Hi Terry,
Found a couple of snippets
"Displacing 280 tonnes,the Transport Ferry Type D was built in quantity between 1942-44. In later years artillery ferries, the famous F-lighters, were converted and built from the standard ferry design."
Got a picture in another book showing a plain barge, apparently twin engined - speed given as 10-12 knots. Armament 2 - 4 x 88mm flak guns plus a 20mm 4 gun flakvierling installed on tower in centre. The pic also shows the barge carrying several vehicles.
Scanner sulking at the moment so cant include pic.
Hope this is some help
Regards
longshanks