Marine Propeller Shaft Diameter Calculator
Enter the horsepower carried by one shaft, rated rpm, reduction ratio, shaft material strength, safety factor, and candidate diameter. The calculator estimates the torsional shaft diameter and checks whether the entered shaft size has enough capacity.
Next 1/8-inch shaft size1.75 in
- Shaft speed
- 1,500
- Transmitted torque (lb-in)
- 10,504.17
- Minimum calculated diameter (in)
- 1.75
- Existing-shaft utilization
- 99.83%
- Maximum power for entered diameter (hp)
- 250.44
- Required torsional strength (psi)
- 49,912.54
- Diameter check
- Pass: entered diameter meets the calculated torsional minimum
Uses horsepower carried by one shaft and the entered torsional strength as the governing material value.
Does not apply allowances for keyways, tapers, fatigue, corrosion, bearing spacing, bending, or classification rules.
How to use this calculator
- Enter the rated horsepower carried by this shaft, dividing total power for twin-screw installations when appropriate.
- Enter the engine rpm at rated power and the transmission reduction ratio.
- Replace the default torsional strength and safety factor with values from the governing standard and certified material data.
- Enter an existing or proposed shaft diameter to check utilization, maximum supportable horsepower, and required strength.
How the propeller shaft diameter is calculated
This calculator applies the solid circular shaft torsion relationship used in the ABYC P-6 propeller shaft sizing procedure. It is a torsional sizing estimate for one shaft, so a twin-screw boat should be calculated with the horsepower carried by each shaft rather than total vessel horsepower.
The shaft speed is found from the engine speed and transmission reduction ratio:
N = Ne / R
Working torque is then calculated as:
T = 63025 * P / N
The minimum solid shaft diameter is:
D = (321000 * P * F / (St * N))^(1/3)
- P is the horsepower carried by one shaft.
- Ne is engine speed at rated power, in rpm.
- R is the transmission reduction ratio.
- N is shaft speed, in rpm.
- St is the minimum torsional strength entered for the shaft material, in psi.
- F is the design safety factor.
The suggested standard size is the calculated diameter rounded upward to the next one-eighth inch: Dstd = ceil(8D) / 8. The calculator does not round a small existing shaft into compliance. It reports a pass only when the entered diameter is at least the calculated minimum.
What changes the result most
Power, shaft rpm, material strength, and safety factor drive the diameter. Diameter changes with the cube root of those terms, so doubling horsepower does not double shaft diameter, but low shaft rpm from a large reduction ratio can still produce a much larger requirement. Higher certified torsional strength lowers the calculated diameter only when that strength is permitted by the applicable standard and suitable for the actual shaft condition.
The reverse checks answer repower and survey questions. Existing-shaft utilization is 100 * (D / De)^3. Maximum supportable power is St * N * De^3 / (321000 * F). Required torsional strength is 321000 * P * F / (N * De^3).
What this calculator leaves out
This is not a complete propulsion shaft design. It does not account for keyways, tapers, threads, corrosion allowance, welds, stress concentrations, fatigue, material defects, bending from propeller weight, thrust, alignment, whirling, unsupported length, bearing spacing, or classification-society requirements. Final selection should be checked against the current standard, certified shaft documentation, and a qualified marine engineer, naval architect, shaft supplier, or surveyor.
Worked example
Suppose one shaft carries 250 hp from an engine rated at 3,000 rpm through a 2.00:1 reduction. The shaft speed is 3000 / 2.00 = 1500 rpm, and torque is 63025 * 250 / 1500 = 10504 lb-in.
With 50,000 psi torsional strength and a safety factor of 5, the diameter formula gives (321000 * 250 * 5 / (50000 * 1500))^(1/3) = 1.749 in. Rounding up to the next one-eighth inch gives 1.75 in. An entered 1.75 in shaft has about 99.8% utilization, supports about 250.4 hp under the same assumptions, and passes the torsional diameter check.
Common questions
Should I enter engine horsepower or shaft horsepower?
Enter the rated power carried by one shaft. For a single inboard installation, that is often the engine rating unless losses or a governing standard require another value. For twin-screw boats, calculate each shaft separately with the power carried by that shaft.
How do I find my transmission reduction ratio?
The ratio is usually on the marine gear nameplate, build sheet, survey, or transmission manual. A 2.00:1 gear means the shaft turns once for every two engine revolutions, so a 3,000 rpm engine produces 1,500 shaft rpm.
What torsional strength should I use for stainless shafting?
Use the minimum certified torsional strength allowed for the shaft alloy, heat, diameter, and governing standard. Do not substitute tensile strength unless the applicable standard or shaft supplier gives an approved conversion or value for torsional strength.
Why is the next standard shaft size larger than the calculated diameter?
The formula returns a theoretical minimum diameter. Shafting is commonly selected from standard fractional sizes, so the calculator rounds the result upward to the next one-eighth inch rather than selecting a size below the minimum.
Can I use this calculator when repowering an inboard boat?
Yes, the utilization, maximum horsepower, and required-strength rows are intended for repower screening. They do not replace a shafting review because higher power can also affect coupling fit, keyways, propeller loading, bearings, alignment, and shaft whirling.
Does a keyway require a larger propeller shaft?
A keyway can reduce effective section and create a stress concentration. This calculator does not apply a keyway correction, so the final shaft size should be checked by the governing standard, the shaft supplier, or a qualified marine professional.