Drivetrain Alignment & Shaft Record PDFCoupling, Runout & Stern Gear Log

A misaligned drivetrain wears the cutless bearing, the shaft seal and the gearbox output bearing, quietly, and nobody notices until something leaks or rumbles.

It is also one of the few faults on a boat that leaves a paper trail if anyone bothers to keep one. A coupling gap measured at four quadrants, dated, with the hours and the conditions beside it, tells you whether the shaft line has moved. The same measurement taken once and remembered vaguely tells you nothing at all.

This is that record. A reference page you fill in from your own documentation, then measurement sessions kept across years, with a column in every one of them for who held the gauge.

Reading a dial indicator and slipping feeler gauges into a coupling gap are within reach of a careful owner. Adjusting an alignment is not: it means jacking engine mounts against a shaft line, it is easy to make worse, and the cost of getting it wrong is a gearbox.

So this document is built to sit on both sides of that line. It records what you measured and what your yard measured, with the name in every session. Used before a lift, it tells your engineer exactly where to look. Used after, it proves what changed, and the vibration page, recorded at the same RPM before and after, lets the work be judged on something other than an invoice.

In the Fillable edition every line has a field, sized for a thumb rather than a stylus. Type the four gaps or the indicator readings; in Adobe Acrobat Reader, free on computer, iPhone and Android, both variances, every TIR, the offset, the deviation from your limit, the wear rate and the hours left to your limit calculate themselves. The Pencil & Print edition is ruled for handwriting in GoodNotes or Notability, or on a clipboard beside the shaft. Both come in A4 and US Letter and work offline.

A drivetrain can sit comfortably inside tolerance on the vertical axis and outside it on the horizontal. Averaging four coupling gaps into one number hides exactly that, which is why this record asks for all four and has you work out the top to bottom and side to side differences separately, then turn them into an angle, while the parallel offset comes from a separate reading on the flange rim, because angular misalignment and parallel offset are different faults with different causes.

The cruising boat that motors out of every harbour. Your shaft line accumulates hours in short bursts under load, in and out of gear, often astern in a marina. Sessions taken afloat, after the hull has settled, give you the trend; anything measured ashore is recorded as ashore and compared only with other ashore sessions. The preparation page matters more than you would expect here, because a hull that has been ashore a fortnight sits differently from one lifted that morning.

The racing boat with a folding prop and very few engine hours. You motor rarely, so wear is slow, but you load the hull hard and rig tension moves the structure the shaft line is bolted to. Record after a season of hard sailing rather than on a calendar, and note the rig state and tankage in the preparation page every time. That is the column that will explain a reading you cannot otherwise account for.

The motorsailer or the boat that lives under power. Cutless bearing play is your working number. Eight sessions of it, with the method named each time, turns a bearing replacement into something you schedule with a lift you were taking anyway instead of one you pay for twice.

The shared or chartered boat. The name column is the point. When four people use the boat and one of them reports a vibration, a record that says who measured, at what RPM, on what date, is the difference between a diagnosis and an argument.

The boat about to change hands. Six coupling sessions on paper, in one hand, with sources cited, answers the survey question about the drivetrain before it is asked. Very few boats can do that.

This document records, structures and compares measurements that you or your engineer take. It publishes no tolerance, no wear limit and no torque figure, and it reaches no conclusion about any installation.

Alignment adjustment is not an owner task. Nothing in this document instructs you to slacken, jack or adjust an engine mount, to move a shaft line, or to dismantle any part of a drivetrain. Where a reading falls outside the limit you transcribed, the next step belongs to a qualified marine engineer working from the builder's own procedure for your installation.

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2026 Edition, Revision 2.1, AML-MECH-2026-02. Apex Marine Labs designs the organisational process behind good seamanship. Our documents record and organise; they do not replace a certified marine survey, a service manual, or a qualified marine engineer.