Propulsion shaft alignment principles

Author, Brett Weintz. Updated 4 April 2020

Alignment calculations involve deriving bearing loads versus offset by means of a computer mathematical (beam element) model as well as optimisation logic/ iterations. The objective of ship propulsion shaft alignment measurement is a satisfactory load distribution between all, and across each, bearing(s) supporting the complete propeller shaftline.

This page provides discussion of ship propulsion shaft alignment principles includes; the function of each particular bearing, conditions that affect alignment as well as sterntube aft/ A-bracket bearing operational problems.

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You can find the following Brabon Engineering Services site pages and case studies regarding ship propulsion shaft bearings:

• Shaft alignment measurements Brabon Engineering Services undertake.
• Shaft alignment measurement details discussion page. Includes factors affecting derived results/ uncertainty.
• Shaft alignment principles discussion (this page).

• Shaft alignment measurements case study.
• Strain gauge shaft alignment measurement case study.
• Sterntube aft bearing machining offset accuracy case study.
• Fair curve alignment design example.
• Sterntube bearing problem case study 1.
• Sterntube bearing problem case study 2.

• Ship sterntube aft bearing operations and problems discussion page.
• Hydrodynamic bearing design and damage discussion page.
• Ship vibration (shaft dynamics) discussion page.

Ship propulsion shaftline bearings

On a propulsion shaftline each bearing typically has particular functions and loading patterns, see Figure 1 below.

Sterntube aft/ A-bracket bearing

A ship’s stern tube aft bearing is probably one of the most severely loaded applications of a plain cylindrical sleeve bearing with oil bath/ water hydrodynamic lubrication. The bearing reacts most of the mean and varying transverse hydrodynamic propulsion forces and moments on the cantilevered propeller. (The stern tube forward bearing plus any shaft bending takes the balance of the forces.) The stern tube aft bearing is generally at least two diameters in length (four diameters for a staved type design) to control the peak oil film pressure as well as to provide damping for the shaft (by means of the total oil film area).

Given the length, it is important to ensure a satisfactory load bias between the aft and forward ends of the bearing in operation (particularly during manoeuvring). An adverse load bias has an associated risk of edge loading at the extreme aft end, 6 o’clock position leading to a thermal wipe or fatigue damage (frequently encountered). Conversely, a heavy load bias to the forward end can permit ‘propeller’ mode lateral vibration of the shaft, although this is less common than damage at the aft end.

Note that the mean propulsive thrust generally does not act at the centre of the propeller. That is, the thrust eccentricity is generally offset above the centre and results in an additional (mean plus vibratory) moment acting on the propeller/ shaft. (However, thrust eccentricity depends on the inflow wake generated by hull form aft.) Thrust force is usually greater than the propeller weight, thus the (mean plus vibratory) variations of the sterntube aft bearing load are typically significant relative to the static/ stopped loads.

Sterntube forward bearing

If a stern tube forward bearing is not fitted, then the aft intermediate bearing adopts this role. This bearing provides stability for the propeller shaft against vibration under the varying hydrodynamic propulsive forces and moments acting on the propeller. A portion of the transverse propulsion forces and moments on the propeller appear on the bearing. Given the fluctuating harmonic forces and moments on the propeller, the stern tube forward bearing is subject to load variations. It is important that the bearing has a positive load under all operating conditions, i.e. draughts, so as to avoid vibration and consequential bearing fatigue damage.

Intermediate bearing (forward)

That is the first bearing aft of the propulsion engine, gearing or electric motor.  This bearing controls the load distribution between the aft two/ three bearings of the propulsion prime mover.  Note that the prime mover and its foundation are subject to (relative) thermal growth between the cool and warm/ normal operating conditions. Hull deflections affecting the main engine relative to the aft bearing may also occur on vessel subject to large changes in draughts. It is important that the aftmost main bearing of the prime mover is not overloaded in the warm condition, or unloaded in the cool condition.

Do you suspect a problem with a vessel’s sterntube bearing?

