Shell hits v armour

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paul.mercer
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Shell hits v armour

Post by paul.mercer » Mon Feb 25, 2019 5:35 pm

Gentlemen,
It seems that whenever there are discussions about the of merits of one ship over another in a battle it will inevitably turn to points over shell size/velocity/angle of impact/penetration/strength of armour etc. and tests made by various Navies of the effect of fire from large calibre guns against armour plate. What puzzles me as a complete amateur on these subjects is this, I have always presumed (rightly or wrongly) that as there would be extreme difficulty in hitting a single piece of armoured plate from normal battle ranges (say 10 miles or so), any tests would have been at fairly close range and at a fairly flat trajectory and the results would be correlated and then used for calculations as to what would happen if the shell hit that thickness of armour at long range.
What I cannot understand is how anyone can possibly calculate what a shell will actually do when fired from ship to ship, if for instance the firing ship is maneuvering or rolling and pitching in a heavy sea, surely the angle or trajectory that the shell leaves the barrel will be different to that it might have been when the trigger is pulled. Likewise, when the shell hits the other ship its angle and place of impact will depend entirely on what angle that ship will be when it is also maneuvering and rolling around in the same heavy sea, therefore whether it penetrates the armour or not is to some extent matter of luck and all the calculations made during tests are actually theoretical. Am I correct or talking nonsense?
If so, please don’t shoot me to pieces!

RobertsonN
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Re: Shell hits v armour

Post by RobertsonN » Mon Feb 25, 2019 7:55 pm

I think you are correct in saying that chance could play a big role in naval engagements. The form of the hull, pitch, roll and yaw all affect the overall angle of incidence. The striking velocity will depend on the nominal muzzle velocity, state of wear of the barrel, charge temperature, relative velocities of the firing ship and the target, air density, winds, among other factors. A lot depends on what point of a ship is struck. Some hits are relatively harmless while others may cause crucial damage, to fire control systems, for example. Another factor is hull structure, in addition to armor, such as beams and pillars or equipment, which may slow down a shell, or deflect a slow moving shell in a direction that is more or less damaging. In addition, shells striking decks at very oblique angles may develop increasing yaw (nose up and not in line with the shell trajectory), which makes further penetration more difficult.

You allude to another point, namely whether the motion of the armor plate during the penetration process makes a difference from the same shell striking a stationary plate. Plates in acceptance tests and tests to determine whether a given ship's protection is adequate against a given shell at specific ranges were naturally always stationary. Where a striking shell might be travelling at 1500 fps and the target ship at 27 knots (about 45 fps), the latter is 3% of the former. Whether this is enough to make a difference I do not know. Maybe this uncertainty was so small in relation to all the other uncertainties mentioned in the previous paragraph that it was just ignored,

Neil Robertson

paul.mercer
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Re: Shell hits v armour

Post by paul.mercer » Wed Feb 27, 2019 10:29 am

Hi Neil,
Many thanks for your detailed reply. Would it be safe to say that in fact all the arguments and mathematics put forward re shell v armour when debating the merits of one ship against another are based on probability, but are purely theoretical as no-one can reliably say what happens when a shell hits a ship due to its movement at the time of impact and that tests against a piece of armour plate provide useful but not definitive data, therefore whether two ships of comparative size and power win or lose in a fight is almost a matter of luck?

RobertsonN
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Re: Shell hits v armour

Post by RobertsonN » Wed Feb 27, 2019 8:05 pm

Paul,

I do not know if target motion at the time of impact was a significant factor in armor penetration, but this question seems to have escaped detailed analysis up to now.

Often, tthe advantage of light was decisive, such as at Colonel. This advantage involved chance, that of the relative locations of the two forces when they sighted each other. Command decisions, either before a battle or at its outset, can heavily weight the odds in the favour of one force or the other. For example, the realization that the Germans fired faster at Dogger Bank than the British battlecruiser force led the latter to abandon safe ammunition practices, which made their ships much more vulnerable at Jutland. At the Battle of the River Plate, Langsdorf threw away the principal advantage of his ship by choosing to fight a short range action, where the smaller British guns were effective, rather than a long range action where his much heavier guns were much more effective than those of his opponents.

Especially in an action between fairly evenly matched opponents, chance plays a big role. A good example of how to quantify this type of situation is the analysis based on statistics and probability theory given by Marco Santarini of the Denmark Strait battle in his book Bismarck and Hood. Alan Zimm gives a similar type of account of the Japanese attack on Pearl Harbour in his book Attack on Pearl Harbour. Included in this is an operational analysis investigation of the probability of various scenarios. The example of a Japanese midget submarine having penetrated the harbour and torpedoed a battleship is given. This basically involves working out what chain(s) of events had to occur for this end result to happen. One then assigns a probability for each event in the chain. Finally, all the individual probabilities are multiplied together to obtain the final probability of the result. As some of the individual probabilities are somewhat subjective, it is usual to insert different values for these and see whether they make much difference to the final result. If they do not the result is said to be 'robust'. There are also two interesting pieces about the Battle of the River Plate in Warship 2018, one by Alan Zimm, the other by Bill Jurens,

Neil Robertson

Iranon
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Re: Shell hits v armour

Post by Iranon » Thu Feb 28, 2019 11:37 am

Fortunately, you don*t have to shoot at a target 20km away to replicate conditions at that range.
You need an accurate range table to hit anything anyway, so you should know what velocity the shell will have remaining and its angle of fall. This can be replicated with a reduced charge and angling the armour plate to be tested.

What happens to the firing ship isn't interesting here - if the angles change enough to be relevant for terminal ballistics, there won't be a hit.
What happens to the target at the time of impact does matter though. If it heels (temporarily or not), that'll change the impact angle. If it moves at speed, a wave trough may expose parts much below the nominal water line. And varies navies calculated different zones of immunity depending on target angle. The British and Americans for example published graphs plotting target inclination to range and marked penetration limits, with a curve for belts and a flat line for decks. Example for KGV against Tirpitz: expected belt penetration limit at 12k yd for 60° to 16k for 90° and of course back to 12k for 120°. Flat lines for expected deck penetrations, 31k yd over machinery and just below 35k over magazines.

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