Underway Replenishment doesn't seem to work correctly, unless I'm completely misunderstanding how it's supposed to work.
Assuming it's supposed to be the % of stationary refueling speed a tanker is capable of during transit, it's currently way to low.
Ex: Tug I is a fleet with a tanker and Tug moving together. Tanker has "Refueling Capability: 60 000 litres per hour" and I got "Underway Replenishment 30%".
After about one month of flying they've covered 2.6b km, and the Tug range has gone down about 2.4b km. So it has been refueled a bit.
But the tanker should have been able to fully refuel the tug several times over during this period, as even at 30% speed refueling should't take more then 10 days or so.
Tanker was probably build before Underway Replenishment was researched if that matters.
Tanker is in a sub-fleet in the Tugs fleet, set to "Refuel own fleet". And it is working somewhat, only way slow.
It's a 9.1/9.2 (not 100% sure) game continued in 9.3