Why the Royal Navy K class was so dangerous

What was the worst submarine design of the First World War?

The Royal Navy K class, because its steam-turbine machinery and multiple retractable funnels made diving dangerously complex.

Earlier steam-powered submarines could work reliably with simpler machinery. The K class combined much larger steam turbines with multiple retractable funnels, creating a complicated sequence of closures before a safe dive. A failure in that sequence could defeat the whole arrangement. Nuclear submarines also use steam, but their enclosed system avoids the same funnel problem.