@david_megginson I've had 30 years gap between the first time I read it and now. Even better this time around, I'd say.
Cannot judge the tech aspect. It is not relevant to the story, imho.
Well, it is the writer's skill which makes us cheer for an u-boot crew. Many a book hero was in fact a bad guy.
It is interesting how both the book and film are popular with military/war fans (and with some right-wing elements too, perhaps), despite the author having always considered his work "anti war."