Engine Alignment and Bedplate Flexure

Engine alignment is the precision setup of a marine slow-speed two-stroke engine’s crankshaft and shafting so that bearings carry their design loads and shaft elements remain free of torsional and bending stress beyond their rated levels. The engine sits on a bedplate chocked to the hull double-bottom; both the bedplate and the surrounding hull deform under loading, weight, temperature, and trim. Crankshaft deflections (web movements between adjacent throws) measured during alignment checks reveal the cumulative effect of all these distortions. This article covers alignment measurement, tolerance, hull deformation, chocking practices, and ongoing monitoring. Visit the home page or browse the calculator catalogue for related propulsion engineering tools.

Contents

Background

A modern marine slow-speed two-stroke engine has a crankshaft up to 18 m long supported on 7 to 14 main bearings. The crankshaft must rotate freely with each bearing carrying approximately its design load. Misalignment, where one or more bearings carries more or less than its design share, produces accelerated bearing wear, fatigue cracking of crankshaft webs, and vibration that propagates through the engine structure.

Alignment is set during installation: the bedplate is positioned in the hull, the crankshaft is laid in, the shaft connections are made to intermediate and tail shafts, and the entire system is shimmed and chocked to a measured straightness. The achieved alignment is verified by crankshaft deflection measurements (also called K-meter readings) at each crank position.

Alignment then drifts in service:

Hull deformation: ship loading, temperature, and trim distort the double-bottom under the engine
Bedplate flexure: thermal cycling, load, and time produce slow distortions
Bearing wear: progressive wear of main bearings shifts crankshaft journal positions
Chock degradation: epoxy chocks crack or shrink; cast iron chocks shift

Periodic alignment checks (typically annually or at major overhauls) detect drift and inform corrective action. This article describes the alignment process, the measurement methods, the tolerance requirements, and the operational considerations.

Crankshaft deflection

What it measures

Crankshaft deflection is the change in distance between two adjacent crank webs (on either side of a crank throw) as the crankshaft is rotated. For a perfectly aligned, perfectly stiff crankshaft on perfect bearings, the web spacing is constant at every crank angle. In practice, the spacing varies as the crankshaft moves through its rotation, with the variation indicating bearing alignment.

Measurement

A deflection meter (also called K-meter) is a precision dial gauge inserted between two crank webs. The crankshaft is rotated through 360 degrees; the meter reads the web spacing at intervals (typically every 90 degrees: top, bottom, port, starboard).

The four readings (top, bottom, port, starboard) at each crank position are recorded. The differences (top − bottom, port − starboard) are the deflections.

Modern engines use electronic deflection meters with digital displays and data logging.

Tolerance

Manufacturer tolerances on crankshaft deflection are typically 1/10,000 of stroke, or 0.10 mm for a 1,000 mm stroke. Tolerances are tighter for newer engines and looser for older designs.

Class societies impose alignment limits as well; typical limits are similar to manufacturer tolerances or slightly looser.

Pattern interpretation

The pattern of deflections across cylinders reveals the alignment fault:

Uniform high reading at top of all cylinders: bedplate sagging in the middle (likely hull hogging)
Uniform high reading at bottom of all cylinders: bedplate hogging in the middle (likely hull sagging)
Asymmetric port-starboard readings: bedplate listed or twisted
Single cylinder anomalous: localised bearing problem

Interpreting the pattern allows correction by either chocking adjustment, hull condition change (cargo, ballast), or bearing service.

Alignment process

New-build alignment

Bedplate positioning: the bedplate is lowered into the engine room and positioned on temporary supports
Initial straightness check: the bedplate’s main-bearing pockets are checked for straightness using piano-wire or laser alignment
Bedplate chocking: epoxy chocks are poured between bedplate and hull tank top; or cast iron chocks are machined and shimmed
Crankshaft installation: the crankshaft is laid into the bearings; bearings are temporarily set with shims
Initial deflection check: deflection readings are taken; bearing shims are adjusted
Final deflection check: after final bearing setting, deflection readings are recorded as the new-build baseline
Shaft connection: intermediate shafts are connected; deflections are checked again to confirm shaft alignment is consistent with engine alignment
Sea trial verification: deflection is measured during sea trial in operating conditions

Service alignment check

In service, alignment is checked:

At major overhauls (typically every 30,000 to 40,000 hours)
Annually or biennially in some maintenance programmes
After events that may have shifted alignment (grounding, severe collision, structural repair, drydocking)

Service deflections are compared to the new-build baseline; significant drift triggers investigation.

Hull deformation effects

Static deformation

The hull double-bottom under the engine deflects under:

Ship displacement (wet vs dry condition)
Cargo distribution
Ballast condition
Temperature differentials (engine room hot, surrounding cold)
Hull plating elasticity over the length of the engine

Static deformation can shift bedplate alignment by several tenths of millimetres between the dry-dock condition and full-load afloat. Alignment tolerances must accommodate this drift.

Dynamic deformation

Wave action induces hull girder bending: hogging (centre rising) and sagging (centre falling) cycles. The cycle period is the wave encounter period (typically 5 to 15 seconds in seaway conditions). Resulting bedplate deformations are smaller than static drifts but cumulative over time.

