Vibrations often reveal mechanical changes earlier than temperature readings or alarm thresholds. The reason is physically straightforward: mechanical wear – whether on bearings, gears or structural elements – first produces altered vibration patterns before it manifests as increased friction heat or performance loss. The lead time can be weeks to months.
On main engines, the relevant vibration sources are numerous: main bearings, crosshead bearings, chain drive, turbocharger, driven generators and auxiliary equipment. Each of these sources has a characteristic frequency spectrum. An experienced analyst can read from the vibration signal whether the cause is a bearing problem, imbalance, misalignment or structural resonance.
Practice also shows the limitations, however: not every vibration change is an indication of damage. Changes in engine load, speed, sea state and even cargo loading influence vibration values. That is why normalisation to comparable operating conditions is critical – an aspect frequently neglected in practice.
A fixed measurement strategy with defined positions and reproducible conditions is advisable. For main engines, a set of 8–12 measurement points is recommended: one point per main bearing (radial horizontal and vertical), turbocharger bearing housing, free shaft end and foundation. Points should be permanently marked or fitted with fixed-mount measurement adapters.
Measurements should be taken under constant conditions: same engine load (ideally sea-going normal load, 75–85% MCR), same draught and calm sea. In practice, this cannot always be achieved perfectly, but documenting the boundary conditions for each measurement enables later correction.
Measurement frequency depends on criticality. For main engines, monthly measurements are a good compromise between effort and information value. For known pre-existing damage or following repairs, the frequency should be increased to weekly. Continuous online systems (permanent sensors with data logger) are the best solution for critical assets but not always economical.
Elevated vibrations can originate from multiple sources: imbalance, bearing problems and structural resonances. On main engines, the most common causes are: main bearing wear (increased clearance leading to low-frequency impulses), crosshead bearing problems (characterised by load-dependent impulses), turbocharger imbalance (high frequencies in the range of TC speed) and chain drive tension (specific chain frequency and harmonics).
Structural resonances are a particular topic with main engines. The firing frequency of the engine and its harmonics can coincide with natural frequencies of the foundation, double bottom or superstructure, causing amplified vibrations that do not indicate machine damage but represent a design issue. The distinction requires experience and frequency-selective analysis (FFT).
A frequently overlooked aspect is the shaft line. Misalignment between engine and gearbox/propeller shaft produces elevated vibrations at the forward and aft engine bearings as well as at the thrust block. This misalignment can change over the vessel’s lifetime – through hull deformation, different loading conditions or temperature effects on the foundation. Regular shaft alignment checks (jack-up tests) should be integrated into maintenance planning.
For attributing causes to readings, the ISO 10816 framework has proven effective as a reference, supplemented by manufacturer-specific thresholds from MAN or WinGD. IACS UR M68 additionally provides guideline values for permissible machinery vibrations on vessels.
Vibration monitoring delivers the greatest benefit when linked to PMS and fault history. The vibration value alone says: “something has changed.” Only the link to maintenance history permits the question: “does this fit what we know?” If the last main bearing was overhauled 20,000 hours ago and vibration at that bearing is rising, the relevance for action is immediately clear.
In fleet practice, vibration monitoring should be integrated into the PMS calendar – as a standalone job with defined intervals, responsibilities and documentation requirements. The Chief Engineer performs the measurements, the superintendent evaluates the trends and decides on actions.
For fleets with more than three vessels, commissioning a specialised vibration analysis service that performs quarterly measurements and delivers reports with action recommendations is worthwhile. Costs are typically EUR 2,000–4,000 per vessel per year – a marginal investment compared to an unplanned main bearing replacement.
Three measurement methods dominate in maritime vibration monitoring. Broadband vibration velocity (Velocity RMS, 10–1,000 Hz) provides a general overview of machine condition and is the best entry parameter. Vibration acceleration (Acceleration, 500–20,000 Hz) captures high-frequency signals indicating rolling bearing damage and gear faults. Envelope analysis (Envelope Demodulation) is specifically optimised for early detection of rolling bearing damage.
For main engines with plain bearings, vibration velocity is the most relevant parameter. An increase in RMS velocity of more than 50% above the reference value at the same load requires closer investigation. An increase of more than 100% requires immediate action – load reduction and technical assessment.
FFT analysis (Fast Fourier Transform) decomposes the vibration signal into its frequency components, enabling attribution to specific sources. The rotational frequency of the engine and its harmonics (2x, 3x etc.) show imbalance and misalignment. The firing frequency and its harmonics show combustion irregularities. Turbocharger frequencies typically appear in the 150–300 Hz range for two-stroke engines.
Portable instruments such as the FAG Detector III, SKF Microlog or Brüel & Kjær Vibro-Pen are practical for maritime use and cost between EUR 3,000 and 15,000. Data storage and trend evaluation is done via the associated software ashore.
A container vessel with MAN B&W 8K98MC-C performed monthly vibration measurements on all main bearings. Over three months, bearing No. 5 showed a steady increase in vibration velocity from 4.2 to 6.8 mm/s RMS – while all other bearings remained stable. Bearing temperature only showed a slight increase of 2°C in the third month.
Early detection enabled a planned inspection at the next docking, which confirmed increased bearing clearance. The bearing replacement could be carried out within the docking scope – without additional off-hire days. Had they relied solely on temperature, the damage would probably have been detected weeks later – possibly at sea.
The decision should be based on trend changes, not absolute values alone. The following traffic-light logic has proven effective: Green – stable values within historical range, no action required. Amber – rising trend over two or more measurement cycles, intensified monitoring and root cause analysis. Red – increase exceeding 50% above reference or sudden jump, load reduction and technical assessment within 48 hours.
Important: reference values must be machine- and position-specific. A “good” value at bearing 1 may already be elevated at bearing 7, because the stiffness conditions at the foundation differ. The baseline measurement after commissioning or major repair forms the reference.
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