What it is
ISO 20283 is the standard series for vibration measurement on ships. Part 2 covers hull and structural vibration. Part 4 covers the propulsion plant — two- and four-stroke diesels, turbines, electric drives, gearboxes, shafting and propellers, including azimuth and Voith-Schneider units. Part 5 covers habitability in accommodation and work spaces, superseding ISO 6954:2000.
How it works
Part 4 defines instrumentation, measurement locations and data processing, then deliberately declines to give numerical limits. It refers the reader to the rules of the responsible classification society or the applicable IACS unified requirement. The split is the point: ISO supplies the method, class supplies the verdict.
Why shore limits do not transfer
A hull is not a foundation. Shipboard machinery almost always sits on what ISO 20816-3 calls a flexible support, so applying rigid-support criteria will fail a healthy pump. There is no steady state either — swell, draught, loading condition and main engine order all move the baseline. The dominant excitation is the propeller blade passing frequency rather than shaft rate. And torsional shaft vibration, the mode that actually breaks shafting, is invisible to a hull-mounted accelerometer: it requires strain-gauge telemetry and yields a barred speed range written into the ship's operating documents.