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PRM 125 Marine Gearbox

Posted: 3 January 2023

PRM 125 Gearbox

Simple mechanical operation, lightweight, compact & rugged.

The PRM125 gearbox is designed for use in pleasure craft where a simple lightweight transmission is required.

The cone clutches for both ahead and astern are mechanically operated and require no adjustment.

The gearbox is purpose built to operate with standard rotation engines i.e. providing a clockwise rotation of the gearbox input shaft when viewed on input. In forward drive the output shaft rotates opposite to input shaft therefore requiring a right-hand propeller.

The reduction ratios offered are 2.00:1, 2.47:1 and 3.06:1 with a common astern ratio of 2.44:1

The heavy duty aluminium gearcase is externally ribbed for strength and heat dissipation and therefore requires no oil cooler.

Direction actuation is by a lever with positive detent positions for ahead, neutral and astern. It is suitable for use with proprietary remote control operating systems which must be of the single lever dual action type, i.e. MORSE MT3 or SL3 which operates the engine throttle in conjunction with the gearlever.


Nominal Power Ratings

 Model  Ahead Ratio
 Pleasure  Light Commercial
 kW  BHP  kW BHP
  2.09:1   1.31  1.75  0.71  0.95
  125D2.5   2.47:1  1.04  1.39  0.71  0.95
  125D3   3.06:1  0.86  1.15  0.60  0.80
 Max. Input Power
   52  70  36 48

Maximum operating speeds: 5000 rev/min

Note: These powers are expressed in BHP and kW per 100 rev/min engine operating speed, and are measured at the engine flywheel. Ratings have been established to ensure the long trouble free life of the gearbox which should not, therefore, be used at powers in excess of those shown.



Oil Cooling

The normal operating temperature should be in the range 50° to 95° using a good quality Automatic Transmission Fluid. 



Propeller Free Wheeling

The PRM125 output shaft can be rotated continuously with the gearbox in neutral. It is therefore not necessary to fit a propshaft brake when running with the engine shut down, for example when sailing.  



Approximate Weight & Oil Capacity  

Approximate dry weight:16 kg (31 lb)

Oil Capacity 0.4 litres: (0.7 pints)

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