I looked for CNG bottle re-testing regulations on the 'net, and found little applying to boats, per se. I wonder if the remaining boats from the 80's with CNG are a vanishingly small % data point?
I did find this article from one vendor:
https://www.gielle.it/periodic-testing-of-gas-cylinders/#:~:text=All compressed gas cylinders have,safe and fit for purpose.
I would guess, based on conversations with divers using tanks, over the years, that "regular" recertification, whether 3 or 10 years, is a must. A troubling guess is that your insurer will Really Want to know that you are following 'best practices'.... if you can figure out what they are. Like it or not, it's probably the
insurers that hold a 'whip hand' in this situation. (As they do in so much stuff related to maintenance...)
If I had this tank question to ponder, I would check with my insurer and also a trusted surveyor.
Apropos of whatever, way way back when we were first boat shopping, I recall seeing - can't remember which - an Ericson or a Olson with a CNG tank installed under the cockpit. The CNG high pressure involved was one of several factors that made us move on, when boat shopping.
Not closely related, but still sort of relevant, we have had to have our two 5# extinguishers "hydro'd" every XX years according to the tag on the metal bottle. Of course the pressure is waaay lower, but this info is always checked on when we get them annually inspected and re-tagged.
Apologies for contributing to thread drift, but it does seem related.
