Short answer: Main engine alignment is the positioning of a yacht’s main engine, and its gearbox where fitted, on its mounts, chocks and foundation so that the output flange or coupling lines up with the shaftline within the coupling and gearbox manufacturer’s limits. The engine is set cold with any offsets the manufacturer specifies for thermal growth and mount settling, the supports are checked for soft foot and even contact, and the result is confirmed by readings across the coupling, running checks and a sea trial. It is the machinery side of the job; the shaftline itself is covered by shaft alignment.
Main engine alignment sets the installed relationship between the main engine, mounts, chocks, foundation, gearbox or output flange and the shaftline that receives the power. It is a machinery-side decision, but the effect reaches the whole propulsion system.
During a refit it becomes important after engine removal, mount replacement, foundation repair, gearbox work, shaftline work, vibration findings or a sea-trial concern. The job is to place the engine where it carries load cleanly, protects the connected components and gives the owner side a defensible redelivery record.
Engine Alignment and Shaft Alignment
| Main engine alignment | Shaft alignment | |
|---|---|---|
| Covers | Engine and gearbox position, mounts, chocks, foundation, output flange | Shaftline from output flange through bearings, stern tube and brackets to the propeller |
| Typical adjustments | Mount height and position, shims or chocks, engine bed corrections | Bearing positions and offsets, bracket and stern-tube line |
| Typical checks | Coupling offset and angle, soft foot, mount condition, and crankshaft deflections on larger engines where the manufacturer specifies them, to confirm the engine frame has not been distorted by chocking or bolting | Sag and gap, laser sighting, bearing loads, run-out |
The two meet at the gearbox output or coupling. The usual order is to set the shaftline first and then bring the engine and gearbox to it, because the shaftline position is fixed by the stern tube and bearings while the engine can be moved on its mounts.
How the Engine Is Aligned in Practice
- Check the supports. Inspect mounts, chocks, holding-down bolts and the engine bed before taking any alignment reading.
- Remove soft foot and let the mounts settle. On rigid installations, make sure every foot seats evenly so the engine does not twist when bolts are tightened; on resilient mounts, check that each mount carries its specified share and has settled under the engine’s weight for the period the mount maker specifies.
- Read across the coupling. Measure offset and angle between the engine or gearbox flange and the shaft flange with dial indicators or a laser system.
- Adjust. Move the engine and, where required by the drivetrain arrangement, the gearbox, with shims or adjustable chocks, to the target values, including any cold offsets the manufacturer specifies for thermal growth and mount settling.
- Fit or pour chocks. With steel chocks, machine and fit them to the measured gap; with resin chocks, hold the engine on its jacking screws, pour once the position is final and let the resin cure to the maker’s procedure before bolt load is applied.
- Tighten and re-check. Torque the holding-down bolts to specification and repeat the readings; the position has to hold after clamping.
- Confirm afloat and running. Re-check after launch, then under load at the sea trial.
Engine alignment depends on the mounts, chocks and foundation as much as on the coupling reading.
Mounts and Chocks Carry the Load
Mounts and chocks are part of the alignment, not just supports. They carry engine weight, absorb or transmit vibration, set height and control movement.
| Support type | What to know for alignment |
|---|---|
| Resilient (flexible) mounts | Common on yachts for noise and vibration isolation; the rubber elements settle and age, so the manufacturer often specifies an initial offset and periodic re-checks; worn or uneven mounts change the reading |
| Rigid steel chocks | Machined to fit between engine feet and bed; need full, even contact |
| Resin (epoxy) chocks | Poured in place between feet and bed after alignment, so they take the exact shape of the gap; must be a class-approved product, poured and cured to the maker’s procedure, with chock thickness, area and bolt tension within the maker’s limits |
Worn mounts, uneven compression, tired rubber, loose fasteners, poor shim contact, damaged chocks or a disturbed holding-down arrangement all change the reading. A familiar refit trap: a yacht comes in for vibration or coupling noise, the first reading points to alignment, and the engine is adjusted to a number that moves again after load, heat or use, because the support condition was never checked.
