The PRM90 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 2.04:1 and 2.50:1 with a common astern ratio of 2.5:1
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 gear lever.
Nominal Power Ratings, PRM 90 Marine Gearbox
|Model||Ahead Ratio||Pleasure||Light Commercial|
|Max. Input Power||37||50||25||33|
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.
The normal operating temperature should be in the range 50° to 95° using a good quality Automatic Transmission Fluid.
Propeller Free Wheeling
The PRM90 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: 13kg (28.7lb)
Oil Capacity: 0.3 litres (0.5 pint)
Flexible Input Coupling for PRM90
|Part Number||Outside Diameter||Mounting Hole Pattern||Remarks|
|mm.||in.||No.||Diameter||Pitch Circle Dia.|
|MT8109||215.9||8.50||6||8.74||0.344||200.0||7.875||SAE 6 ½ in|
|MT8110||241.3||9.50||8||8.74||0.344||222.25||8.750||SAE 7 ½ in|
|MT8113||151.5||5.965||8||6.50||0.256||140.0||5.512||Hawker Siddeley Marine|
|MT8115||190.0||7.48||6||8.10||0.320||170.0||6.693||Nanni / Yanmar|
Adaptor Flanges For PRM90
|MT8083S/A||B/W (Velvet Drive) adaptor||1.4||3.1|
Other Accessories For PRM90
|MT3054||Tail shaft half coupling (pilot bored)||1.3||2.9|
|MT3056||Tail shaft flexible coupling||0.7||1.5|
Service Classification Definitions – Pleasure
Limited to craft used exclusively for pleasure boating; operation at full engine throttle should not exceed 5% of total time, with balance of usage at 90% of full throttle engine speed, or less. Maximum operation 500 hours per year. The selection of PRM90 according to this classification for any commercial boat is not approved by PRM Newage Ltd.
Service Classification Definitions – Light Commercial
Planing or semi-displacement craft used in pleasure or commercial applications may qualify for light commercial rating if annual usage is less than 1500 hours and full throttle operation is limited, with most operating time at partial throttle.
- It is essential for the engine, transmission model, reduction ratio and propeller size to be correctly matched so that the engine can attain its rated speed appropriate to the relevant service classification without labouring.
- It is also necessary to ensure the torsional compatibility of the complete propulsion system from engine through to propeller, since disregarding this may result in gear noise, particularly at low speed operation, and may even result in damage to the engine as well as the transmission components.
PRM Newage Ltd will provide all possible information and assistance to help find solutions to potential torsional problems, but it is the ultimate responsibility of the person assembling the drive and driven equipment to ensure that they are torsionally compatible.