Washington

[Thorsten Wahl]

alecsandros the meaning stability you use here is not the meaning of the ballistics term "stability", wich describes the ability of a projectile to maintain its orientation within flight, despite disturbances.

[lwd]

...or it could have been anything but the fact is that it was 3 straddles and 3 hits.

That was essentially my point.

Then how do you explain the hits, straddles, and near straddles?

good luck maybe?

For one straddle/hit/near straddle that wouldn't be an unreasonable speculation however given 3 hits, 3 straddles, and more "effective" salvos it becomes very problematic.

I dont know of any "near straddles", just 2 straddles + 1 other probable resulting in 3 hits, total salvos 18.

I strongly suspect that's what they meant by "effective salvo". It would really help to have the form that contains info each salvo by PoW.

PoW fired 18 salvos and 55 rounds that works out to just a bit over 3 rounds per salvo. Furthermore her first salvo was with 6 guns and her second with 5 I believe. If you take out these two ranging salvos then we have 16 salves and 44 rounds which is 2.75 rounds per salvo. The implication is clear she could have had a good range and still not straddled on a number of occasions. It's even possible that one or more of the hits were on salvos that didn't straddle.

ummm, getting hits without straddling the taget, that is what I call good luck!

For any particular salvo it would indeed be good luck. However if you are only firing two shells if you get a straddle the only way to get a hit is an underwater hit or one that passes through the ships upper works. With one shell you obviously can't straddle yet there are indeed records of hits. Look at the number of ships sunk by submarine deck guns for instance. Even with 3 shells if o ne hits the odds on a straddle (not counting the cases mentioned above) is still only 50%. Which would indeed suggest that it isn't at all unreasonable to think that one of PoW hits may have come from a salvo that didn't straddle.

[lwd]

PoW fired 18 salvos and 55 rounds that works out to just a bit over 3 rounds per salvo..

NO.
21 salvos and 59 rounds.

The numbers I quoted don't include the last three "salvoes" where a total of 4 rounds were fired under local control by Y turret. That should have been rather obvious from the discussion up to this point. It's also worth pointing out that I didn't post this originally.

[Wm Jurens]

Regarding stability vs caliber, Alecsandros wrote:

"I think it's pretty self explanatory.
An obvious empirical example comes from GKDOS100, where shell with larger caliber give smaller dispersion at longer range than their smaller caliber cousins. Assuming same gun and mount characteristics, the obvious explanation is shell trajectory - and the way it is influenced on the way by the external ballistics variables...

And I have a distinct feeling of Deja Vu. Have we discussed this before ?"

I don't recall having discussed this before, but certainly could be wrong. The relationship between dispersion, range, and caliber that you note is not due to projectile stability per se, but largely reflects the fact that to reach any given range, a smaller projectile (with a smaller ballistic coefficient) will usually have a more highly curved, hence longer, trajectory. Stability per se remains the same from bullet to bullet, but for a smaller projectile the trajectory and the time of flight will be longer and there is therefore more time and distance over which small disturbances can work. This does not mean that the smaller projectile is inherently less-stable, only that because in theoretical terms the stability can only be optimized for a single range, that a similarly-stable projectile will have more time and distance over which to go off course a bit.

For spin-stabilized projectiles, one can nominalize stability for a given range band by changing the twist of the rifling; in general terms, it will be found that the higher the angle of departure, the slower the best possible twist will be. A lot also has to do with projectile length; so far as stability is concerned, longer bullets, e.g. 4.5 caliber long projectiles, will generally do less-well than shorter ones as range increases.

Bill Jurens

[alecsandros]

alecsandros the meaning stability you use here is not the meaning of the ballistics term "stability", wich describes the ability of a projectile to maintain its orientation within flight, despite disturbances.

you mean the orientation of the nose in respect to the expected trajectory ?

[Pandora]

...or it could have been anything but the fact is that it was 3 straddles and 3 hits.

That was essentially my point.

...then do we agree that 3 hits out of 3 straddles requires a lot of luck?

Then how do you explain the hits, straddles, and near straddles?

good luck maybe?

For one straddle/hit/near straddle that wouldn't be an unreasonable speculation however given 3 hits, 3 straddles, and more "effective" salvos it becomes very problematic.

I dont have that impression but you are entitled to their own opinion

I dont know of any "near straddles", just 2 straddles + 1 other probable resulting in 3 hits, total salvos 18.

I strongly suspect that's what they meant by "effective salvo". It would really help to have the form that contains info each salvo by PoW.

haven't you read it? all other salvos were seen as short or over.

