Propeller Shaft Diameter Calculator
Calculate the minimum required diameter of a solid marine propeller shaft from shaft horsepower, shaft RPM, allowable torsional stress, and a design factor. Results are shown in inches and millimeters.
Propeller Shaft Results
Select an equal or larger commercially available shaft diameter after accounting for keyways, tapers, corrosion allowance, bending loads, and applicable marine standards.
How is propeller shaft diameter calculated?
A preliminary solid propeller shaft diameter can be estimated from the horsepower transmitted by the shaft, shaft RPM, and allowable torsional shear stress of the shaft material.
A commonly used marine shaft sizing relationship is based on the torsional capacity of a solid circular shaft:
$$D = \sqrt[3]{ \frac{HP \times 321000} {RPM \times S} }$$
The formula gives the shaft diameter in inches when horsepower is expressed in hp, RPM is revolutions per minute, and allowable torsional shear stress is expressed in psi. A published marine shaft-sizing reference gives this relationship with the 321,000 constant. :contentReference[oaicite:0]{index=0}
- D: Calculated solid shaft diameter in inches.
- HP: Horsepower transmitted through the shaft.
- RPM: Propeller shaft rotational speed.
- S: Allowable torsional shear stress in psi.
- 321000: Constant used with these units in the empirical shaft-sizing equation.
Why does shaft RPM affect diameter?
For a given amount of transmitted horsepower, a slower shaft must transmit more torque. Because shaft torque capacity increases strongly with diameter, reducing shaft RPM can require a larger shaft even though the engine horsepower is unchanged.
$$T \propto \frac{HP}{RPM}$$
$$D \propto T^{1/3}$$
Why is a gearbox ratio important?
A reduction gearbox decreases propeller shaft RPM while increasing the torque available at the propeller. This calculator therefore uses the engine RPM and reduction ratio to determine the propeller shaft RPM.
For example, with a 2:1 reduction, a 2,400 RPM engine produces an approximately 1,200 RPM propeller shaft speed.
What is allowable torsional shear stress?
Allowable torsional shear stress is the stress value permitted in the shaft during the design calculation. It depends on the shaft material, heat treatment, manufacturing condition, applicable design rules, loading and required safety margin.
The default 8,000 psi value in this calculator is provided only as an example starting value. It should not be assumed to be the correct allowable stress for a particular shaft material.
What does the design factor do?
The design factor increases the horsepower used in the sizing calculation. Because diameter varies with the cube root of the design load, increasing the design factor produces a smaller proportional increase in required diameter.
$$HP_{design} = HP_{shaft} \times F$$
$$D_{design} = \sqrt[3]{ \frac{HP_{design} \times 321000} {RPM \times S} }$$
Does this calculate the final propeller shaft size?
No. This is a preliminary torsional sizing calculator for a solid shaft. Real propeller shaft design may also need to consider bending stress, shaft length, bearing spacing, critical speed, fatigue, keyways, tapers, corrosion, propeller thrust, coupling loads, material properties and classification or regulatory requirements.
In other words, the calculated number should be treated as a preliminary minimum diameter rather than a final engineering specification.
What happens if the shaft has a keyway?
A keyway removes material from the shaft and creates a local stress concentration. Therefore, the actual required shaft size can be greater than the simple solid-shaft torsional calculation suggests. The shaft should not automatically be reduced to the calculated diameter just because the remaining shaft appears to satisfy the formula.