What was the cause of most early steamboat boiler explosions?

The primary cause was simple ignorance. Designers did not know the tensile, compressive or shear strengths of metals. Operating engineers did not know the effects of scaling, mud, etc. Feedwater pumps were connected to propelling engine crossheads or valve gear and there was no way to inject feedwater when the engines were stopped. Steam pressure gauges were either bad or non-existent. Safety valves could be overloaded by engineers who did not know what pressure they were carrying in any event. If an accident stopped the engines the boilers went on making more steam.

But mainly it was ignorance. Ignorance of the forces with which they were dealing, ignorance of the strength of materials and ignorance of the necessity for cleanliness. Manufacturers did not know how to consistently make iron, copper or steel uniformly strong.

Greed also played a part. Owners too tightfisted or too poor to pay for good equipment and competent help asked for trouble.

Probably all of the above. Believe it of not, the physics and mechanics of why boilers exploded was not even understood in the very early days. There were no materials testing or standards, and no understanding of how to design a boiler to resist a given pressure. It was all emperical, by eye engineering. The iron steam piping that was recovered from the 1832 steamer HEROINE has a wall thickness of almost an inch: since they hadn't the knowledge to make consistant material or to design it, they compensated by making it heavy. Or they made it light and cheap and took their chances. Pressure gauges didn't come into widespread use until after the 1870s, nor did any means of determining how much water was in the boilers. Before the "doctor" was invented, the feed water pumps ran off the pitmans or the wheel shafts, so if the wheels weren't turning, there was no boiler feed. Boilers were known to be so flimsy that you could see them "breathe"- expand and contract with the stroke of the engines, or so leaky at the seams that the engineers drove cypress shingles into the joints to stop the leaks. Most of the early boats had no cutoff, so they ran full pressure all the time. To get more power for more speed, or to drive over a bar, they held down the relief valve to build more pressure. (Can you imagine driving a flexible hulled boat with brittle cast iron steam piping full speed into a gravel bar in an attempt to get over it? It was standard proceedure.) There was a reason that the stop or hook on end of the relief valve arm was known as the "death hook".

I understand that with the institutiion of Federal regulation, as opposed to state regulations, which could easily be circumvented, at least the operational aspect of the steamboat was brought under control. Hence, many fewer explosions. The technical aspect was probably steadily improving at the same time, hence fewer explosions. Does anybody know when the "code" for boiler construction came into being?

Probably Capt. Bates does. I think it was part of the 1853 regulation act, but I'm not positive. Of course it took a number of years before they were able to make that act really effective in practice.

Louis C. Hunter's book says that Congress started trying to regulate steamboats in 1832, but that the act of 1852 was the first effective regulation. That act did include hydrostatic testing of boilers, maximum pressures allowed and inspection of boiler plate at the point of manufacture. Examination and licensing of engineers was also provided for, as was adeguate administration for enforcement of the act. The steamboat industry resisted it's passage.

The Steamboat Act of 1838 was badly flawed and was unenforceable. The Steamboat Act of 1853 specified shell thicknesses and safety appliances and required licensed engineers. This law had teeth in the form of prison penalties, fines and losses of license and provided for inspectors to enforce the rules - for passenger vessels only. A few disasters when uninspected boats exploded near passenger vessels brought on additions to the laws and they were eventually applied to all. At first the laws applied only to pilots and engineers, but were soon extended to mates and masters. As usual, there was a lot of resistance to the regulations.

In 1845 many steamboat engineers and others in the business in the Cincinnati area requested to congress for a better law regarding steamboat boilers. There are about 200 names listed on this document. The reference is House Doc 25, serial set 482. Dec 11, 1845. They range from the mayor to a foundry owner to Engineers and Pilots.