PRM marine gearbox power tables indicate the operating limits of each model according to reduction ratio, engine speed and type of service.
PRM technical tables indicate the operating limits of marine gearboxes according to the reduction ratio, engine speed and type of service.
The values are often expressed in kW or BHP per 100 rpm at the flywheel. This method makes it possible to calculate the permitted power at different engine speeds, but it may not be immediately clear to users who are unfamiliar with the PRM rating system.
This guide explains how to read the power rating tables for both mechanical and hydraulic PRM marine gearboxes, using the PRM 60 as a practical example.
To verify compatibility, it is not sufficient to compare the engine power with a single value. The rating per 100 rpm, maximum input power, maximum input speed and type of service must all be respected.
Which Information Is Included in PRM Tables?
PRM tables may include:
- gearbox model;
- reduction ratio;
- ahead ratio;
- astern ratio;
- permitted power per 100 rpm;
- values in kW and BHP;
- type of service;
- maximum input power;
- maximum permitted input speed.
Each value must refer to the actual gearbox version and reduction ratio being considered.
What Does “kW per 100 rpm” Mean?
A value expressed in kW per 100 rpm is not the maximum power that the gearbox can accept under all conditions.
It indicates how much power may be transmitted for every 100 revolutions per minute of engine speed.
To calculate the permitted power at a specific engine speed, multiply the table value by the engine speed and divide the result by 100.
How to Calculate the Permitted Power
Use the following formula:
Permitted power in kW = PRM rating in kW per 100 rpm × engine speed ÷ 100
For example, with a rating of 0.73 kW per 100 rpm and an engine speed of 3,000 rpm:
0,73 × 3.000 ÷ 100 = 21,9 kW
The permitted power at that speed is therefore 21.9 kW, provided that all the other PRM limits are also respected.
BHP and kW: Which Column Should Be Used?
PRM may state power in:
- BHP, brake horsepower;
- kW, kilowatts.
To avoid unnecessary conversions and possible errors, it is preferable to use the column corresponding to the unit used for the engine power rating.
Do not confuse:
- BHP;
- HP;
- metric horsepower or CV;
- kW.
When the engine power is stated in CV or HP and the PRM table is read in kW, the value must be converted correctly before comparison.
Type of Gearbox Service
The permitted power changes according to the intended duty.
Pleasure Craft Use
This normally refers to intermittent operation, where the engine does not run continuously at maximum power.
The permitted ratings may therefore be higher than those specified for more continuous commercial operation.
Light Commercial Use
This category involves more demanding operating conditions and longer periods of use.
For this reason, at the same engine speed and with the same gearbox model, the permitted ratings are generally lower than for pleasure craft use.
The service category should be selected according to the actual operation of the vessel, not simply according to whether it is privately or commercially owned.
A privately owned vessel used for many hours at high power may require a more conservative assessment than a typical leisure application.
Practical Example: PRM 60
The values shown in PRM marine gearbox power tables must always be referred to the exact model, reduction ratio and service category.
The PRM 60 is a mechanical marine gearbox available with several reduction ratios.
The permitted ratings vary according to:
- selected reduction ratio;
- engine speed;
- pleasure craft or light commercial use;
- maximum input power.
How to Read the PRM 60 Table
To carry out the calculation:
- identify the gearbox model and reduction ratio;
- select the correct service category;
- read the value in kW per 100 rpm;
- multiply it by the maximum engine speed;
- divide the result by 100;
- compare the result with the engine power;
- check the maximum input power and maximum permitted input speed.
Example with PRM 60D1.5
Assume the following:
- gearbox: PRM 60D1.5;
- service: pleasure craft;
- engine speed: 3,000 rpm;
- PRM rating: 0.73 kW per 100 rpm.
The calculation is:
0,73 × 3.000 ÷ 100 = 21,9 kW
At 3,000 rpm, the calculated limit is therefore 21.9 kW.
An engine producing 18 kW at 3,000 rpm falls within the calculated limit.
An engine producing 25 kW at the same speed exceeds the permitted rating for that version, even if the general maximum input power for the gearbox is higher.
