7d • General discussion
Propeller shafts
Most inboard powered boats, both pleasure yachts and workboats, are equipped with a stainless steel propeller shaft. There are a variety of stainless steel alloys that the shaft may be made out of, including Aquamet of various numbers.
Commercial vessels and large yachts subject to class society regulations are required to remove their propeller shaft and inspect it every 5 years. This requirement forces the inspector to visually inspect both the shaft and it's bearings for wear and damage.
But, smaller yachts and many smaller work boats are not subject to this inspection requirement, leaving their owners to decide how and when they inspect the shaft.
A common occurrence with stainless steel shafts is corrosion. Yes stainless, not stain proof, it corrodes. The shaft is particularly prone to corrosion inside the stern tube areas where stagnant water sits. This water becomes oxygen depleted, and accelerates the corrosion. The shaft will develop deep and jagged pitting. This takes place in the hidden areas of the stern tube, and cannot be inspected without removing the shaft.
Below is a photo of a shaft which had too much time between inspections, and where the pitting was not properly inspected and evaluated during its last inspection. The pit propegated into a crack, and eventually the shaft broke off, forcing expensive repairs.
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Propeller shafts
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