Hi,

submarinehunt Wrote:
-------------------------------------------------------
> I was having a quick read of the VII manual. They
> say that the oxygen and carbondioxide absorbers
> allow 72 hours supply. I am a bit perplexed. I
> presume that in mid sea, there was no way to
> replensih oxygen or the co2 absorber chemicals. So
> doing the maths, one could then , assuming 8 hours
> submerged, would limit patrols to only 8 days or
> so ? I thought many u boat patrols were much
> longer.

You are right - there was no way to replenish oxygen and co2 absorbers while at sea.

However you must note, that period 72 hours was given as maximum, uninterrupted period of staying submerged. Such long period of staying underwater was unusual. In fact, limiting factor was amount of energy in batteries (and the amount of the compressed air). The batteries were drained much faster, than 3 days of uninterrupted cruise underwater (except the cases, when the main electric motors were not used, for instance while laying at the bottom).

Using calculation formulas given in manual (for type VIIC, but they can be easily modified for type IXC boat), you can see, that (without using oxygen and chemical absorbers), the critical level of co2 (that means 5%) is achieved after ~18 hours.

And that was most common scenario - pursued underwater U-Boat had to surface due to exhausted batteries after maximum 24 hours, so there was no need to use air renewal measures (or they were used a little). While recharging batteries, the hull was thoroughly ventilated and the air was completely replaced - and the boat could stay underwater for next ~18 hours.

As I said before, in some exceptional cases, when the boat was at the bottom, and there was a little usage of the electrical energy, the length of staying underwater depended on the amount of oxygen and co2 level, and air renewal measures were used.

--
Regards
Maciek


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