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Propeller Unbalance Calculator
Calculate permissible propeller unbalance, centrifugal force, eccentricity, and per-blade allowance per ISO 21940. Essential for marine shaft vibration control.
Results
Permissible Propeller Unbalance (ISO 21940)
The permissible unbalance for a propeller is calculated using the same ISO 21940-11 formula used for all rotating machinery:
Where G is the balance grade (mm/s), m is propeller mass (kg), and ω = 2π·n/60 is angular velocity (rad/s).
Centrifugal Force from Unbalance
The centrifugal force generated by unbalance at operating speed:
- U — unbalance (g·mm)
- ω — angular velocity (rad/s)
- F — centrifugal force (N)
Per-Blade Allowance
For symmetrical propellers, the total permissible unbalance is divided equally among all blades:
Typical Balance Grades for Propellers
| Grade | Application |
|---|---|
| G40 | Slow-speed workboat propellers, non-critical |
| G16 | Standard marine propellers, cargo vessels |
| G6.3 | High-speed craft, navy vessels, passenger ferries |
| G2.5 | Precision propellers, low-noise submarines |
Practical Example
Given: Mass = 500 kg, Speed = 300 RPM, Grade = G16, 4 blades
ω = 2π × 300 / 60 = 31.42 rad/s
Uper = 16 × 500 × 1000 / 31.42 = 254,620 g·mm
Eccentricity = 16 × 1000 / 31.42 = 509.2 μm
F = 254,620 / 1,000,000 × 31.42² = 251.5 N
Per blade: 254,620 / 4 = 63,655 g·mm
⚠️ Note: Marine propellers operate in harsh conditions. Fouling, blade damage, and cavitation erosion can alter balance over time. Periodic vibration monitoring is recommended to detect developing unbalance.
Unbalanced propellers generate centrifugal forces at shaft speed that transmit through bearings into the hull. This causes excessive vibration, accelerated stern tube bearing and seal wear, fatigue cracking in shafting, and passenger/crew discomfort. Proper balancing per ISO 21940 minimizes these effects.
G16 is the most common grade for standard marine propellers (cargo vessels, tankers). G6.3 is used for high-speed craft, navy vessels, and passenger ferries where vibration must be minimized. G40 may suffice for slow-speed workboat propellers.
The total permissible unbalance is divided equally among all blades. For a 4-blade propeller with 1000 g·mm total allowance, each blade may have up to 250 g·mm. This assumes blades are symmetrically arranged around the hub.
Centrifugal force F = U × ω² / 1,000,000 (N), where U is unbalance in g·mm and ω is angular velocity in rad/s. Even small unbalances create significant forces at higher RPM — force increases with the square of speed.
Yes, in-situ propeller balancing is possible using portable balancing instruments like Vibromera's Balanset series. Trial weights are attached, vibration is measured, and correction masses are calculated — avoiding costly drydocking.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments for marine propellers, shafts, and rotating machinery. Achieve ISO 21940-11 compliance on-site.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