PoW fired 18 salvos and 55 rounds that works out to just a bit over 3 rounds per salvo. Furthermore her first salvo was with 6 guns and her second with 5 I believe. If you take out these two ranging salvos then we have 16 salves and 44 rounds which is 2.75 rounds per salvo. The implication is clear she could have had a good range and still not straddled on a number of occasions. It's even possible that one or more of the hits were on salvos that didn't straddle.

ummm, getting hits without straddling the taget, that is what I call good luck!

For any particular salvo it would indeed be good luck. However if you are only firing two shells if you get a straddle the only way to get a hit is an underwater hit or one that passes through the ships upper works. With one shell you obviously can't straddle yet there are indeed records of hits. Look at the number of ships sunk by submarine deck guns for instance. Even with 3 shells if one hits the odds on a straddle (not counting the cases mentioned above) is still only 50%.

exactly and since the odds are so low, even if you straddle, to get a hit luck has a lot to do.

Which would indeed suggest that it isn't at all unreasonable to think that one of PoW hits may have come from a salvo that didn't straddle.

problem is that all PoW salvos that didn't straddle were reported as short or over.

[lwd]

...or it could have been anything but the fact is that it was 3 straddles and 3 hits.

That was essentially my point.

...then do we agree that 3 hits out of 3 straddles requires a lot of luck?

NO. We agree that there were 3 hits and 3 straddles as I have stated it's not at all clear that all 3 hits were assoicated with the straddles or even that there were 3 straddles (isn't one of them questionable?).

.. haven't you read it? all other salvos were seen as short or over.

And all the shell splashes from each of these salvoes was accounted for? Or could there for instance been a 3 gun salvo where 2 shorts were observed and no longs or conversely one where 2 longs were observed and no shorts? I haven't seen the detailed descritpion of each salvo by any means and no one has linked it to my knowledge or reported it in this thread.

exactly and since the odds are so low, even if you straddle, to get a hit luck has a lot to do.

LIkewise not to hit is a matter of luck.

[alecsandros]

For spin-stabilized projectiles, one can nominalize stability for a given range band by changing the twist of the rifling; in general terms, it will be found that the higher the angle of departure, the slower the best possible twist will be. A lot also has to do with projectile length; so far as stability is concerned, longer bullets, e.g. 4.5 caliber long projectiles, will generally do less-well than shorter ones as range increases.

Bill Jurens

Hello Bill,
Thanks for the insights,

Just a quick though - doesn't the smaller mass of the smaller projectile make the wind turbulences more evident ? I'm guessing a 1000kg shell mantains it's trajectory better than a 100kg shell from the same family of shells, when facing high-winds of 30-40m/s ?

Thanks,
Alex

[lwd]

Just guessing but if the shapes are the same then the ballistic coeffiecents should at least be close. Givent that the force of the wind will be porportional to the area presented which is going to vary as the square of the linear dimensions and the acceleration will vary as this force divided by the mass which will vary as the cube of the linear dimensions. So all other factors (including velocity) being the same I think you are correct the larger projectiles hould experiance less "windage".

[Wm Jurens]

Alecsandros:

You are correct insofar as a smaller projectile will have a much smaller mass than a larger one will, and will therefore be somewhat more susceptible to buffeting. The effects would not be that great unless atmospheric conditions were extreme. I doubt if anyone would be shooting anything at all in 40 m/s wind velocities; if my calculations are correct that's equivalent to about 90 miles per hour.

IWD:

You are confusing drag coefficient with ballistic coefficient. The ballistic coefficient (here described in somewhat simplifed form for the purposes of this discussion) is proportional to mass/(drag coefficient * cal^2) and increases very rapidly with caliber. Projectiles of the same shape might be expected to have similar drag coefficients as the drag coefficient is, in most cases, basically independent of size, but the drag coefficent is not really closely linked to the ballistic coefficient at all. Drag is defined by projectile shape. In practical terms -- as in the simplified formula above -- the ballistic coefficient is really related to both caliber and shape, caliber controlling the mass and shape controlling the drag coefficient.

It is, overall, correct to state that in general smaller projectiles are somewhat less accurate than larger ones, and although some of this is due to the factors you mention, the primary cause of increased dispersion can be generally related to the need for a smaller projectile to traverse a longer trajectory length and experience a longer time of flight to reach a given range.

My original question related to the issue of projectile stability to describe this phenomenon, but in reality stability, as the term is conventionally applied, is not very closely linked to the ballistic coefficient or the drag coefficient at all; it's mostly controlled by spin -- or, in the case of longer projectiles -- via fins.

