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PRM 750C Inline Marine Gearbox (Ahead Ratio 1.935:1)

Part No: PRM-750C2

Full hydraulic operation: in-line output shaftThis inline variant of the PRM750 gearbox can be used to directly replace the Velvet Drive 72C transmission with the addition of coupling MT5030... Read more


Full hydraulic operation: in-line output shaft

This inline variant of the PRM750 gearbox can be used to directly replace the Velvet Drive 72C transmission with the addition of coupling MT5030 to keep the overall length the same so complete interchangeability is simple.

The PRM750C marine gearbox is purpose built for use in both pleasure craft and commercial boats; its twin countershaft design provides separate oil-operated multi-disc clutches (which need no adjustment) for ahead and astern drive allowing full rated power to be transmitted continuously in either direction.

A choice of reduction ratios available are 1.09:1, 1.459:1, 1.935:1, 2.565:1 and 2.904:1 all of which can provide either left-hand or right-hand propeller rotation in "ahead", making the gearbox particularly well suited to twin engine installations.

The robust and reliable hydraulic system uses lubricating oil of the same viscosity as that used in the engine, and helps ensure good boat handling by its rapid response to movements of the operating lever.

The operating lever has a positive neutral detent which assists the setting-up of the remote control operating unit whilst for added security, to guard against the unlikely event of hydraulic failure all gearboxes incorporate a mechanical lock-up device so the boat can be brought safely back to port.

The gearcase is constructed of high grade cast iron, internally ribbed for rigidity and strength, and consists of two separate halves to facilitate servicing, the oil pump and hydraulic control valves being externally mounted for easy accessibility.

A special feature of the PRM750C is the direct drive power take-off which is available as an optional extra; this will power a hydraulic pump to SAE J744C type 'B' specification, thus providing an economical and space efficient means of driving on-board machinery.

An electronically operated trolling valve can be fitted, which will allow variable speed of the propeller to zero whilst allowing a maximum engine speed of up to 1200 rpm.

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.

Working Oil Pressure

Minimum - 30.34 bar (440 PSI), Maximum - 33.44 bar (485 PSI). Two tapped holes 1/8" BSP on the top, and M18 x 1.5 on the side of the valve block are provided so that the pressure gauge can be fitted if required.

Oil Cooling

The normal operating temperature of the oil should be in the 50°C - 80°C range and should not be permitted to exceed 90°C. An oil cooler is necessary to ensure that correct operating temperatures are maintained, and two 3/8" BSP connections are provided on the valve block to allow it to be fitted. The size of the cooler required depends on a number of factors including the transmitted horsepower, operating speed, duty cycle, inlet water temperature and ambient temperature.

Propeller Thrust

Both ahead and astern thrust is carried by the output shaft bearings which are of adequate capacity for all factory approved ratings.

PRM750 Gearbox spline: 26 teeth 20/40 DP. PRM 750 Gearbox coupling: 6", 6 holes, 4¾" PCD, 3" female register. Oil Capacity: 3 litres (5.28 pints).

PRM 750C is new part number for the PRM 750CB range of marine gearboxes.

Capacity Description Oil Capacity: 3.0 Litres (5.28 pints) (plus the amount required to fill the cooling circuit)
Material High Grade Cast Iron Gearcase
Weight 93kg (205 lbs) (excluding adaptor, drive coupling and oil cooler)
Brand PRM
Gearbox Manufacturer PRM
Country of Origin United Kingdom
Hydraulic or Mechanical Hydraulic
Shaft Arrangement Inline
Warranty 2 Years
Gearbox Coupling 6", 6 holes, 4-3/4" PCD, 3" female register
Gearbox Spline 26 teeth 20/40 DP
Nominal Power Rating Heavy Commercial 7.43 BHP / 100 RPM
Nominal Power Rating Light Commercial 7.85 BHP / 100 RPM
Nominal Power Rating Pleasure 9.48 BHP / 100 RPM
Ratio Ahead Stern Description 2.565:1
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PRM 750C Inline Marine Gearbox (Ahead Ratio 1.935:1)


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