I noticed in the FAI Rules 7.4.8, that Fixed Tubes can be reloaded in 2 Game Turns per Tube. As Turns are 3 Minutes, this would suggest a Tube can be reloaded in 6 Minutes. This is at odds with the correct time which was a good 30 Minutes to reload each Tube, and this was in WW II U-boats! Source: "U-boats Under the Swastika," by Jak P Malmann Showell, Ian Allan, Shepperton 1998, page 90.
No doubt the WW I submarines would have taken at least as long. So a Boat having fired her 4 Tubes would take around 2 Hours, in real time, to reload 4 Tubes.
A surface Ship with fixed Tubes, having much more space, would likely have had the equipment to do it in less time. I would be interested in a source for that if anyone knows of one.