Guide 06 Marine engineering and ship repair
Marine shaft alignment explained
Shaft alignment is the static condition of a ship's propulsion shaft line at its bearings, calculated, set on the blocks in dry dock, and confirmed afloat. The aim is a set of bearing loads that stays acceptable from the stern tube bearing to the engine, across loaded, ballast and hot running conditions.
- Topic
- Marine engineering and ship repair
- Checked
- 23 September 2026
- Sources
- 3
- Questions
- 6
01
What shaft alignment sets
The alignment calculation fixes the height and offset of every bearing in the shaft line, from the stern tube bearing through any intermediate (plummer) bearings to the engine or gearbox bearings. The target is a load distribution where no bearing is overloaded and none is left unloaded, or lifted clear of its shell, in the conditions the ship actually runs in.
Because a hull bends differently empty and loaded, and a diesel engine grows as it heats from cold to running temperature, the same shaft line can be correctly aligned in one condition and out of tolerance in another. The calculation has to account for all of them, not just the static condition on the blocks.
02
Sag and gap: the pre-assembly check
Sag and gap is measured before the shaft couplings are bolted together. With the flanges uncoupled, two readings are taken at the joint: sag, the vertical offset between the two half-flanges, and gap, the angular opening between their faces. Both are checked against the values the alignment calculation produced for that ship and loading condition.
The check is most reliable with the ship resting on blocks in dry dock. Afloat, the hull deflects under the weight of cargo, fuel and ballast, and that deflection changes the sag and gap readings, so an afloat check has to allow for it in the underlying calculation rather than compare raw numbers to the dry-dock target.
Two shaft alignment checks compared
- Sag and gap
- Pre-assembly, flanges uncoupled; most reliable in dry dock on the blocks
- Jack-up test
- Calibrated hydraulic jack measures actual bearing reaction; used to confirm alignment afloat
03
The jack-up test: measuring the real load
The jack-up test measures what a bearing is actually carrying rather than what the drawing predicts. A calibrated hydraulic jack is set up next to the bearing, and as it lifts the shaft a small amount, load and lift are logged together. The bearing reaction is read from the resulting jacking curve, corrected for the distance between the jack and the bearing centreline.
Load cells fitted under the jack sharpen the reading, and strain gauge methods are often run alongside the jack-up test on larger shaft lines. Because it reflects the shaft as built and loaded rather than as drawn, a jack-up result is a direct check on the calculation, not a substitute for it.
04
Why alignment is checked twice
A ship's hull bends differently on the blocks, in ballast and fully loaded, and the engine itself grows several millimetres between cold and running temperature. Bearings that read correctly aligned in dry dock can end up overloaded, or lose their load altogether, once the ship is afloat and the machinery is hot.
That is why alignment is set against calculated values in dry dock and then confirmed afloat, often by jack-up measurement, before the ship goes to sea trials.
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Marine shaft alignment explained
What is shaft alignment on a ship?
Shaft alignment is the static condition of a ship's propulsion shaft line at its bearings, calculated and set with the system at rest. It aims to keep every bearing, including the stern tube bearing and the engine or gearbox bearings, within an acceptable load across loaded, ballast and hot running conditions, not just as built in dry dock.
What is the sag and gap method of shaft alignment?
Sag and gap is a pre-assembly check made with the shaft flanges uncoupled. The vertical offset (sag) and the angular opening (gap) between the mating flanges are measured and compared with the alignment calculation. It is most reliable in dry dock; afloat, hull deflection changes the readings and has to be allowed for.
What is a jack-up test in shaft alignment?
A jack-up test measures a bearing's actual reaction rather than its calculated one. A calibrated hydraulic jack beside the bearing lifts the shaft while load and lift are recorded, and the reaction is read from the jacking curve, corrected for the jack's offset from the bearing centreline. Load cells improve the reading's accuracy.
Why is shaft alignment checked in both dry dock and afloat?
A ship's hull bends differently on the blocks, in ballast and fully loaded, and the engine grows as it heats up. Bearings aligned correctly in dry dock can become overloaded or unloaded once afloat, so alignment is set against calculated values in dock and then confirmed afloat, often by jack-up measurement, before sea trials.
Does the Indian Register of Shipping survey propulsion shaft alignment?
The Indian Register of Shipping (IRS) is a full member of IACS, the International Association of Classification Societies, and surveys the propeller and tube shafts of the ships in its class against IACS Unified Requirements such as UR Z21, which sets the scope, methods and intervals for those surveys.
Can shaft alignment be checked without dry docking?
Some checks can. Sag and gap and jack-up readings can both be taken afloat, but the hull's deflection under cargo, fuel and ballast changes the numbers, so an afloat check has to be corrected against the alignment calculation. A full pre-assembly sag and gap check is normally done with the ship on the blocks.
Checked against
Sources
- 01Shaft alignment, Wärtsilä Encyclopedia of Marine Technology
- 02Propulsion Shaft Alignment, guidance notes, American Bureau of Shipping
- 03About IRClass, Indian Register of Shipping
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
On this site
Where this meets the work
- Shaft line work covered under shafting services
- The marine division
- Hull maintenance done alongside shaft alignment in dry dock
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