“If it be a sin to covet honour, I am the most offending soul alive,” he tells me over breakfast, quoting Henry V. “Faint hearts never fucked a pig” is another of his maxims, which may be a translation from Clausewitz’s On War.

We go through at sunrise and Chief Johnson, who is up on the bridge, tells me it’s the first time he has sailed past Stromboli since he was on the frigate HMS Leander at the start of his naval career 30 years ago.

Climbing the ladders between decks exhausts me; I am forever hitting my head on protruding bits of metal; and once in the control room, while leaning against the periscope, I stumble backwards and accidentally press a button.

Even on a fast-attack sub, if there is nothing to attack and you grow tired of listening to passing whales and pretending to target nearby destroyers, life can get repetitive, so the men slip into other worlds.

Chief Petty Officer Paul “Jakie” Foran, the likable but occasionally terrifying Scot who oversees these tests, expects dedication, and woe betide any trainee (AKA oxygen thief) who is discovered having a cup of tea in the junior rates’ mess when he could be unearthing the secret of the magazine spray drench system.

A few men go “wibble” after years under water; they just can’t stand it any more – the lack of proper sleep, the absence of privacy, the endlessly repeated conversations, the cycle of meals (it’s Wednesday so it must be curry), the unspoken dangers.

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