Dunnigan on Protection, Comments on Chapter 10 of The Fletcher Pratt Book, Pt II

The next major section of chapter 10 of [1] is a quote from James Dunnigan from [2], the gist of this is that the points value of ships in the FPWR do not reflect the true staying power of the ships involved. Now I have no disagreement with this, as I have indicated elsewhere [3]. However it is his argument for this conclusion that is unjustified. Dunnigan claims that the discrepancy is due to the FPWR not accounting for differences in quantity and quality of ship protection. For instance he quotes the average percentage of displacement given over to armour in British and German battleships and battle cruisers in WW1 (he gives these as 35% vs 30% and 30% vs 26% which may be arguable I may come back to this). He also claims that the German armour was better arranged than that of the RN, I will not comment on this, it needs more justification than I have time for at present.

Dunnigan presents as examples Derfflinger, Barham and Lion which he claims have points values under the FPWR of 131,000, 142,000 and 104,000 points respectively. His comment on these is that we might conclude that the Derfflinger was less capable of taking damage than Barham with Lion not far behind. First lets dispose of the last remark; Lion is 27,000 points behind Derfflinger, or Lion is only capable of absorbing 79% of the damage that Derfflinger is capable of absorbing, this is not "not far behind" in these rules Lion has significantly less staying power than Derfflinger (and I may argue later that this difference is in fact excessive for exactly the same reason that I believe that FPWR over rate Barham with respect to Derfflinger). We may as well note here that the percentage of normal displacement devoted to armour in Derfflinger and Barham are about 33-34% and 32% respectively, which given the difference in other characteristics (like area to be armoured per unit displacement) and the uncertainty in what is counted as armour when different people calculate such percentages, are very closely comparable.

Under the FPWR Derfflinger has slightly less staying power (ability to absorb penetrating hit damage for a given loss of capability) than Barham (about 8%). In this Derfflinger is disadvantaged by thinner maximum armour thickness and smaller caliber armament and advantaged by greater speed. None of these should be influencing the ships staying power. I will discuss armour in particular (the arguments against the others are more obvious). As I have defined staying power the armour is obviously irrelevant since the definition refers to penetrating hits. Armour will have already have contributed to the ships survivability by preventing penetration of certain hits at certain ranges (and also accounting for other factors if we so wish) so has no place here (or at least no significant place). This leaves displacement as the only relevant factor in the FPWR ship formula. On ship displacement alone we find that Derfflinger has about 5% less staying power than Barham (and Lion is more or less equivalent to Derfflinger on this measure). I would probably favour a model where staying power were a monotonic function of the displacement devoted to the ship hull plus that devoted to auxiliary machinery but this data is difficult, but not impossible to come by, so I wont pursue this.

On displacement all three of the ships discussed by Dunnigan are closely comparable and this is what we should take as a baseline which we may modify if we feel strongly that one nationality build intrinsically more survivable ships than another. However caution is called for here since in the time frame of Dunnigan’s article Bismark was thought to be disproportionately well protected but current opinion is that its armour was poorly disposed and the Germans had not kept up with armour developments in the US and UK which resulted in inch for inch superior belt amour in US and RN ships (don't know about deck armour as this tended to be a different type from vertical armour).

As an aside we might look at the staying power calculations from a ship modelling tool like SpringSharp. This gives the mass of penetrating non-critical hits to sink for Derfflinger and Barham as 41,100 and 39,600 pounds respectively. Which reverses the order of the displacement ranking, but still leaves them closely comparable. The problem here is that I do not know how SpringSharp models staying power.

In conclusion we may say that if we ignore the poor quality of RN armour piercing shells and the instability of RN cordite and poor propellant handling hygiene the staying power of these three ships is comparable. But in battle Barham would be expected to prevail against Derfflinger because of its' heavier armament and thicker armour.  The result of a Lion/Derfflinger encounter is less clear, the weaker armour of Lion is countered by the weaker armament of Derfflinger (but my money would go on Derfflinger). In reality the outcome of encounters between these ships would depend on factors not accounted for here, specifically the poor explosive hygiene  and fire control on/in British battle cruisers at Jutland, and the poor quality of British shells (both of these were at least partially corrected after Jutland).


References

1. Curry, J., Fletcher Pratt Naval Wargame, the history of wargames project 2011
2. Dunnigan, J., Article in Strategy and Tactics, Vol1, 1967
3. Larham R. Fletcher Pratt Pt 4 (what I think is wrong with the FP Gunnery/Damage models), blog post on this blog:  http://navalwargames.blogspot.com/2011/07/fletcher-pratt-pt-4-what-i-think-is.html, 2011