Trim and list

Trim (longitudinal pitch attitude) and list (transverse roll attitude) shift the gravitational loading on the engine and bedplate. Significant trim or list will affect crankshaft deflection readings; alignment checks are therefore performed at zero trim and list when possible.

Temperature

Engine room temperature affects the thermal expansion of the bedplate and the hull surrounding it. Cold check (engine off, ambient temperature) and hot check (engine warmed up to operating temperature) give different deflection readings; modern practice typically uses cold check.

Chocking practices

Epoxy chocking

Epoxy chocks (typically the Chockfast Orange or similar formulation) are now standard for new-build engine installation. Liquid epoxy is poured into the gap between bedplate and hull, with retaining dams; it cures to a hard, slightly resilient solid that supports the engine while accommodating thermal expansion.

Epoxy chocks last 20 years or more if undisturbed but can crack from impact, hot spots, or heavy thermal cycling. Replacement requires lifting the engine partially or fully, which is a major service event.

Cast iron chocking

Older or some specialty installations use cast iron chocks: precision-machined cast iron blocks shimmed and bolted between bedplate and hull. Cast iron chocks are easier to inspect and replace than epoxy but require more setup time.

Combination chocking

Some installations use a combination: a few critical points with cast iron, the remainder with epoxy. This combines easier inspection with simpler installation overall.

Chock inspection

Chocks are inspected at major overhauls for:

Visible cracks (especially in epoxy)
Looseness or shifting (cast iron)
Local crushing or deformation
Signs of oil leaks or moisture ingress

Chock failure typically produces sudden alignment shifts visible in deflection readings.

Shaft alignment

Beyond the crankshaft itself, the propeller shaft system (intermediate shaft, tail shaft) must be aligned with the engine output flange. Misalignment between engine and shaft produces:

Bending stress in the shafts
Uneven loading on intermediate bearings
Vibration at coupling locations
Wear at the stern tube bearing

Shaft alignment methods

Sag alignment: shaft is lowered slightly at the propeller end to compensate for hull deformation under load
Reverse alignment: shaft is raised slightly to over-correct for expected service deformation
Optical alignment: telescope or laser used to align bearing centres
Strain gauge alignment: bearings instrumented during sea trial to measure actual loads

The method depends on the engine class, hull type, and yard practice.

Coupling tolerances

The coupling between engine output flange and intermediate shaft must be within angular and concentricity tolerance, typically 0.05 to 0.15 mm circumferential variation and 0.05 to 0.15 mm gap variation.

Hot vs cold alignment

The engine deforms when warmed to operating temperature:

Bedplate expands by approximately 1 mm/m for a 100°C rise (steel thermal expansion)
Bedplate flexes from differential temperatures (top hotter than bottom)
Hull surrounding the engine also expands

The result is a different alignment hot vs cold. Manufacturers specify whether tolerances apply to cold check, hot check, or both:

Cold-check spec: typically used for new-build and routine service checks. Deflections measured with engine cold, easy to perform.
Hot-check spec: requires engine running to working temperature, then a brief shutdown for measurement. Used for verification at sea trial.

Both checks are valuable; the patterns may reveal different issues.

Bearing load distribution

The goal of alignment is even bearing load distribution. Each main bearing should carry its share of the crankshaft weight plus dynamic loads from cylinder firings. Misalignment shifts loads:

Some bearings overloaded → faster wear, possible white-metal fatigue
Some bearings underloaded → poor lubrication regime, marginal oil film

Bearing load can be inferred from deflection readings or measured directly via instrumented bearing trial.

White metal fatigue

White metal (the soft tin-based alloy on bearing surfaces) is fatigue-sensitive. Sustained bearing overload (above ~12 MPa typical limit) can produce subsurface fatigue cracking visible as surface pitting after many hours. White metal fatigue is a leading cause of main bearing failures on misaligned engines.

Oil film

An adequate hydrodynamic oil film requires sufficient bearing load to develop. Underloaded bearings have insufficient pressure to maintain a hydrodynamic film, leading to boundary lubrication and accelerated wear.

Service implications

Alignment shift indicators

In service, alignment drift is indicated by:

Crankshaft deflection outside tolerance
Bearing temperatures rising above baseline (sensor data on modern engines)
Bearing oil sample wear metals (white metal, copper) above expected
Shaft vibration measurements showing increased amplitude
Visible bedplate cracks at maintenance

Corrective action

Detected alignment drift may be corrected by:

Chocking adjustment: re-shimming or partial epoxy replacement
Bedplate machining: in extreme cases, machining the bedplate at slipway to restore straightness
Shaft alignment: re-aligning shaft segments to match the new bedplate condition
Bearing replacement: replacing worn bearings as part of broader alignment service

Major realignment is a slipway task, performed during major overhauls or after significant hull repair work.

References

MAN Energy Solutions. (2023). Engine Alignment and Chocking Manual. MAN Energy Solutions.
WinGD. (2023). Engine Installation and Alignment Procedures. Winterthur Gas & Diesel.
IACS. (2018). Unified Requirements M51: Crankshaft Deflection.
Class NK. (2022). Guidelines for Engine Alignment.
Sun, J. & Wang, Y. (2018). Marine Engine Alignment: Theory and Practice. Wiley.