In a timber hull, alignment is a moving target that has to be checked afloat. The structure flexes between dry and wet, and a shaft line set true on the hard will not necessarily be true once the vessel is back in and taken up. The correct method is align on the hard, relaunch, allow the hull to settle, then check and correct.
Why engines come out at all
Sometimes for the engine’s own sake. More often because everything around it needs attention and cannot be reached otherwise: bearers, the timber under them, the bilge below, the stern tube, and the fastenings in a zone that is permanently wet and usually oily. On an older vessel, engine removal is one of the highest-yield inspections available, precisely because nobody has looked there in years.
What is usually found under an engine
Soft bearers
Oil, salt water and fresh water in combination, with no drying and no inspection. Bearer timber is often the worst in the hull.
Wasted fastenings
The zone under and around the engine routinely shows advanced fastening wastage, as covered in fastenings and joint failure.
Blocked limbers
Drainage paths clogged with debris, so water stands against floors instead of running to the pump.
Improvised repairs
Packing shims, added timber and previous alignment corrections that have become structural by accident.
Bearers must be right before anything else
An engine can only stay aligned if what it sits on is stable. Rebuilding or renewing bearers is therefore structural work, done in the structure phase of a refit rather than at installation. Bearers should land into frames and floors, drain properly, and be built in timber suited to a permanently damp, oily environment. Bolting a modern engine onto tired bearers and then chasing alignment is a well-travelled route to recurring vibration.
Fix drainage while it is open
Standing water under an engine is the single most reliable way to destroy the surrounding timber. With the engine out, limber clearance and bilge drainage can be corrected in a couple of hours — work that is essentially impossible once the engine is back in.
The stern tube and bearing
With the shaft withdrawn, the stern tube, bearing and seal are accessible and should be assessed as a set. A worn bearing will destroy a newly aligned shaft; a tired seal will leak regardless of alignment; and a stern tube that has moved in the deadwood is a structural finding rather than a mechanical one. The timber around the tube deserves particular attention, being both permanently wet and heavily loaded.
Alignment method
- Establish the shaft lineFrom the stern tube and bearing, since those define where the shaft must run.
- Set the engine to the shaftNot the reverse. The engine is the movable component.
- Check coupling facesMeasured at multiple points around the coupling, not by eye and not by feel.
- Relaunch and waitAllow the hull to settle and take up before making any judgement about the alignment.
- Re-check afloatUnder normal load and trim, then correct as needed.
- Re-check after runningA short period of operation reveals mount settling and any residual error.
Why the afloat check is not optional
A timber hull supported on blocks is a different shape from the same hull floating. The difference is small but entirely sufficient to move a shaft line beyond tolerance. Yards that align only on the hard produce vessels that vibrate mysteriously and destroy bearings and seals on a regular cycle. Building the afloat check into the programme costs a day and prevents years of recurring cost, which is why it is written into the handover sequence described in relaunch and take-up.
Diagnosing vibration honestly
| Symptom | Common causes | Check before replacing anything |
|---|---|---|
| Vibration rising with revs | Propeller damage or imbalance, bent shaft | Propeller inspection, shaft run-out measurement |
| Vibration at specific revs only | Resonance, mount condition | Mount inspection, resonance identification |
| Vibration with heavy load | Alignment error under load, soft bearers | Afloat alignment check, bearer inspection |
| Noise and heat at the bearing | Bearing wear, insufficient lubrication flow | Bearing clearance, water flow to stern tube |
| New vibration after refit | Alignment set only on the hard | Re-check afloat once taken up |
Sequencing within the refit
Engine and shaft work sits in the systems phase, after structure is signed off and before joinery closes access. Attempting it earlier means aligning to a hull that is still being altered; attempting it later means removing new interior to get an engine through. The full phase logic is described under refit and restoration and in planning a liveaboard refit.