• Increase font size
  • Default font size
  • Decrease font size

Reduction Ratios


Reduction ratios in the gearbox attached to a marine engine are critical to correct propeller sizing as they determine the speed at which the output shaft to which the propeller is attached for any given engine rpm setting.

It is essential to understand the implication of this ratio in both Ahead and Astern.

The power a propeller consumes is a function of the propeller shaft rpm^3 or in english terminology - the shaft speed raised to the third power. For example if the shaft speed was to increase by 10% or 1.1 then the power required would increase by 1.1 x 1.1 x 1.1 = 1.33 or 33%. Propeller sizing is thus very sensistive to shaft speeds and by default reduction ratios.

This applies in both Ahead and Astern which is why it is so critical to correct sizing and choice of propeller to specify the reduction ratio fitted to the engine correctly when ordering the propeller.

Shaft speed is then calculated ( Always at max rpm ) by dividing the engine rpm @ maximum by the reduction ratio to calulate the shaft rpm at engine maximum.

For example - Imagine a Volvo 2030 rated at 3600 rpm max with a Sailrive with a 2.47:1 reduction. Then shaft speed @ maximum engine rpm will be 3600 / 2.47 = 1457.  This can then be entered into Power s Shaft RPM graphs to select the optimal diameter and pitched propeller for this installation.

Note the sizing is almost totally dependent upon the engine and reduction ratio rather than the vessel type, except in the case of catamarans where pitch will generally be increased by 1º to accommodate the higher cruise speeds associated with these types of vessels.

The reduction ratio is generally available on a small brass plate on the side of the gearbox in the format A: x.xx  B: y.yy where A is the Ahead reduction ratio - B is the Reverse reduction ratio.

Where this ratio is not available, and the gearbox is not of the hydraulic type, then by puting the gearbox into Ahead while the motor is stopped and marking the output shaft forward of the stuffing box, turning the motor over slowly with a spanner on the forward crankshaft x times to generate 1 whole turn on the output shaft will allow the calculation of the Ahead ratio in the gearbox.

Refer: Reduction Measuement on these web pages

Saildrives due to the nature of their gear train design all have the same reduction ratio in Ahead and Astern. With just a few models in production simply specifying the Saildrive model # will determine the reduction ratio in the Saildrive.

All Kiwiprops go to maximum pitch of ~ 24º in reverse to accommodate the higher redn ratios typical of Yanmar and some Volvo gearboxes fitted to shaft installations. This will introduce additional loadings in reverse and target the real world manouvering requirement where reverse is not generally used at other than lower engine rpm.