It's important to get a competent surveyor, who's credible by a third party. Had a friend who bought a 50 ft. older steel power cruiser, manufactured by a very credible German yard. Upgraded by a wealthy owner. Looked great. Still they lost thier house, savings and more on hull repairs... ended up giving up on the project. Most surveyors survey what's accessible. Not good enough for steel. Got to get into the tanks and all hull areas. Putting a plate over a rusted hull section is only acceptable as an emergency repair as the corrosion continues underneath. Trust but verify.
You are 100% correct in your assertion that there is no way to visually inspect a steel hull and certify it as to its integrity. When I had the oil field pipe business we had a machine that cost well over a million dollars that was one of the only ways to inspect steel pipe correctly. This machine is called an EMI and is basically an MRI for pipe, it uses several large magnetic coils to generate a field which can "see" through the pipe and detect microscopic defects and flaws in the pipe. This inspection is done on almost all oilfield casing and tubing, in an effort to prevent "blowouts" and reveals the flaws in the pipe. It does a great job, however there is not a good way to run an EMI on a hull, you cannot see the microfractures that come from stress on the material.
There are surveyors who use ultrasound to do hull inspections on steel boats. This is the only way to really get a good idea of the hull integrity, and it is costly. If you are purchasing a used mega-yacht the cost is not prohibitive, if you are purchasing a used sailboat under 40 feet it may be more than most would be willing to spend on a survey, and it takes up to two days to do it. There are not many people here in the states who can do an ultrasound hull survey, so that makes it even more fun.
The stress of an impact on the keel seam of a vessel weighing in at somewhere around 30,000 pounds is going to so far exceed the tensile strength of the material at the point of impact as to be able to cause incredible damage and still not be visible. The damage will be done at the molecular level, and this is something you cannot see or inspect on a survey, but it sure can come back to haunt you.
BS said that the steel he used had some huge million plus pound tensile strength, that is not true. I do not know how anyone can state that and even expect us to believe it, because there is not a single type of steel with a tensile strength that high. This is not an assumption, it is a fact.
If you use A36 steel, 3/16" thickness you have an estimated tensile strength of 36,000 pounds, that is what the 36 represents. Layering it to four layers does not increase the tensile strength at all, it does raise the amount of force needed to penetrate the hull, but not to 1,800,000 psi times four, as asserted by BS. I have no idea how BS came up with that number for tensile strength, but unless he can show me a metallurgical test for the specific hull plating used that gives a tensile strength of 1,800,000 psi I am going to have to say he has had a mathematical error somewhere in his calculations.
To start with 3/16" is too thin for a hull, especially at the keel, I would think that the keel would want to be done in about 3/4 inch steel plate, which is more like what BS is talking about with his four layers of 3/16 inch steel. The thing is that layering is good, but you still only get 36,000 psi you just get four layers of it. I have not done the math yet, but if you have a boat that weighs 36,000 pounds, which would not be at all out of line with a steel boat, I am guessing that when you factor in the speed and angle of the impact you get a good deal more than 36,000 psi, so the best thing to do with a steel hull remains the same as with any other hull....don't hit stuff. Now if Brent has some 1.8 million psi 3/16 inch steel somewhere he needs to start building pressure vessels and tanks with it, and maybe the military might like to have some too.
A36 is a standard low carbon steel, without advanced alloying.
As with most steels, A36 has a density of 7,800 kg/m3 (0.28 lb/cu in). Young's modulus for A36 steel is 200 GPa (29,000,000 psi). A36 steel has a Poisson's ratio of 0.260, and a shear modulus of 79.3 GPa (11,500,000 psi).
A36 steel in plates, bars, and shapes with a thickness of less than 8 in (203 mm) has a minimum yield strength of 36,000 psi (250 MPa) and ultimate tensile strength of 58,000–80,000 psi
(400–550 MPa). Plates thicker than 8 in have a 32,000 psi (220 MPa) yield strength and the same ultimate tensile strength.
A36 bars and shapes maintain their ultimate strength up to 650°F. Afterward, the minimum strength drops off from 58,000 psi: 54,000 psi at 700°F; 45,000 psi at 750°F; 37,000 psi at 800°F. A36 steel has low carbon, that produce high strength to the alloy
Tensile strength and shear strength are not the same thing, and not only that, an impact exerts both tensile (pulling) and shear (cutting or tearing) forces on the point of impact and combined they will poke a hole in your boat. The hardness of the object impacted, the speed of the impact, the weight of the impacting object, and the angle of the impact along with some other factors are what determine the ultimate force of the impact, but steel, wood, concrete, fiberglass or a combination of all the above will not be enough to stop a sea container that fell off a ship from knocking a great big gaping sink your boat hole in the hull of a sailboat if you hit it at the right angle. Sailing a boat is not something that can ever be made 100% safe, there are risks involved, but the rewards far outweigh the risks, if I die while sailing I will have died doing something I enjoyed. If I die while sailing on a steel boat or a wooden boat I will still be dead.