Italy 1870 start

[Walter]

I'm SURE the 500m is wrong

That is what the designer calculated, but calculations can be wrong. You could not pay me enough to get aboard that thing and try it out.

The times are probably for the amount of air for the crew.

Wiki does say regarding the engines: "The heat of the reaction was used to power a steam engine and the oxygen it released could be used for breathing and illumination." so with the engine running, you are increasing the time you can stay under water. 2-3 hours max would be my guess.

[KWorld]

As the mod, I've no issue with using such a vessel for test purposes.  Without better information on the engine and it's capabilities, though, it will be only a test vessel.

[Walter]

If we're going to apply the techs from the previous attempt, I would think that any submarines before the first tech level of 1900 should be 'experimental' and of no combat value.