Actual Conversion of RPM to Knots Submerged
 

In researching for a different project ran across this nice little piece of info from an Admiralty report retrieved from the British Archives:

Type VII C
RPM Speed
60 1.5 kts Dead Slow
112 3.0 kts Slow
160 4.1 kts Half Speed
200 5.23 kts 3/5 Speed
233 6.1 kts 3/4 Speed
280 7.4 kts Full

Type IX C
RPM Speed
60 Dead Slow
90 Silent
110 Slow
160 Half
180 3/4 Speed
210 7/8 kts Full

It goes on to further state that slow speed was actually around 90 RPM's.

Interesting. Could you provide any larger exerpts relating to this and the name of the document?

In that section of the report it goes into detail about the disel's and electric engines. Much to my surprise the disels' on most ships had either a turbo or supercharger added to them (if they didn't come that way from the factory). The actual report is: C.B 04051 (103) from the British Admiralty. There's a lot of other interesting stuff in there, be prepared though it's a long report.

If we're talking about u-boats, every boat starting with the Type VIIB came from the factory with superchargers. This has long been one of my arguments about upgrades in the game, because the maximum surfaced speeds listed in references are of course with superchargers, so the boats could never achieve the speeds that come with the game upgrades. The GWX folks took this into account in 2.1.

But you were talking about submerged speeds, so I apologize for the digression.