42 foot steel round bilge sailboat scantlings

That is good news, re how most of the shell thickness is still 3 mm.
Regarding 'quite a few' localised bad areas and holes, how many approximately would this be for each?
Re the holes, are you planning on welding them up individually?
And approx how large would your largest bad area be, in terms of cutting it out and welding in a new plate?
Re the location of these areas, are they totally random, or do you see a pattern, or any localised concentrations of holes?
Are they mostly in the bilges, or in the topsides as well?
Photos #4 and #6 each show at least four holes / deep pits - do the photos show all the holes / pits in these areas, or are they others not too far away?

 

I would say these are representative holes. There are many more over the hull.
All holes are below the waterline.

not sure about what areas to cut out vs just fill. I understand there are USCG recommendations about not having welds within 100mm of each other. Not sure if that pertains to weld fill holes or more for plate edges.

my plan is to fill as many lone holes as possible, and cut out areas where there are higher density of holes per square meter.

 

In my opinion you should remove the rust before checking the thickness with ultrasound. Measurement, with rust, is misleading.
If the ship is to be inspected by an official entity, you should find out what the inspector's judgment of that entity is. It may not be the same as the one that we, sitting in an armchair, thousands of kilometers from the ship, can have.
Think about renewing plates rather than filling holes, that is the technique that Classification Societies usually require. Keep in mind, also, that for many years now the CS has been asking for an increase in additional thickness, to prevent corrosion, which was not required in the years when the ship was built. This means that, even if the ship maintained the regulatory thicknesses in its day, they would not be acceptable today.

 

Ultrasound measurements reported were on clean, bright metal.

 

You might find the book, Elements of Boat Strength by Dave Gerr useful, it has the information on scantlings you seek as well as information on how to frame, differences between typical steel (A36) and Corten, how to insulate etc. Remember the lead will react with the steel, so you will need to isolate the two metals for the long term, IE for ever, if you place lead inside the steel hull at the bottom.

 

Hello,. I know that what I'm about to say is not what you asked about. But, I remember a very nice little steel hulled sloop (A graduation present when the owner graduated from Johns Hopkins Medical School) which after forty or so years was showing a lot of skin corrosion. The owner sold the boat and was never pleased with the similar sized brand new FRP sloop she bought. In the meantime, someone bought the steel sloop, and after proper preparation, basically reskinned the underbody with fiberglass. I think it worked well.

Cheers,
John