Brabon Engineering Services can help by conducting accurate shaft alignment measurements and independent assessments.

Call for a discussion: +353 87 383 5043
email for a proposal: info@brabon.org
Send a message via our contact page
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Propulsion shaft fair curve alignment

(What are the optimum shaft alignment loads and offsets?)

The (primary) objective of propulsion shaft alignment is satisfactory load distribution between all the bearings. The bearing offsets and shafting geometry/ masses are the design input variable(s). However, the measured/ derived loads form the basis of any assessment.  While bearing loads when running determine satisfactory performance/ life, shaft alignment measurements are conducted in the stopped/ static condition. The general assumption is that applied design guidelines (rules of thumb) result in dynamic running loads (and variations due to structural deformations) being satisfactory when stopped and throughout the operating speed range of the system.

For shafting up to about 200mm diameter, a shaftline design having all bearings in a (straight) line generally provides a satisfactory bearing load distribution. However, the lateral/ beam stiffness of larger diameter shafting (for a similar ratio of span to diameter) associated with greater transmitted power and transverse loads generally necessitates a (coordinated) set of bearing offsets for all bearings to have satisfactory load. Modern design is typically based on fair curve alignment where the offsets are selected to give a smooth shaftline curve with the least number of inflexion points. Many references are available, e.g. [1]. Notwithstanding the selected shaftline design offsets, it should be noted that many sets of bearing offsets can produce a satisfactory distribution of loads among the bearings. For example, the main engine/ gearing may not necessarily be parallel to the sterntube.

[1. Mann G., ‘Analysis of shafting problems using fair curve alignment theory’, Naval Engineers Journal, 1965 Vol 77, p651.]

The design axial locations of the sterntube forward and intermediate bearings should be optimised for minimum (calculated) shaft stiffness in way of each bearing. However in practice, bearing axial positions may be constrained by hull structure and shaftline dismantling considerations, e.g. allowing sufficient longitudinal distance for the propeller shaft to be withdrawn into the engine room.

The alignment (adjustment) process then involves ensuring satisfactory/ adequate support (loading) at each bearing at all aft draughts conditions and with machinery warm/ cool. Counter to intuition, systems having fewer bearings typically present greater challenges in the propulsion shaft alignment process, i.e. achieving a satisfactory load distribution on the vessel, rather than design.  For example, where a system has one intermediate bearing between the sterntube aft bearing and the propulsion engine, the intermediate bearing has to be positioned provide a satisfactory load distribution on the sterntube aft bearing and the aft bearings of the main engine (under operating conditions), thus there may be little margin for adjustment once the propulsion engine has been positioned and chocked.

Fair curve alignment – example 1

The following is an example of ship propulsion shaft fair curve alignment.  Example 1 involves a two-stroke diesel, direct-drive propulsion system.  The system comprises a propeller of mass 14 tonne (immersed) fitted to a propeller shaft 500mm in diameter with centre-to-centre axial distance of 5,000mm (10 diameters) between the sterntube aft and forward bearings.  The propeller shaft is coupled to an intermediate shaft 375mm in diameter which is supported by an intermediate/ plummer bearing 5,000mm forward of the sterntube forward bearing.  The engine crankshaft has an equivalent (continuous shaft) diameter of 330mm.  A mathematical beam element model was generated for analysis.

The shaft/ bearing offsets and calculated corresponding bearing reaction loads (shaft static/ stopped) for two possible sets are shown in Table 1.  The associated shaft shapes are shown in Figure 2.  Note differences between shaft and bearing offsets are based the half clearance in each respective bearing.

Case A involves the main engine crankshaft parallel and offset down from the sterntube in order to generate a load on the sterntube forward bearing, i.e. counteract the static mass-moment of the propeller.

Case B involves the main engine output being in-line with the sterntube and the crankshaft sloped forward-up in order to develop a load on the sterntube forward bearing.  This methodology is popular in some shipyards in the Far-East as it allows the main engine to be pre-positioned early in the construction process (although the engine relative offset changes for vessel supported on dock blocks versus the afloat condition).

