Gearboxes

Why do Propulsion Marine Electric Drives use gearboxes instead of belt reduction?

A gearbox is maintenance free, has an integral thrust bearing, and multiple gear ratios to choose from.    The gearbox affects the personality of the drive to match the boat and the owners requirements.   The gearbox provides an elegant solution to a complex problem.    Motors develop power from rpm, run cooler and more efficiently the faster they turn.   Propellers are more efficient at low rpm.    Gearboxes enables both the propeller and the motor to run efficiently and create more power.      Helical gears are 97% efficient.   The gearbox has the added function of mounting the motor higher than the propeller shaft.

What happens when you change the gearbox ratio?

The gearbox ratio determines the amount of power the system develops.   Lower ratios, closer to 1:1, develop more power.  Higher ratios, closer to 5:1 are more efficient.   Therefore the gearbox ratio determines the balance between power and efficiency.

An electric motor provides power for an application by consuming current at voltage.     Scientifically we know this as the power equation where Power = Current x Voltage.     Lower ratios consume more power at less voltage for the same propeller rpm, whereas higher ratios consume less current at higher voltages for the same propeller rpm.

It is important to remember that lower ratios can develop more total system power than a higher ratio can, however this is done at a cost to low end efficiency because a higher ratio will be turning the propeller with higher voltage and less current and voltage is always more efficient than current.

If you choose too low a ratio you will overwork the electric motor and it will overheat.   Higher ratios run cooler than lower ratios.

How do I decide what ratio I choose?

Propulsion Marine Gearbox ratios start at 1.25: 1 and go past 5:1 in increments of about   .3 : 1     We recommend using a propeller that we have already tested and know the ratio for.

If you would like to use your own propeller, start by knowing at what rpm your existing propeller gave you the top boat speed that you require.   Multiply your existing engine rpm by your gear ratio to get your existing propeller rpm.     Divide the electric motor maximum rpm, by the existing propeller rpm to get your desired ratio.

Electric horsepower is about 3 to 4 times stronger than diesel horsepower.     One horsepower 746 watts so one Kilowatt = 1.34 h.p.    Our 5.5 KW motor consumes 7.37 h.p.    Multiply by 3 to 4 and you get 22 to 29 h.p. which is the diesel equivalent.

How do I change the gearbox?

Remove the motor mount nuts and the flange coupling nuts on one side of the flexible coupling.    Loosen the collar of the dripless packing gland or the stuffing box and slide the shaft aft about an inch.   Remove the electric motor.    Lift out the drop pan mount and the gearbox.   Change the gearbox.   Reinstall.   It takes about one hour with the right tools.