Ram Mark II manual

[Matthew Noonan]

Originally Posted by 45jim

Tolerance in cross section is not a function of location of manufacture but one of process control. Arbitrarily selection of a tolerance of 0-5% and calling it a "rule of thumb" isn't even rational as tolerance changes as surface area increases. Current thickness tolerances are about 2.5% for off the shelf rolled armour (up to 60" width) and most vehicle companies require tolerances closer to 1% (e.g. CMS 21). The standards exist for a reason, ballistic performance can be compromised by stacking tolerance in the wrong direction. Cast standards are also good, examine a cast iron engine block for example.

I wonder if you have misunderstood the notations on the cast US armour spec you elude to. Anyone who manufactures anything to a drawing makes it to the drawing (as close as possible within the confines of the process) so if it says 2" on the drawing then the final product will be 2". If there is a notation on the drawing that states "equivalent to 2" RHA" it most likely relates to the ballistic standard that that section must achieve as RHA is the standard by which all other metallic armours are compared.

The basis curve you attached has nothing to do with armour protection, it is a graphical representation of a simple mathematical calculation of cross section at various angles of attack. This is useful in calculating the reduction of weight achievable by using thinner sloped armour. You can tell this by the nice slope, penetration performance does not generate such a nice predictable curve. It quickly changes to ricochet at high angles of attack. If you were using this curve to predict armour performance you have a problem because as you approach 90 deg the thickness becomes the height of the armour sample.

Worthington knew what he was talking about regarding armour and that using NATO angles (normal angle to zero) is nice but it fails to take into account potential engagement angles in combat.

Many spec sheets from the period will list "Armour basis" you use the basis curve to find out actual, or in reverse if you know the angle of the plate and the actual thickness you could find out what it's basis would be.

The T14 assault tank for example, early spec sheets call for a (125-127 mm) frontal armour basis. The armour basis curve shows us how to find actual from that, which ends up being 50mm @ 60 deg. The actual front plate on the T14 in construction was 50mm at 60 deg. It's upper hull sides called for a 62.5 mm basis, which we know are sloped at 30 deg. Actual thickness was 50 mm. 50 mm @ 30 deg in the armour basis curve is 62.5 mm ( have seen 125 listed and 127 as basis, probably down to whoever was converting the value at the time either as 50 mm or 2 inches)

M6 heavy tank is another example of a vehicle who's exact specs are hard to find, but we do have the called for armour basis which was 127 mm frontal. So the vertical and near vertical surfaces would have to be physically at or near 127 mm, and the upper hull which was sloped at 30 deg would have to be around 101 mm to meet the called for spec.

There's a British AFV situation report update where they state the front of the Hull is 101.6 mm "Actual" as they don't list upper or lower front as location I assume it's for the majorty of the upper hull.

The Americans applied the basis curve to German vehicles as well at times. They knew the actual values, and they wanted to see the basis under their own curve.

http://i.imgur.com/9XPTyC6.jpg


The War Museum replied back to me, they are having trouble locating the files but are going to continue to look, prices are quite decent $6 for the first 20 pages, .30 cents per page after.

The National archives of Australia have some possibly interesting reports as well on the mounting of the 6 pdr in the Ram turret, along with drawings. Could be a possible source with dimension drawings at least for the mantlet and mabye the turret.