The data file for "Large Modern Tanker" incorrectly sets its speed to 12 knots max. This is a bug, because all the game's documentation states that this ship type can do 20 knots.
The best fix for Large Modern Tanker is simply changing the data file to make it 20kts (which without testing, I am guessing is at Data\Sea\NOL_Nippon\NOL_Nippon.cfg).
Cloning it just for a speed bump is overkill, and besides, the Kawasaki style 1930's and wartime 1TL 10k ton tankers, which are what the Large Modern tanker is supposed to represent, are documented in the histories as capable of 18-20kts....so that even if you did a clone just for the speed, you'd still need to go back and fix the Large Modern Tanker.
I think you should do this up in a JSME package along with a minesweeper fix for all of us to enjoy
