old post, but good explanation of the hard coded issues with fuel usage in SH4:
https://www.subsim.com/radioroom/sho...7&postcount=45
since SH4 is based on the SH3 code, it assumes all subs only have 2 engines like a U-boat, as opposed to the 4 engines in fleet boats, so ahead 2/3rds (8-9 knots) is the most efficient speed since the game seems to assume only one engine is running.
Problem is that it seems fleet boats usually travelled at 12-15 knots between their base and patrol areas. This was efficient since the boat was only running on 3 engines, but the game assumes the boat is running on both U-boat engines, therefore all 4 fleet boat engines.
There is no easy solution to this problem, only workarounds.