“Propeller shaft seal leak - not unheard of during shakedown/sea trials.”
Let me see ... the world has been building ships with propeller shafts that go thru the hull for roughly 150 years. It would seem to me that such technology should be very mature and not present an issue in this day and time.
The world produces a lot of stuff that has been produced a long time - very few of those manufacturing processes are 100% defect free. Processes that produce fewer parts in a given time span (such as ships, MRIs, CTs, etc) actually have a higher defect rate than those that produce a greater number of parts.
The highest end automated electronic manufacturing lines run around .01-.3% defect rates. Most manufacturing processes that are not automated and have a human component to them run at a 3-sigma first pass yield which means ~94.5% are good first time off the line and the remaining 5.5% are not.
That doesn’t include issues that are design related for which the manufacturing process is being used as a sorting process - such as one example from my past when it was far cheaper (Millions/year) to use manufacturing tests in order to sort capacitors to determine which will work in an RF tuned circuit then it was to buy capacitors pre-sorted to a very tight tolerance.
How many houses has mankind built? How many still have details that require fixing prior to being turned-over upon first sale? My point in all this is that there is no such thing as perfection when producing a product - we can strive for it, there are whole industries about how to do it better, but perfection is achieved only by God. The greater number of opportunities in a complex piece of equipment the more likely the probability that something will need corrected. In fact every ship running through final trials comes back to port with a list of things that are either mandated to be fixed prior to final turnover or nice to haves.
Ever worked on two different VINs for the same model of car? Ever notice how one is just a little different from the other? That’s not just personality - though it lends itself to our treating each one differently - it’s variance in the manufacturing process.
Yes, we’ve been doing it for more than 150 years. Doesn’t mean we’ve eliminated manufacturing variance or perfected mankind. I could go on this topic for hours it’s what I do, but I’m already sounding preachy based on what I’ve already written.