Boat Diesel – Alignment of the propeller shaft

Whether the engine is mated to its drive shaft by a flexible coupling or connected directly to the propeller shaft flange, it must be carefully aligned. The engine output must run both square and concentric with the mating propeller shaft flange.

Although shafts, couplings and flanges are machined to precise tolerances, perfect alignment is often difficult to achieve. Nevertheless, significant misalignment will result in excessive wear of the bearings. If the drive flange is not square the propeller shaft will vibrate, causing rapid wear of the bearings. Bent shafts and flanges should be repaired ashore.

Engine mounts isolate vibration from the hull and allow the engine position to be adjusted for accurate alignment. The top nut is loosened and the lower nut turned to raise or lower the engine.

Alignment of the engine is carried out by adjusting the engine mounting bolts for position and height, to achieve close contact on all parts of the connecting flanges. You can test for close contact by trying to insert a feeler gauge between the flanges. The flanges must also be aligned, up and down and sideways.

Perfect alignment between engine and shaft is difficult to achieve, but excessive misalignment will reduce bearing life.

Various types of shaft seal are available, some water lubricated. This Vetus system will prevent water leakage into the hull and does not rely on a messy, grease-filled stuffing box.

Tightening the flange bolts on this old-fashioned stuffing box compresses a fibre wadding and, together with a gland greaser, will prevent most leaks around the shaft. Overtightening will damage the shaft.

Adjustments are made by moving the engine up or down using the lower nuts on the extended threaded rods on top of the engine mounts. When the engine is finally aligned, tighten the top nuts and use a lock washer.

Saildrive units are self-aligning, but the mountings still need to be properly set up to spread the loads correctly over the hull.

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