Why is propeller balancing important for marine vessels?

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.

What balance grade G should I use for a marine propeller?

G16 is the most common grade for marine propellers. G6.3 is used for high-speed or navy vessels where vibration limits are tighter. G40 may be acceptable for slow-speed workboat propellers.

How is per-blade allowance calculated?

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 of unbalance. This assumes blades are symmetrically arranged.

What centrifugal force does propeller unbalance create?

Centrifugal force F = U × ω² / 1,000,000 where U is unbalance in g·mm and ω is angular velocity in rad/s. Even small unbalances at high RPM create significant forces — for example, 5000 g·mm at 300 RPM produces about 5 N, but at 3000 RPM it produces about 500 N.

Can I balance a propeller on the shaft without removing it?

Yes, in-situ (on-site) propeller balancing is possible using portable balancing instruments like Vibromera's Balanset series. Trial weights are attached to the propeller, vibration response is measured, and correction masses are calculated. This avoids costly drydocking.

Free Engineering Tool

Propeller Unbalance Calculator

Calculate permissible propeller unbalance, centrifugal force, eccentricity, and per-blade allowance per ISO 21940. Essential for marine shaft vibration control.

ISO 21940-11 Marine Propellers Per-Blade Allowance

Propeller Mass (kg)

Number of Blades

Propeller Diameter (mm)

Shaft Speed (RPM)

Balance Grade G

Quick presets

Results

Permissible Unbalance

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Specific Unbalance (Eccentricity)

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Centrifugal Force at Permissible Unbalance

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Per-Blade Allowance

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Angular Velocity ω

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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

Example — 4-Blade Marine Propeller

Given: Mass = 500 kg, Speed = 300 RPM, Grade = G16, 4 blades

ω = 2π × 300 / 60 = 31.42 rad/s

U_per = 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.

Propeller Unbalance Calculator | Marine Shaft Vibration

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Permissible Propeller Unbalance (ISO 21940)

Centrifugal Force from Unbalance

Per-Blade Allowance

Typical Balance Grades for Propellers

Practical Example