Gearboxes, sterndrives and shafts
ZF and Twin Disc gearboxes, sterndrive bellows, stern glands, shafts and alignment.
How often should engine alignment be checked?
After any engine work, grounding, or mount replacement, and periodically on older boats as mounts settle. It is cheap relative to the wear it prevents.
Full guide: Engine Alignment and Coupling
Can alignment be done on the hard?
It should not be finalised there. Hull shape changes between the hard stand and the water, so alignment is set afloat once the boat has settled.
What are the symptoms of misalignment?
Vibration, a shaft seal that will not stop leaking, a cutless bearing wearing quickly, or repeated gearbox output seal failure.
My boat vibrates — is it the propeller or the alignment?
Either, and it needs checking rather than guessing. A damaged or unbalanced prop and a misaligned shaft feel similar from the helm.
Do engine mounts need replacing at the same time?
If they are old, sagged or hardened, yes. Aligning on tired mounts gives a result that does not last.
My gearbox slips — does it need a rebuild?
Check the oil level, the oil type, the cooler and the cable adjustment first. Partial engagement from a badly adjusted cable is a common cause and a cheap fix, though left long enough it burns the clutches and then it does need a rebuild.
Full guide: Marine Gearbox and Transmission Service
What oil does a marine gearbox take?
It depends on the box — some take engine oil, some ATF. Using the wrong one causes slipping. Check the plate or the manual rather than assuming.
How often should gearbox oil be changed?
On the manufacturer's interval, typically alongside an engine service. It is the most commonly forgotten item on the drivetrain.
There's a rattle at idle that stops in gear — is the gearbox failing?
That is the classic failed damper plate symptom, not a gearbox fault — though reaching it usually means separating the box from the engine.
Why is my gearbox overheating?
Most often a blocked raw-water oil cooler, low oil, or slipping clutches generating heat. All three should be checked together.
How do I know if the cutless bearing is worn?
With the boat hauled, try to move the shaft vertically in the bearing. Perceptible play means it is due.
Full guide: Propeller Shaft and Cutless Bearing Service
Is my vibration the propeller or the bearing?
Either, plus alignment or a bent shaft. All four are checked at the same haul-out, which is why they are done together.
How long does a cutless bearing last?
Highly variable — years in clean water with good alignment, far less with sand, silt or misalignment.
Can the shaft be reused if it is scored?
Sometimes, depending on where and how deep. Scoring at the gland from an over-tightened stuffing box is common and may be recoverable.
Should I replace the shaft anode every haul-out?
Yes, or whenever it is around half consumed.
Should my stern gland drip?
A traditional packing gland should — a few drips per minute under way. A dripless seal should not, and any water from one needs investigating.
Full guide: Shaft Seal and Stern Gland Service
How often should a dripless seal bellows be replaced?
On the manufacturer's interval, typically measured in years. It is rubber under compression below the waterline, and a failure can sink a boat.
My stuffing box is hot — is that normal?
No. It means it is over-tightened and the packing is not being cooled. Back it off; a scored shaft is an expensive consequence.
Can I repack a stuffing box myself?
Many owners do, at haul-out. Doing it afloat is possible on some installations but is not a beginner's job.
Why does my seal squeal after launching?
Usually trapped air. Many dripless seals need venting after launch, or they run dry.
How often should sterndrive bellows be replaced?
On the manufacturer's interval — commonly a few years — regardless of hours. They fail from age and UV, not use, and a split drive bellows lets water into the boat.
Full guide: Sterndrive and Outdrive Service
My drive oil is milky — what does that mean?
Water has entered past a seal. Act promptly; seawater in a gear case destroys bearings and gears quickly.
Can sterndrive work be done in the water?
Some inspection, but proper servicing needs the boat hauled and the drive removed.
Are sterndrives higher maintenance than shaft drives?
Yes, meaningfully. More wear items, more anodes, and more of them below the waterline. The trade-off is efficiency and packaging.
What does a rumbling noise from the transom mean?
Commonly a failing gimbal bearing, usually the consequence of a bellows that let water in. Have it checked before it takes the U-joints and shaft with it.