Foundation Condition and Soft Foot
The foundation under the engine sets the real reference. Previous welding, insert repair, corrosion, coating build-up, local distortion, weak fastener seating and old impact damage all affect how the engine sits. After heavy refit work, such as engine removal, structural repair, tank work or heat input near the bed, the foundation is inspected before alignment.
Soft foot is the condition where one foot or support does not sit evenly with the others. The engine then twists or settles as bolts are tightened, and the flange reading changes with clamping force. It comes from uneven bed surfaces, mount condition, shim stack problems, distorted feet or coating build-up, and it has to be resolved before a reading is worth recording.
Cold Readings, Thermal Growth and Running Behaviour
A cold static reading is a snapshot before heat, thrust and sea condition enter the picture. Engines and gearboxes grow as they warm up, and resilient mounts move under torque and settle over time, so manufacturers often give cold alignment targets that are offset from zero. The record states the condition of each reading: dry dock or afloat, tank loading, recent running and mount temperature.
Alignment earns confidence when static geometry agrees with running behaviour: engine load, gearbox temperature, vibration, coupling response, mount movement, bearing temperature, seal condition and noise across the rpm range. Readings and trial notes turn a workshop adjustment into evidence, which is why this measured layer sits with tests and surveying. Running behaviour also separates alignment from related causes such as propeller condition, shaftline geometry, bearing wear, engine condition or foundation movement.
Running readings confirm whether the cold alignment holds under load.
Sea Trial and the Engine Alignment Record
The sea trial is where main engine alignment faces real propulsion demand: the engine is loaded, the gearbox transmits torque and the yacht shows whether the installation behaves cleanly across the operating range.
A useful record identifies the yacht’s condition, work completed, mount and chock status, foundation findings, measurement method, cold readings, adjustments, running observations, sea-trial notes, remaining restrictions and final acceptance. If vibration, leakage, bearing heat or mount movement appears after delivery, the technical team has a baseline. On larger projects this work sits with machinery and equipment and project management, because alignment affects lifting, access, specialist attendance, launch preparation and trial planning.
Frequently Asked Questions
What is main engine alignment on a yacht?
Main engine alignment, sometimes called motor alignment, is positioning the main engine and gearbox on their mounts, chocks and foundation so that the output flange lines up with the shaftline within the manufacturer’s limits, and confirming that position under load.
How do you align an engine to a propeller shaft?
Set the shaftline first, then check the engine supports, remove soft foot and let resilient mounts settle, read offset and angle across the coupling, adjust the engine on its mounts or shims to the target (including any cold offsets for thermal growth), torque the bolts and re-check. Confirm afloat and at the sea trial.
How is main engine alignment different from shaft alignment?
Engine alignment positions the machinery: engine, gearbox, mounts, chocks and foundation. Shaft alignment sets the shaftline: couplings, bearings, stern tube, brackets and propeller. They meet at the output flange.
Why is engine alignment done with offsets when cold?
Because the engine and gearbox grow with heat and flexible mounts move under torque and settle. Manufacturers often specify cold targets so that the alignment is correct at operating condition.
Do engine mounts and chocks affect alignment?
Yes. Mount compression and age, chock contact, fastener condition, shim quality and support stiffness change how the engine sits and how the output flange meets the shaftline.
What is soft foot?
A support that does not sit evenly with the others, so the engine twists or settles when bolts are tightened. Readings taken with soft foot present are unreliable.
When is main engine alignment checked during a refit?
After engine removal, mount replacement, gearbox work, foundation repair or shaftline work, after launch, and when vibration, coupling noise or a sea-trial concern points to the machinery position.
How often should engine alignment be checked?
There is no single interval. Follow the engine, gearbox and mount manufacturers’ guidance, and re-check after any work that disturbs the engine, its mounts or the shaftline, or when symptoms appear. Resilient mounts in particular may need re-checking as they settle.