The bending moment distribution along the shaftline is slightly greater in Case B versus Case A, Figure 3.  However, the (absolute) maximum that occurs in way of the sterntube aft bearing is identical in both cases.

The International Association of Classification Societies (IACS) unified requirements for machinery installations specified requirements are limited to the sterntube aft/ A-bracket bearing length (relative to the shaft diameter) and maximum nominal pressure for the bearing material to be used. Notwithstanding, the Class society for the vessel typically require the design shaft alignment for review and approval.

While the fundamental theory of shaft alignment is straightforward, there are many subtle factors that can affect the bearing load distribution. Each vessel/ class shafting arrangement tends to have a unique set of factors and considerations for the alignment condition.

Take the steps to solving your propulsion shaft bearing reliability problem.

Call for a discussion: +353 87 383 5043
email for a proposal: info@brabon.org
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Hull structure effects on propulsion shaft alignment

Large vessels, say in excess of 200,000 DWT, typically have a large diameter shaftline with high stiffness (in beam bending) relative to the aft hull global bending and shear stiffness (as compared to vessels of lesser size). In particular, the hull area between the main engine, aft end and the sterntube/ aft peak tank typically has low stiffness due to the fine/ narrow hull section around the height of the shaftline and optimised scantling sizes versus the rigid deep frames and longitudinals of the main engine foundation as well as the continuous frames supporting the sterntube within the aft peak. Thus, flexure of the vessel with changing (draught) conditions can result in relative movement between the main engine and the sterntube as well as any intermediate bearings.

It was found in the 1970s that a draughts (aft) change from ballast to laden (deeper) generated a corresponding greater (static) pressure on the hull in way of the main engine and the resulting hull deflections would lift the main engine relative to the sterntube [2]. The resultant variations in shaftline bearing loads are more significant in tankers, bulk carriers and similar vessels having a short shaftline due to the short distance from the sterntube to the main engine and the (nearly direct) influence of the relative movement on bearing loads. For vessels with longer shaftlines the variation in offset (between the main engine and sterntube) occurs progressively over say two intermediate bearings with lower associated bearing load deviations.

[2. Volcy G.C., Memoirs of a marine troubleshooter, IMarE, 1995.]

Thus, large tankers and bulk carriers having significant changes in draughts between the ballast and laden conditions are likely to experience significant deviations in relative offsets between the sterntube bearing(s) and the main engine, main bearings, with corresponding changes in bearing loads. The bearing load deviations are greatest at the aftmost main bearing of the engine as well as the sterntube forward bearing.

During design the axial locations of the sterntube forward and intermediate bearings should be optimised for minimum shaft stiffness in way of each bearing. During construction alignment it is usual to set a light load at the aftmost main bearing with the vessel at light ballast and the engine cool. For large tankers, bulk carriers and similar, the propulsion shaft alignment during construction should be conducted at the normal ballast condition (at a deep water berth). For vessels where the shaftline is supported by two, or more, intermediate bearings there is less imperative to arrange a very light load at the aftmost main bearing as any load variation would be mainly limited to the effect of the main engine foundation (relative) thermal growth.

Where a sterntube forward bearing is fitted, hull deflexions typically have minimal effect on the sterntube aft bearing load distribution/ bias. (Although, the sterntube forward bearing is affected.) If a sterntube forward bearing is not fitted, the sterntube aft bearing load distribution is (primarily) controlled by the intermediate bearing, then changes in draughts can result in a significant variation in the sterntube aft bearing load distribution.

As noted above, for large tankers, bulk carriers and similar, the construction alignment should be conducted at the normal ballast condition (measurements should also include concentricity of the propeller shaft in the forward end of the sterntube). Indeed, all measurements should be conducted with the vessel afloat as the intermediate bearing foundation (all bearings forward of the sterntube) will deflect due to forces from the dock blocks. Confirmatory bearing load measurements should be conducted at the laden and ballast draughts with the machinery warm (hull deflections at each bearing versus draught are difficult to predict).

