Here is a picture of a dummy R Class battleship made from wood and canvas in Scapa Flow.
Fleet Tenders A and B were set up as two R class Battleships to give the Luftwaffe the impression ships of the Home Fleet were still safely anchored at Scapa Flow.
On May 22 in 1941, the Luftwaffe were fooled, flew home after a reconnaissance over Scapa, and reported to Admiral Lutjens in Bismarck, that Home Fleet battleships were still anchored and were not at sea.
Fleet Tender C was a merchant ship disguised as the Carrier Hermes. On her way to Chatham to be reconverted to a merchant ship, she struck a wreck, and then collected three torpedoes from German E-boats.
This ship was finally bombed to destruction by a Canadian Squadron from Coastal Command.
The old Battleship HMS Centurion was converted to a look alike KGV battleship, and spent a lot of time at Plymouth.
These dummy ships at Scapa Flow had a tendency to drag their anchors in any sort of storm, and thus became a menace to the real Home Fleet.
I understand this initiative to build dummy ships was one of Winston Churchill's brainwaves, not well known over WW2.