Prop shaft Alignment

Poor propeller shaft alignment can lead to excessive shaft drag as well as accelerated bearing and shaft wear.

- Steve D’ Antonio

Nearly all marine engines are designed to be adjustable, to a point, relative to the propeller shaft by using adjustment or jacking nuts on their motor mounts. The adjustment should be thought of as no more than finetuning, the maximum range of vertical travel being limited to no more than an inch or two at the most, which is substantial when one considers that alignment is typically measured to just a few one-thousandths of an inch. Thus, the responsibility rests with the naval architect and boatbuilder to ensure that the design and execution of the engine installation guarantees near-perfect alignment with the shaft before the motor mounts are adjusted.

The other alignment

There is another side to the alignment story; it involves the support and position of the propeller shaft relative to its bearings, the shaft log (the tube through which the shaft passes) and the engine. While engine alignment is, or is expected to be, well understood by most marine industry professionals, shaft alignment is, on the other hand, far more esoteric and well understood by far fewer in the industry. In my experience, precious few truly understand its importance and the consequences of misalignment, and fewer still understand the techniques and processes involved in making adjustments and corrections.

Contrary to popular belief, shaft misalignment rarely leads to vibration, as the bow or offset induced in a shaft by misaligned bearings is constant. Improper shaft alignment can lead to excessive shaft drag and a resultant increase in fuel consumption, as well as accelerated bearing and shaft wear.