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FINDINGS LIBRARY · UNDERWATER GEAR

Shaft misalignment: how surveyors spot it and how engine alignment is checked

UPDATED 10 NOVEMBER 2026 · 3 EXAMPLE FINDINGS · 2 SOURCES

What engine to shaft misalignment does to a boat, the signs a surveyor sees on the sea trial and haul-out, how alignment is checked at the coupling, the 0.001 inch per inch rule, and three example findings graded A, B and C.

AT A GLANCE
Shaft misalignment
USUAL GRADE
A for a shaft visibly whipping or a loose strut, and for engine mounts that let the engine jump under load. B for vibration, a fast-wearing cutless bearing or a shaft seal that keeps leaking, with no sign of a cause yet found. C for a recommendation to check alignment after a refit or a new engine
FOUND WITH
Watching the shaft turn at idle and cruise, feeling for vibration on the sea trial, checking the cutless bearing and strut on the haul-out, and a back-down test of the engine mounts
JUDGED AGAINST
Steve D'Antonio in Cruising World gives about 0.001 inch of gap at the coupling faces for every inch of coupling diameter. David Pascoe accepted up to 1/8 inch of shaft runout on shafts 1-1/2 inch and under
REPAIR COST
A mechanic realigning on the engine mounts afloat, more if mounts, the cutless bearing or a bent shaft need replacing. No published figure to quote

The engine, the gearbox coupling, the shaft, the shaft seal and the cutless bearing in the strut all have to line up. When they don't, the shaft bends a little on every turn. The cutless bearing wears fast, the shaft seal leaks, the boat vibrates, and in the end the gearbox or the shaft fails.

How a surveyor spots it

A surveyor doesn't usually measure alignment. The signs show up on the sea trial and the haul-out.

Under way. Vibration through the sole that rises with rpm. Watch the shaft between the seal and the strut at idle and cruise: a visible whip means something is out. The late surveyor David Pascoe accepted up to 1/8 inch of runout at idle on shafts of 1-1/2 inch and under.

Engine mounts. Pascoe's back-down test: a short, hard burst ahead and astern while someone watches the engine. If the engine or a mount moves more than 1/8 inch in any direction, the mounts have failed. See failed engine mounts.

Out of the water. A worn cutless bearing, especially worn on one side; a strut that moves when struck with a block; a galled shaft. See worn cutless bearing.

Records. A new engine, new mounts, a grounding or a refit without a record of realignment.

How alignment is checked

At the coupling, with the boat afloat. Steve D'Antonio describes the method: separate the coupling faces, check the gap with a feeler gauge, rotate the shaft half a turn and check again. His limit is about 0.001 inch for every inch of coupling face diameter, so no more than 0.005 inch on a 5 inch coupling. The engine is moved on its mounts to close the gap.

Both D'Antonio and Pascoe say final alignment is done afloat, because the hull changes shape when hauled. D'Antonio estimates that well over half the boats he sees have some misalignment. Pascoe warned that a coupling check alone can give a false sense that all is well, so the strut, shaft and mounts get checked too.

What decides the grade

A whipping shaft or a loose strut is an A, and so are mounts that let the engine jump: the shaft or gearbox can fail under way. Vibration, a fast-wearing cutless bearing and a seal that won't stop leaking point to a B. A missing record after an engine change is a C.

Shaft misalignment: example findings

The same defect written up at each grade, the way Axsail writes it from what the surveyor says on board. The grade follows what was found, so use these as wording, then grade what you see.

UNDERWATER GEARA
Starboard shaft, strut and cutless bearing

Shaft whips visibly at cruise rpm between the shaft seal and strut. Strut moves when struck with a block. Cutless bearing worn with about 3 mm of play.

Recommendation: Do not run the starboard engine above idle until the strut is refastened, the shaft checked for straightness and the cutless bearing replaced, then realign afloat.

UNDERWATER GEARB
Port shaft and engine

Vibration felt through the cockpit sole above 1,800 rpm on the sea trial. Shaft seal dripping steadily. Cutless bearing on this side more worn than starboard.

Recommendation: Have engine to shaft alignment checked and corrected afloat within 12 months, and replace the cutless bearing at the next haul-out.

UNDERWATER GEARC
Engine and shaft coupling

Engine replaced by the present owner. No record of alignment after the new engine was fitted. Shaft runs true at idle, no vibration on the sea trial.

Recommendation: Have alignment checked at the coupling afloat at the next service, and after each haul-out.

ASafety. Safety or compliance deficiency. Fix before the vessel is next underway.
B12 months. Deficiency that needs timely attention. Fix within about 12 months.
CRoutine. Note or observation, cosmetic or maintenance. Routine maintenance at the owner's convenience.

What repair involves

Realignment is a mechanic's job afloat: loosen the mounts, adjust and shim, recheck the coupling. If mounts have sagged they're replaced first. A coupling face that isn't square to the shaft goes to a shaft shop, and a bent shaft is straightened or replaced. Replace the cutless bearing at the same time if it's worn.

Photos to take

The cutless bearing and strut out of the water, the shaft seal, the engine mounts and a short video of the shaft turning. With Axsail, say the rpm where vibration started as you note it on the sea trial.

Writing it up

Say what you saw and felt, at which rpm, the state of the cutless bearing, strut, seal and mounts, and whether there's a record of alignment. Recommend a check afloat by a mechanic; a surveyor's job is to show that alignment is suspect.

Frequently asked

What are the symptoms of shaft misalignment?

Vibration and noise first. Steve D'Antonio lists premature cutless bearing wear, transmission bearing failure and shaft breakage. David Pascoe added stuffing boxes that wear and leak, and bent shafts.

What is the tolerance for engine to shaft alignment?

Steve D'Antonio in Cruising World gives a maximum of about 0.001 inch of misalignment for every inch of coupling face diameter, so no more than 0.005 inch on a 5 inch coupling. Less is better.

Should alignment be done in or out of the water?

Afloat. Both D'Antonio and Pascoe say final alignment is done with the boat in the water, because a hull changes shape when it's hauled.

Is a feeler gauge check at the coupling enough?

Not on its own, in Pascoe's view. He warned it can give a false impression that all is well, and checked the whole drive line, including shaft runout, the strut and the engine mounts.

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