Mr Duck, I am glad that you have had success with your methodology and it is obvious that you stay on top of maintenance on your boat. I certainly didn’t mean to pick on your post although it was the most convenient one to address some issues I saw in several posts in this thread.
Let me just comment on a couple of your observations for the benefit of others who are interested in steel hulls and might not be so hand-on as you are and will rely on shipyards to do their maintenance. Also, this might be handy for those interested in purchasing a steel hull.
On a large doubler plate, plug welds may indeed provide more adhesion of the doubler to the hull plate. But it still will not constitute a permanent repair with any (and I mean not a single one) authority in cases where there would be a need for inspection. So, if the same boat was used in charter, the doubler repair, if found, would be a reason to revoke a certificate of inspection if it had not been replaced with an insert within a year of installation. So the question should be, would you repair a boat to a lesser standard? If so, what would the standard be? Who sets that standard? If the hull was any other material, would a repair to a lesser standard than ‘as original’ be tolerated? If no, then why tolerated for steel? There is no reason other than it is ‘more convenient’.
I have seen corrosion continue after being doubled. There is such a a thing as anaerobic corrosion, especially in the presence of stray electric current. With a doubler, how would you know? It would be hidden behind the doubler, out of view. That’s why there is a time limit on how long a doubler temporary repair can be approved.
I don’t think commercial requirements are any more stringent than for non commercial. They are just published for all to see and they set a minimum standard for shipyard repair and they are enforced. Further, I don’t believe that steel hull pleasure boat hull thickness is much thicker than necessary for structural integrity. Mostly I see 1/4” bottom plate on vessels under 60’. This is significantly less strong that say 2” of well laid FRP. Further, pitting is usually not readily visible until it is about 1/8” deep and by that time it has eaten into 50% of the original scantling. Remember, the standard is 25% (1/16”) loss of thickness before the plate should be replaced, not perforated through.
If a steel boat were to go aground and become damaged, the insurance company would be required to replace the damaged steel, not fit doubler plates over it. If they tried that, owners would howl in outrage. So why should corrosion damage be treated otherwise?
Finally, remember that steel can corrode from both sides. The only prevention is to be well coated on a properly prepared surface. Paint on top of rust does nothing. Corrosion, where found needs to be arrested, dug out to the root, coated with appropriate primer for the environment, either internal or external, and finished with a tough topcoat.
There is nothing to be afraid of on a steel hulled boat and it’s main advantage in my opinion is the ability to be repaired in remote environments. If I had to have a boat for world traveling and it was going to be greater than say about 75’, I would certainly consider steel as this was my world for 40 years. But my choice for a 52’ coastal cruiser was fiberglass. It just made more sense to me. Plenty others have different opinions and that’s a good thing.