[TorpX]

No, the variables are the same. Any ship trial is conducted with as light a load as possible, and as much power as possible, the goal being to make the ship look as good as possible to the potential buyer (in this case of course the navy). In the case of a submarine it would require the batteries be fully charged of course, and the engines are pushed absolutely as fast as they'll go. With that in mind I would insist that 20.25 knots was the best speed on the best day, maybe getting to 21 if you're lucky.

You may be right, but it is still hard for me to believe they would push engines far past their limits in peacetime trials. Or maybe the boats became lighter during refits?

...claimed to have made 23.5 knots using "150% overload", however that works with a diesel engine, but was officially credited with a world speed record for it.

I don't know enough about this sort of thing to even guess. 150% does sound incredible.

You talked about the accuracy during an attack, but an attack takes place at 2 knots, not 20. Are the instruments accurate at higher speeds?

I skimmed through the manual for the Bendix log. I didn't see any standard for accuracy, but they did calibrate them at 2 speeds minimum (low and high) and at as many as possible, time permitting. They had to be good not just during a submerged attack, but throughout an approach, surfaced or submerged. The maintainace/ calibration proceedures were very involved. They were obviously considered important.

And that's the crux of the matter. On the one hand anecdotal evidence is always suspect, especially given possible variations in measuring equipment. On the other hand anecdotal evidence is vital because sometimes it really is true.

Agree.

So it's an impasse. But I will leave you with one question: If it's realistic to allow the special ability to boost the speed that much, is it still realistic to not have any chance of an engine breakdown at the worst possible moment? To my mind you can't have one without the other and still claim realism.

I agree completely. The magic-engines that never break down is a weak point in the game. So is the rocket-like acceleration of everything. What is the speed advantage of the special abilities anyway? I had assumed it was a mod-dependent thing. I've never had it in the game, so I never gave it much thought before.