Don,
to be strict, this is revolution counter, not RPM counter.
In my opinion, the turn-over value indicates that it was supposed to read off every 24 hour, and then the values were sum-up.
I cannot give you any schedule-periods, but engine revolutions as well as working ours were recorded in the
KTB Maschine.
Some U-Boat KTBs had attaches: radio logs, maps, torpedo reports or
engineering logs (
KTB Maschine). In the engineering logs were recorded all information related with mechanical equipment of the U-Boat (among the others data related with diesel engines: fuel consumption, working time and so on). At the uboatarchive.net website are available some engineering logs.
For example:
http://www.uboatarchive.net/U-107/KTB107-6KTBMaschine.htm | Port | Motor | | Stb | Motor | |
| Days | Hours | Min | Days | Hours | Min |
Total operating time since commissioning: | 220 | 18 | 33 | 221 | 15 | 30 |
Total operating time during the last patrol: | 37 | 7 | 37 | 37 | 22 | 15 |
Revolutions since commissioning: | 83 | 148 | 596 | 84 | 922 | 374 |
Revolutions during the last patrol: | 13 | 398 | 180 | 14 | 144 | 920 |
http://www.uboatarchive.net/U-518/KTB518-2KTBMaschine.htmstb engine
operating time since commissioning: 2603 hours 4 min
operating time during last patrol: 943 hours 56 min
revolutions since commissioning: 33,349,060
revolutions during last patrol: 11,341,510
port engine
operating time since commissioning: 2598 hours 10 min
operating time during last patrol: 960 hours 28 min
revolutions since commissioning: 36,043,720
revolutions during last patrol: 11,412,440
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Regards
Maciek