Originally Posted by RichardS
Thanks. Basically what I thought. But do you know how much noise and/or drag they added to a submarine? I would think a lot, but I never been to sea.
WW 2 era submarines were very noisy by today's standards. The machinery was not silenced in any particular way and made a lot of ambient noise. The vent slots and all that top clutter added to a submarine's underwater noise as well giving lots of things that could cause cavitation to occur as it moved along.
Add in prop noise as the propellers were not designed to avoid cavitation either.
The vents and top clutter add alot of drag as does the boat simply not being very streamlined. WW 2 boats were optimized for surface operations, their being able to submerge was more a defense mechanism than one intended for normal operations. Offensively, subs tried as much as possible to attack on the surface with submerged attacks being a less optimal alternative.
Of course, given sonar and hydrophones of the period detection on just ambient noise alone was not overly likely. Sonar was high frequency and the detection range of sounds was resultingly relatively short. Hydrophones only worked well for submarines where their being submerged and moving relatively slowly allowed them to operate. But, again, they are limited to ambient noise that is detectable by human hearing.
You can see the results of cleaning up a WW 2 era submarine in the German Type XXI or US GUPPY conversions. Their speed more than doubles underwater when given a clean hull with few penetrations and no top clutter.