With a dive time of around 5 minutes (with the record being 3 minutes 25 seconds which was claimed by ) it allowed the captain the luxury of being able to walk around the superstructure to ensure that the funnels were securely folded. The last, improved, boat, "K26" was completed slowly, being commissioned in 1923. She had six bow torpedo tubes but retained the 18-inch beam tubes. Her higher casing almost cured the problems of seawater entering the boiler room, and improved ballast tank arrangements cut the diving time to 3 minutes 12 seconds to get to . She also had an increased maximum diving depth of .