Used in any era from Ancient Roman onwards..there´s a few still being operated (newly built ones that is..not ones from the ancient romans)
Quotefrom Wiki ;
"A ship mill is a type of watermill. The milling and grinding technology and the drive (waterwheel) are built on a floating platform on this type of mill.
Its first recorded use dates back to mid-6th century AD Italy"
As with the Horse-Powered Ferryboat, These Mills come (came) in a wide range of designs which gives some freedom in creating my own Version
As far as I can tell, this type of Mill would be "parked" further up stream from where it was to be used and when the grain was loaded for Milling, it would be set adrift (using ropes tied to the bank so it didn´t drift away entirely) until it reached a Point where the river flow was strongest and the wheel would be more efficient. When the grain was milled, it would be pulled back to the riverbank or serviced by boats.
Others seemed to have been attached to Long poles and pushed out into Position, to ropes stretched across the river from one bank to the other and pulled out into Position or permanently attached to some sort of ramp/Dockside
Medieval Ship mills in a painting from 1411, The Martyrdom of St Ursula at Cologne
For a really good write up on Boat mills, Hanging Mills, Bridge mills and all thier varients go HERE. In fact..if your interested in weird and wonderfull machines and "forgotten" Technology, the site is well worth a browse..I mean..who wants to miss out on learning about windpowered chinese wheelbarrows..? :-D
As ever, it´s all scrap material, mostly Card with bits of insulation foam and paperclips, heads of Pins for bolts etc.
Obviously this will end up in Bodstonia...and it´s shown in "Action" HERE