TheMarcksPlan wrote: ↑14 Jul 2020 20:30
T21's underwater endurance figures are given on Wikipedia and elsewhere.
Thanks, although the figures you quoted seem to be the aspirational design speed/endurance as per this quote from the page:
This design promised an underwater speed of 18 knots for 1.5 hours or 12 to 14 knots for 10 hours with 4000 hp = 2942 kW. A speed of 5 knots was expected for 60 hours. In the course of the planning, the boat eventually grew to a size of 1600 tons.
Whereas actually achieved speed/endurance on main motors during trials is reported as:
Thus U 3507 reached 16.8 kn for 20 minutes and 16.5 kn for 50 minutes (admiralty constant C = 175) on 30 November 1944.
It's also worth noting this phrase:
It was calculated that with 5 kn of dipped march with schleichmotoren every 24 hours 3 hours battery charge with snorkel were sufficient to keep the charge level between 60 and 90%.
Unfortunately, the endurance statistics don't tell us what the battery capacity was at the beginning of the speed/endurance trial run and at the end of the run.
TheMarcksPlan wrote: ↑14 Jul 2020 20:30
The Seraph had lower endurance because it was an older design.
Yes, it was an older design but it had gone through a large overhaul to allow it to approach the speed expected by the British of the Type XXI as described in the article I linked to:
The modifications included the uprating of the main motors from 1,400 to 1,600 shaft horsepower at full power, the fitting of coarser pitch T Class propellers that allowed the extra power to be converted to thrust, and the installation of a high capacity battery to extend her endurance. In addition, to reduce her drag Seraph was streamlined by fairing off apertures, such as, anchor holes, torpedo tubes and one-third of the free flood holes, as well as reducing the size of her hydroplanes. The forward planes were also fixed in the "out" position and given a more powerful control mechanism. The forward periscope, radar mast, Oerlikon anti-aircraft guns and the 3" deck gun were removed, and finally the profile of the conning tower was reduced.
Obviously, the Type XXI performance was impressive for the period and if the Germans had been able to deploy large numbers of Type XXI's much earlier in the war there would have been larger Allied shipping losses.
TheMarcksPlan wrote: ↑15 Jul 2020 16:59
So an escort chasing a T21 sprinting away from contact cannot use Foxer; the T21 can kill the escorts with Gnats.
Really? Did the Gnat not need to be fired in the general direction of a target? Could the Gnat be fired at the submarine's max speed and then loop back to attack a following escort?
TheMarcksPlan wrote: ↑15 Jul 2020 16:43
The idea that a 9-knot convoy can evade a 17-knot sub as well as a 7-knot sub is just facially absurd.
"facially absurd" - does that mean one of these -
If the Allies could still read German U-boat traffic then wouldn't evasive routing of a supposed 17 knot submarine in the Atlantic Ocean be just as effective as evasive routing away from a 7 knot submarine? You do realise how big the north Atlantic is and how small the detection range of the Type XXI was?
Regards
Tom