I assume you have a high aspect rudder? Mine is low aspect (less leverage) plus it has a fair bit bigger shaft, hence the naval architect who designed my boat to a CE A rating stipulated the use of tube. Was your boat designed to a CE A rating? Possibly your NA ran the figures and it would have never passed a CE rating, but that does not mean mine has not.
Think of all the other boats out there whos rudders are about ready to break on old boats? The fact is I sailed this boat from the USA to Brisbane
last year with this rudder shaft (passed survey
but how much can you really tell without pulling the rudder). And even with this corrosion
it did not break. So going by that an exact replica will last another 20 years. This one has NOT failed. It may last another 20 years for all we know, but I am not going to try and prove a point as to how much structural integrity it has left. I will be miles ahead of all the other boats out there with 20 year old corroded shafts whose owners are oblivious to the damage as you were. Many people would probably just put the old one back in then sell the boat!
Do you think yours would have broken if you replaced it with the same size tube prior to the disaster? How old was it when it broke? Clearly stainless rudder shafts are a consumable item and they need to be replaced. The material degrades over time and as you found out and you can not tell if there are probs. We replace our rigging
every 10 years and it is the same material only its not immersed in stagnant sea water! This whole thing would make me very hesitant to own an old boat with an old SS spade rudder of any design.