft.
This locks the gear teeth of all three components together. They rotate as a single unit.

For reverse, the planet gear ring is released from the ring gear and held stationary.
The sun gear now causes the planet gears to spin.
The spinning planet gears now rotate the ring gear in the opposite direction.

Just put a brake onto the planet carrier and you have reverse gear!