About this detail of the Tiger
The Tiger's gun produced fumes when the breech was opened after firing. To clear the air in the turret, a ventilator fan was put in the turret roof. According to the Turret Manual, this fan was rated at 12 cubic meters of air per minute.
In the first kind of turret, the ventilator was in the rear right of the roof. This photo, taken by Kenneth Tyler, shows the Tiger in Bovington museum.
This is the precise position of the fan on the turret roof.
There was a protective 15mm armour plate on top. It was supported on six washers above a round housing.
Six flat-ended bolts ran through the plate, the washers, the housing and the hull roof. They were nutted inside the roof, the nuts holding a safety cover. This photo inside of the Bovington vehicle. According to the Turret Manual there were two kinds of internal cover for the fan; a cast one, and a sheet-metal one. This is the sheet-metal cover. The original power switch is visible, mounted on the roof.
This diagram shows the approximate dimensions of the external parts. The housing is dished and so it has small drain holes for rainwater. I don't know how many drain holes it had; there were at least two, probably six.
A loose metal cap was provided to seal the fan opening so that the tank could drive underwater. This cap was NOT a rain guard or an armour cover. The cap was locked in place with six hooks tightened by wing nuts. It had a rubber ring underneath to form a watertight seal.
When the cap was on, the fan did not work. Many modern drawings and models depict the tank with this cap in place. But it is rare to see the cap in photographs of tanks in service. As far as I can tell, the cap was in place when the vehicle was delivered from the factory. After removing and replacing it a few times, the crew would figure out that they didn't need it (perhaps by reading the manual!) They would probably then stow it in the turret bin.
[2] Survey of Tiger 250122, at Bovington museum, by David Byrden
[3] Survey of Tiger 250122, at Bovington, by Hilary Louis Doyle