Polyurea coating on the outside of a steel ship hull

[westernman]

Where this may be the case, it can only be due to one or more of the following - poor performing older type coatings, the wrong choice of coating, poor/inadequate preparation of the steel or inadequate application of the two pack epoxy coatings. This could be lack of coverage, thickness or number of coats.
The conditions that the exterior of the hull on a steel boat are subjected to, are far more arduous than those experienced by the inside. So with correctly applied modern epoxy coatings, the logic would be that the inside corrosion protection should perform at least as well as the exterior. Probably better, because it won’t be subjected to mechanical damage!
The other big factor in preventing corrosion on a steel boat, is the design and methodology adopted during construction. Think of the mild steel hull and superstructure being a piece of steel which is totally encapsulated in a plastic coating of even thickness throughout. If that protective skin is penetrated anywhere, you invite the opportunity for a corrosion cell to commence.
So - no square edges to finished steel in order to maintain an even thickness of epoxy , no tapped holes for fixings, where holes for through bolts can’t be avoided - they should be drilled oversized (e.g. 10mm for a 6mm bolt) all edges epoxy coated during the painting process and a plastic shouldered washer used to prevent the thread of the bolt from damaging the epoxy.

There’s a channel on YouTube (Saving Blue Steel) worth having a look at. This is the story of a couple attempting to save a very corroded steel 14m motor cruiser which was originally professionally built by a yard in the Netherlands. A good pedigree you would think, but wow! It’s seems to be a good example of how not to build a steel boat! Still has its original teak deck which is fixed to the hull with literally hundreds and hundreds of set screws drilled and tapped into the steel deck! All the deck fittings seem to be attached in the same way too!

The trouble is that the internal coating is very often poorly done by the original builder.
Out of sight, out of mind until it bubbles through to the outside.

 

[ducked]

The trouble is that the internal coating is very often poorly done by the original builder.
Out of sight, out of mind until it bubbles through to the outside.

Perhaps one might usefully recognise that imperfection is likely, and try and minimise the amount of destruction required to access the inside of the hull for inspection and remediation.

Easier said than done, of course, but I'd bet it is often neither said nor done.

 

[[194497]]

Steel is a really great material for building boats, but once they're built, it's a pain in the arse to live with... The trouble is that you want ready access to the whole of the inside of the hull for inspection and maintenance, but you also want a good thickness of insulation and a vapour barrier on the warm side, and ideally the insulation would be fixed without using mechanical fasteners onto steel, so that you're not scraping off paint each time you unbolt or unscrew them. It's possible, but it needs a lot of very careful detailing to be done just right.