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.

You've probably had a bad day when you check your oil and it looks like this.

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I grew up sailing and playing on boats. I sailed across the Atlantic at age 12, with my family on our 36' sailboat. I raced dinghies and offshore throughout my teens and twenties. I spent my free time fishing inshore and offshore. During a university sabbatical I worked on super yachts. I got my 100 Ton captain license at 19. Through some luck I found myself working as an apprentice marine surveyor in New Orleans, inspecting tugs, barges, ships, OSV's, and cargo. I've bought and sold boats. I've broken most things on boats, and fixed them too. I've been in crap weather, dealt with after hurricane salvages, fires, sinkings, as well as tragic marine accidents involving fatalities. While I haven't done or seen everything, I have done and seen a lot. Let's talk, yachts, tugs, fishing, repairs or marine insurance.