Ship comparisons

Saphroneth

Banned
I've been reading Warrior to Dreadnought recently, and at one point in the footnotes D.K.Brown mentions a formula to compare the impact of guns of different types. That is, that the impact and damage of a shell is theoretically the cube of the shell weight but in practice more like the square of the shell weight.



It sounds like this is a useful parameter for comparing different kinds of ships across different countries - among other things, it reflects the generally understood case that a heavy slow shell is better than a small fast shell even for the same gun type.




So for gun effectiveness, might I suggest that this is at least a reasonable approximation:


Number of salvoes/shells per minute (use mid value) times square of shell weight times shell muzzle velocity.

Of course for very large ships with lots of armour then the penetration table approach works somewhat better. But this could be useful for comparing DDs and CLs, as well as MGBs.




For example:


Tribal class DD
8 4.7" mk IX
10-12 rounds per minute (use 11)
Shell weight 22.7 kg
Muzzle velocity 808 mps

Per-gun effectiveness value
4.58 million
Ship effectiveness value
36.6 million


1936-class DD

Main battery:
5 5" guns


15-18 rounds per minute (use 17)
Shell weight 28 kg
Muzzle velocity 830 mps


Per-gun effectiveness value
11 million
Ship effectiveness value
55 million


From this we get that the 1936 class is somewhat better than a Tribal - but that if they had the same ROF they'd not be much different.

I'd be interested to see if this actually works out.
 
This is one of the components for figuring the effects of artillery fires on hardened targets like a concrete bunker, a bridge, or buildings. This combined with tests, and battlefield data were combined into "Effects Tables" we used for calculating quantities of ammunition & numbers of cannon to gain various levels of effects on targets. The Soviet Army equivalent were called "Nomograms" in the documents we used.

Main weakness in your calculation is a allowance for actual number of direct or damaging hits. For cannon fires at ranges much beyond 25% of the cannons maximum range the number of actual hits is amazingly low. Plus it varies widely from the technical differences of a model of cannon, and the fire control system used. i.e.: One ship may expect a 5.2% hit rate at 20,000 meters range & another a %.8% hit rate at that range. That leaving aside conditional variables which could halve those hit rates.

Other than that you are on the right track.
 
Isn't the main issues with guns actually hitting the target?

Especially light fast craft without battleship fire control, where most of the shells will miss, after all to kill any DD will only take a very few hits to the critical parts of the engine room with any 4"-6" shells? (they have no protection so all you need to do is hit?)

it reflects the generally understood case that a heavy slow shell is better than a small fast shell even for the same gun type.
Would the fast light shell not have more chance of a hitting? assuming same ROF etc....

Regarding ROF how do you really work them out in fat calm with good crew or with average tired crew in a winter gale off Nordkapp? I think comparing guns and rate of fire is very hard between navy's...
 

Saphroneth

Banned
Here I'm using NavWeaps.

And obviously a fast heavy shell's better than a slow heavy one, but given the choice between a small increase in hitting probability or a large increase in damage the heavy option's usually better.


Remember that director firing (even DDs had some) means that naval guns don't vary much in accuracy for the same crew.

Now, modelling hit rate is really dang hard... but this handles most everything else I hope.
 
And obviously a fast heavy shell's better than a slow heavy one, but given the choice between a small increase in hitting probability or a large increase in damage the heavy option's usually better
But if the hit rate is very low (as in fast craft like WWII DDs) say 1% then adding a small amount (say 1%) to hit rate gets you doubled to 2% hits.
adding say 10% to the damage will add far less as it only effects the hits so gets you to 1.1% ?

Remember that director firing means that naval guns don't vary much in accuracy for the same crew.
But different nationalities directors vary hugely for hit rates as do different ages and types within a navy. (ie 20s v 30s v 40s with radar or BB v CL v DD) I would add that some ships crew seam to be much better at using them than other crews of sister ships.
 

LordKalvert

Banned
All kinds of things go into this-

rate of fire (it expands rapidly in the 1890s)

range

reloading- again a lot of early guns can't be easily reloaded at certain elevation

accuracy

shell weight- here the explosive power of a shell grows much more than the size of the weight. Simplifying things here- the shell and the fuse don't grow in direct ratio to the shell weight leaving a greater weight for the explosive charge
 

Saphroneth

Banned
But if the hit rate is very low (as in fast craft like WWII DDs) say 1% then adding a small amount (say 1%) to hit rate gets you doubled to 2% hits.
adding say 10% to the damage will add far less as it only effects the hits so gets you to 1.1% ?

But different nationalities directors vary hugely for hit rates as do different ages and types within a navy. (ie 20s v 30s v 40s with radar or BB v CL v DD) I would add that some ships crew seam to be much better at using them than other crews of sister ships.


As to the first point - yes, but I meant a relative increase. 10% extra hit probability, that kind of thing.

Basically I hope to avoid the situations where we simply have to say "25 destroyers vs 15" - can we at least tell something about the 25?
 
Your figures are a little bit out.

5/38 cal gun. shell 24.5kg muzzle velocity 760mps, sustained rate of fire at low angles more like 14 or 15 rpm but possibly lower in reality as director firing reduces rpm.

4.7/45 cal gun. shell 22.7kg muzzle velocity 800mps, sustained rate of fire 10 at surface targets, 12 rpm at air targets but again probably lower in reality when under Director control. In the RN guns were usually fired by the Director either by voice command or remotely, not sure how USN guns were fired early on I think it was by a visual signal (Red Light?) to the gunner then late in the war in both Navies it went totally automatic.

Ammunition supply was better in the USN but the RN tended to have more ready to use rounds so in a short sharp fight which the RN usually got into actual real life firing rates would be similar. In a longer action the better ammo supply and power assisted loading of the USN versus the RN longer supply route and crew exhaustion would mean the USN rate would overtake the RN.
 
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I make it
rpm x shell² x mv x number of guns

Tribal 10 x 515 x 800 x 8 = 32.96 million

1936 15 x 600 x 760 x 5 = 34.20 million
 
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