 The waterline armour belt of Tegetthoff measured 280 millimetres ( 11 in ) thick between the midpoints of the fore and aft barbettes and thinned to 150 millimetres ( 5 @.@ 9 in ) further towards the bow and stern , but did not reach either the bow or the stern . It was continued to the bow by a small patch of 110 – 130 @-@ millimetre ( 4 – 5 in ) armour . The upper armour belt had a maximum thickness of 180 millimetres ( 7 @.@ 1 in ) , but it thinned to 110 millimetres ( 4 @.@ 3 in ) from the forward barbette all the way to the bow . The casemate armour was also 180 millimetres ( 7 @.@ 1 in ) thick . The sides of the main gun turrets , barbettes and main conning tower were protected by 280 millimetres ( 11 in ) of armour , except for the turret and conning tower roofs which were 60 to 150 millimetres ( 2 to 6 in ) thick . The thickness of the decks ranged from 30 to 48 millimetres ( 1 to 2 in ) in two layers . The underwater protection system consisted of the extension of the double bottom up to the lower edge of the waterline armour belt , with a thin 10 @-@ millimetre ( 0 @.@ 4 in ) plate acting as the outermost bulkhead . It was backed by a torpedo bulkhead that consisted of two layered 25 @-@ millimetre plates . The total thickness of this system was only 1 @.@ 6 metres ( 5 ft 3 in ) which made it incapable of containing a torpedo warhead detonation or mine explosion without rupturing . 