Hope this helps...

Bill Jurens

[dunmunro]

PoW gunnery report:
http://www.hmshood.org.uk/reference/off ... 09guns.htm

PoW salvo ranges with short/over/straddle per salvo:
http://www.hmshood.org.uk/reference/off ... encIVa.gif

PoW salvo rounds/salvo:
http://www.sfu.ca/~dmunro/PoW_salvo_a.jpg

[dunmunro]

For those of you who haven't read it, or haven't read the updated version, here is Bill Jurens article on the sinking of the Hood:

http://www.navweaps.com/index_inro/INRO_Hood_p1.htm

[Byron Angel]

Just guessing but if the shapes are the same then the ballistic coeffiecents should at least be close. Givent that the force of the wind will be porportional to the area presented which is going to vary as the square of the linear dimensions and the acceleration will vary as this force divided by the mass which will vary as the cube of the linear dimensions. So all other factors (including velocity) being the same I think you are correct the larger projectiles hould experiance less "windage".

..... The rules of thumb as I understand them were that, all other things being equal, dispersion varied approximately directly with range and inversely and directly with gun caliber. In other words, at a range (say, 16.000 yards) where a 12in salvo from a well calibrated gun battery could achieve a 100 yard pattern, the best that could be achieved by a similarly calibrated 6in battery would be about 200 yards, and that both patterns would be doubled in dimensions at twice the range.


B

[wm Jurens]

Regarding projectile stability.

I have been having great difficulty posting to this board lately. The following is a re-written re-post of material submitted yesterday, which apparently did not make it on to the thread. This post may stand on its own, but if the other appears as well, there will be some redundancy. I do hope this one makes it.

Alecsandros wrote:

"Just a quick though - doesn't the smaller mass of the smaller projectile make the wind turbulences more evident ? I'm guessing a 1000kg shell mantains it's trajectory better than a 100kg shell from the same family of shells, when facing high-winds of 30-40m/s ?"

Yes, all things being equal, a smaller projectile will be more affected by local turbulence. This is not really a stability issue however, as the term is commonly used, and the differences overall are not large. I doubt if anyone would be shooting in 35 m/s winds anyway, as -- if my equations are correct -- this amounts to a wind of about 80 miles per hour. That's a special case. Basically, stability -- at least as traditionally defined -- is nearly independent of caliber.

To IWD:

You are apparently confusing the drag coefficient and the ballistic coefficient. The ballistic coefficient may be simply defined essentially as mass/(drag coefficient * cal^2). The drag coefficient takes into account the effects of projectile shape and is essentially independent of projectile mass. The caliber has a very great effect on the mass, so larger projectiles will have a much greater ballistic coefficient even though the drag coefficient (which is dependent upon shape) may remain the same. It is incorrect to conclude that projectiles of the same shape but of differing caliber will have (nearly) identical ballistic coefficients. As an aside, it's worth noting that In spin-stabilized projectiles, 'stability', as the term is normally used, is essentially independent of drag coefficient or ballistic coefficient.

As noted earlier, in properly designed gun systems, dispersion is primarily related to trajectory length and (what is nearly the same thing) time-of-flight. The effect of projectile caliber, per se -- though by no means non-existent -- is rather small. The increase in dispersion at range is primarily due to the fact that smaller projectiles, for comparable initial velocities, suffer proportionately more drag and therefore require a larger angle of departure to reach any given range. This results in a longer trajectory and an increased time of flight. If range were measured in projectile calibers instead of absolute units such as yards or meters, little difference would be apparent.

Hope this helps...

Bill Jurens

[alecsandros]

Yes, all things being equal, a smaller projectile will be more affected by local turbulence. This is not really a stability issue however, as the term is commonly used, and the differences overall are not large. I doubt if anyone would be shooting in 35 m/s winds anyway, as -- if my equations are correct -- this amounts to a wind of about 80 miles per hour. That's a special case. Basically, stability -- at least as traditionally defined -- is nearly independent of caliber.



Bill Jurens

Thanks Bill,
Just to clarify :
The 40m/s wind I was thinking would occur higher up on the trajectory, not at the initial firing point (basicaly a few meters elevation). The high-point of a naval shell trajectory can easily be several kilometers high, and there are other meteo conditions up in the air...

--

Of course, you are using stability in the ballistic sense of the word, as Thorsten pointed out.
I was reffering to "stability" as in "pattern consistency"... because the discussion concerned the hit ratio of Prinz Eugen compared to Prince of Wales.

Cheers,
Alex