It should be noted that filling the aft peak tank in order to increase the aft draught and improve the propeller immersion in the ballast condition on a vessel not fitted with a sterntube forward bearing and having a flexible hull, say in excess of 100,000 DWT, could also result in an adverse variation of the sterntube aft bearing load distribution. If there is no service experience of operation in this condition, then the risks should be carefully considered. For example, underway tests could be conducted with the aft peak tank only partially filled. Shaftline bearing load measurements with and without the aft peak tank filled may be available from original construction.

Where problems occur in service with the sterntube aft bearing, say across a class of vessels, it may be prudent to consider conducting measurements with a view to optimising the alignment condition as well as design features that improve the loading margin/ tolerance of the sterntube aft bearing. Note that a dual-slope sterntube aft bearing is only suggested as beneficial in a limited range of vessels, e.g. very large tankers and bulk carriers as noted above. A dual slope (sterntube aft) bearing typically also requires additional alignment measurements in order to verify a satisfactory load distribution on the bearing in the normal ballast and laden conditions (if the bearing is fitted as part of a repair the measurements may be conducted before the vessel is dry-docked). In most other cases the modification is unlikely to derive any benefit.

It is popular to depict the propeller shaft in the sterntube aft bearing as hogged/ curved downwards towards the propeller. However this is only the case with the shaft static and at low speeds. At the service speed the propeller shaft is typically sagging/ curved upwards towards the propeller, i.e. inverted, due to the hydrodynamic forces/ moments usually resulting in a thrust eccentricity biased upwards from the propeller centre.

New vessel construction shaft alignment procedure

During ship construction the aft hull blocks are assembled complete with foundations for the main engine(s), reduction gearing/ prime mover and intermediate bearings. The aft most block with the stern frame is positioned prior to being joined so that the sterntube axis is aligned at the design offset to each foundation (using a system of laser or optical telescope and targets). If the vessel is fitted with a flexi-tube, then the axis of the sterntube bearings is aligned to the design offsets at each foundation prior to the flexi-tube being resin chocked in the stern frame.

When the sterntube and engine room foundation are in place the main propulsion machinery is fleeted in on temporary adjustable chocks. The propeller shaft is fitted in the sterntube and the intermediate shaft(s) are brought into the engine room, although not coupled. The vessel is then launched having all major structural sections and machinery.

With the vessel afloat (at the light ballast condition), the intermediate shafting and bearing(s) are initially positioned relative to the forward end of the propeller shaft by the ‘gap and sag’ process. This is based on the relative radial and angular offsets between each successive pair of flanges from aft to forward (using calculated values for the shaftline uncoupled state). The preliminary alignment of the main propulsion engine/ reduction gearing is similarly conducted by means of the ‘gap and sag’ between the intermediate shaft forward flange and the prime mover output flange. On completion of preliminary alignment the shaftline flange fastener holes are match drilled/ bored and the shafting coupled.

Final propulsion shaft alignment is conducted on the basis of measured bearing loads with the vessel afloat. Static (shaft stopped) loads on the accessible bearings are typically measured by the jacking method. The measured loads are assessed against calculated design values. However, it is generally necessary to assume a satisfactory slope and resultant load distribution on the sterntube aft bearing when assessing the (jacked) loads on the sterntube forward and intermediate bearings. (Unless shaft bending strain measurements have been conducted.) With satisfactory bearing loads and main engine deflections the machinery is resin chocked and fixed down. Confirmatory measurements are conducted during shipbuilder’s trials with the main machinery warm as well as for the vessel at (normal) ballast and laden draughts.

As previously noted, systems having fewer shaftline bearings typically present a greater alignment challenge. In particular, systems having no sterntube forward bearing and only one intermediate bearing. In this case, the intermediate bearing position has to provide a satisfactory load distribution on the sterntube aft bearing as well as the propulsion engine aft bearings (under operating conditions). Thus, there is little margin for adjustment once the propulsion engine has been positioned and chocked.

