Greetings Denis
the Top Speed of the Coromuel, which is the ferry the Albatross IV is based on has a top speed of 16 knots
http://www.faktaomfartyg.nu/coromuel_1973.htm
Other Papenburg sisters as you say have a higher top speed, and I have asked for another version on the Channel Ferries livery - see the White Marlin (possibly also with the windows on the bow, but that might be too much as it would require a rebuild of the model), with that version having a higher top speed
See Viking IV, later Earl Granville
http://www.faktaomfartyg.nu/viking_4_1973.htm
19.5 knots
http://www.shipspotting.com/gallery/photo.php?lid=1220605
Leaving Portsmouth!