At one time I read that it is not just armor but additional internal bracing which does take volume. However, I could well be conflating something else, and that sounds more like a handwave to me, but it could help with the explanation.
You really do not have any additional bracing for vehicle armor if it is either welded or cast armor. If you have riveted armor, then you do have the metal framework that the rivets are placed in, which is essentially nil as it becomes part of the armor framework. On a large warship, you will have additional frames located behind the armor, but as there are ship frames there already to support the hull, it does not mean additional volume taken up. The frame spacing might be two feet instead of four, but you are not loosing any usable volume.
Basically, it is mixing mass and volume. Armor has a lot of mass but not that much volume for the mass. The armor on the battleship
HMS Inflexible of the Victorian Era was composed of 2 thicknesses of 12 inch wrought iron plate with 8.5 inches of teak backing each plate and all of the riveted to two 1 inch thicknesses of wrought iron plate. Wrought iron plate weighed about 40 pounds per square foot of one inch plate. The total weight of wrought iron per square foot of armor on the
Inflexible was 40 pounds times 26 inches or 1040 pounds of wrought iron per square foot along with 17 inches of teak at about 100 pounds for a square foot. Without counting the wrought iron girders reinforcing the teak and part of the backing, you are looking at 1140 pounds of armor per SQUARE FOOT of surface at the water line. That 43 inch thickness of armor has never been exceeded by any ship before or since, and represents about one-tenth of a cubic meter of armor per square foot of surface. I apologize for mixing measurement values, but it makes the math a bit easier. The one-tenth cubic meter per square feet of surface means that 135 square feet of surface would equate to one Traveller dTon of volume. That one Traveller dton of volume would have a mass of armor on the
Inflexible of 135 times 1140 pounds or 153,900 pounds, or 69.795 meter tons. How much volume would that take up under the Traveller 5 Vehicle Maker sustem?
The total weight of armor on the
HMS Inflexible of the 1870s was s 3,155 tons, which would have been long tons of 2240 pounds, heavier than the 2205 pounds equivalent to a metric ton. so 3205 metric tons of armor. Again, what would be the volume of that under the Traveller 5 Vehicle Maker system? Based on the comments by Admiral King in his report on European warships, all that armor did not significantly take up that much additional framing volume over what the ship required for adequate framing.
I will be including a cross-sectional image of an M-4 Sherman tank in my upcoming image book, and I would invite all those saying that armor requires additional framing to carefully examine the cross-section and tell me where the added framing is. I might even put in a cross-section of the framing of the
Inflexible for the same reason.
An to further add to the fun, on modern submarines, the framing is external to the interior hull, and takes up no interior volume. The US submarines in World War 2 traded a reduction in lead ballast for a thicker pressure hull with no change in framing. The thicker pressure hull of alloy steel doubled the safe diving depth for the submarines over the previously used mild steel.