Brabon Engineering Services can assist in cases of suspected alignment problems with the following:

• Detailed ship shaft alignment modelling/ design calculations of bearing load distribution.
• Bore alignment offsets using the Taylor-Hobson micro-alignment telescope and targets.
• Bearing loads by shaft jacking using strain gauge load cell and displacement transducer.
• Main engine, main bearing loads by crankshaft jacking. Main engine crank web deflections.
• Bearing loads by shaftline bending strains (strain gauges).
• Measurement of alignment/ offsets of elastic couplings and cardan shafts as well as gap and sag of uncoupled shaft sections.
• Assessment/ interpretation of measured bearing loads (versus design).
• Calculations of bearing offset adjustments and predicted load changes for an improved local alignment and/ or optimised shaftline bearing load distribution.

Sterntube aft bearing problems

(Why is propeller shaft alignment important?)

Indications of bearing distress would include a rapid (exponential) rise in bearing temperature above the alert level. For oil lubricated, whitemetal lined type bearings a typical alert temperature would be 65°. Note that non-metallic bearings have lower thermal conductivity, thus tend to operate at a slightly higher temperature and have a slower rate of cooling. Similarly, where the support liner of a metallic bearings is set in (epoxy) chock, the bearing may operate at a slightly higher temperature. In cases of severe damage, the lube oil may become contaminated and emulsified within a short time along with an increase in vibration of the aft end. Bearing temperature may be lower during steady motoring after a damage incident (due to possible increased clearance), however, the bearing temperature may be more sensitive to manoeuvring or heavy ship motions.

In the event of (suspected) bearing damage the following is suggested:

• Reduce shaft speed. The shaft should be maintained at the highest speed that still allows a decreasing bearing temperature (towards typical). The shaft should not be stopped. Notwithstanding, navigational safety must take precedence over all other considerations.
• Change-over the duty/ stand-by lube circulation pumps and inspect the suction strainer. Reflective flakes or the acrid smell of burnt phenolic would be respective indications of damage to whitemetal or non-metallic bearings.

Do you suspect a problem with a vessel’s sterntube bearing?
Brabon Engineering Services would be pleased to review your condition monitoring data and provide a free honest and expert bearing damage risk assessment.

Call for a discussion: +353 87 383 5043
email for a proposal: info@brabon.org
Send a message via our contact page
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Propulsion shaft alignment bearing repairs

If the decision is made to conduct a repair docking, then the following measurements should be considered:

• Confirmatory bearing load measurements prior to docking, i.e. with the vessel afloat.
• Visual inspection of the damaged bearing. This can inform the damage mechanism and general rectification action, although it should be noted that cumulative progression can obliterate the initial area of damage.
• Optical/ laser bore offset measurements, i.e. to derive relative slope, of the (damaged) bearing surface as well as the bearing housing (after bearing extraction).
• The above may indicate adjustments to the design offsets (and slope) of the replacement bearing in order to improve the bearing load distribution.
• It may be prudent to also consider options that improve the operating margin of the bearing. These could include; directing the fresh lube oil supply to the aft end of the bearing and/ or raising the lubricant washways from 9/ 3 o’clock to 10/ 2 o’clock to increase the arc of the lower working surface. Additional temperature sensors may also be fitted in the bearing in order to monitor and better understand the relationship of manoeuvring speed/ applied helm versus bearing temperature.

Further information

The following case studies are available on request:
1. Sterntube aft bearing whitemetal damage (and associated corrective bearing offset adjustments).
2. Stern seal and A-bracket bearing problems.

Discussion of alignment principles can be found in various industry prepared references, such as British Ship Research Association (BSRA). These are generally available from the Institute of Marine Engineering, Science and Technology (IMarEST) library. Classification societies also provide (limited) guidance on shaftline alignment design and practice.

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The above notes are applicable to typical generic events, however, please note that every actual event is unique and requires individual consideration. Any action taken upon the information on this website is strictly at your own risk. Brabon Engineering Services are not responsible in any way whatsoever for your use of the information.