Example with PRM 60D2
Assume the following:
- gearbox: PRM 60D2;
- service: pleasure craft;
- engine speed: 3,000 rpm;
- PRM rating: 0.63 kW per 100 rpm
0,63 × 3.000 ÷ 100 = 18,9 kW
At the same engine speed, the permitted power is therefore lower than for the PRM 60D1.5.
This shows that identifying the gearbox only as a PRM 60 is not sufficient. The exact reduction ratio must also be known.
Maximum Input Power
The result obtained using the formula must not exceed the maximum input power stated by PRM.
For example:
0,73 × 4.000 ÷ 100 = 29,2 kW
If the PRM maximum input power is 26 kW, the effective limit remains 26 kW.
The lower of the following values must always be used:
- the result calculated from the rating per 100 rpm;
- the maximum input power;
- any additional limit applicable to the service category.
The calculation never permits the maximum declared input power of the gearbox to be exceeded.
Maximum Input Speed
Each gearbox also has a maximum permitted input speed.
Compatibility is therefore not confirmed simply because the engine power falls within the calculated limit.
An engine may have relatively low power but an operating speed above the gearbox limit.
The following must all be respected:
- power rating per 100 rpm;
- maximum input power;
- maximum input speed;
- service category.
Does the Reduction Ratio Affect the Permitted Power?
It can, because PRM tables may specify different ratings for different reduction ratios.
The reduction ratio also determines propeller speed in relation to engine speed.
For example, with a 2:1 reduction ratio:
an engine speed of 3,000 rpm corresponds theoretically to approximately 1,500 rpm at the gearbox output.
The ratio must not be selected solely according to the permitted power. It must also be compatible with:
- propeller diameter and pitch;
- required propeller speed;
- vessel characteristics;
- required torque;
- propulsion system;
- recommendations from the vessel designer or manufacturer.
Power and Torque Are Not the Same
Power describes the work produced by the engine over time, while torque represents the rotational force transmitted through the shaft.
Two engines with the same power may have different torque values, especially when they reach rated power at different engine speeds.
An engine producing a certain power at low speed generally transmits more torque than an engine producing the same power at a higher speed.
For demanding or unusual applications, it may therefore also be necessary to verify:
- maximum engine torque;
- engine speed at maximum torque;
- number of cylinders;
- torsional characteristics;
- type of operation;
- frequency of gear engagement.
Which Engine Power Value Should Be Used?
The power value used for comparison must represent the power actually available at the gearbox input.
Depending on the engine documentation, the following values may be stated:
- flywheel power;
- shaft power;
- rated power;
- maximum power;
- intermittent power;
- continuous power.
PRM tables refer to the power transmitted to the gearbox. The selected engine value must therefore be technically comparable with the PRM rating.
Common Mistakes to Avoid
Non scegliere una trasmissione PRM basandoti soltanto su:
- potenza massima riportata nel titolo della scheda;
- modello generale della serie;
- rapporto di riduzione;
- regime massimo;
- utilizzo indicato genericamente come “diporto”;
- confronto con una trasmissione già installata;
- dimensioni esterne;
- diametro della flangia;
- potenza del motore senza relativo regime.
A single matching value is not enough to confirm that the gearbox is suitable.
View the Technical Data for Individual PRM Models
Mechanical Marine Gearboxes
Hydraulic Marine Gearboxes
Ratings vary according to model, reduction ratio and type of service. The specific table for the gearbox being considered must always be used.
How to Request a Check from Scandiesel
Unsure Which PRM Gearbox Is Suitable?
Please send us:
- engine manufacturer and model;
- power and unit of measurement;
- engine speed corresponding to the stated power;
- maximum torque, when available;
- type of service;
- PRM model being considered;
- required reduction ratio;
- propeller and application details, when available.
We will review the available technical information to help identify the most suitable model and reduction ratio for the application.
Please send us:
- marca e modello del motore;
- potenza e relativa unità di misura;
- regime corrispondente alla potenza dichiarata;
- coppia massima, se disponibile;
- tipo di utilizzo;
- modello PRM considerato;
- rapporto di riduzione;
- dati dell’elica e dell’applicazione, quando disponibili.
We will review the available technical information to help identify the most suitable model and reduction ratio for the application.