Naval History Forums

Had a question comparing these two common Japanese destroyer weapons of ww2.

The 5" Fires a heavier shell, the gun itself is about 33% heavier than the 3.9" Gun.

But the dual mount of the 3.9" is actually about 2000kg heavier than the 5" dual mount, about 32.5k Kg for the 5" to 34.5k Kg for the 3.9".

Any idea why? My thoughts are perhaps a beefier mount and engine to accomodate a faster traverse/elevation speed?

No one has any ideas here I guess?

No, not really. What I can say is the USN 5"/50 was too heavy to be a practical AA weapon. It had slow elevation and traverse, and a slow rate of fire compared to the 5"/38. The IJN may have been dealing with a similar state of affairs.

The IJN 3.9in was designed for all angle loading with power ramming and a loading tray. The 5in/50 used fixed angle loading, with no loading tray and was hand rammed.

Dunmunro has it right, IMO. The 3.9in was Japan's first true DP weapon.

According to Campbell -
Max mount elevation 90deg; elevation @ 16deg/sec; train @ 12-16deg/sec; RoF 15-21 rpmpg.
MV 3314 ft/sec; with a fixed round 44in in length. Wow.

B

No, not really. What I can say is the USN 5"/50 was too heavy to be a practical AA weapon. It had slow elevation and traverse, and a slow rate of fire compared to the 5"/38. The IJN may have been dealing with a similar state of affairs.

Fully aware of that, the Japanese 5"/50 was a DP weapon in name only, slow rate of fire and bad traverse/elevation speeds greatly limited it's ability as an AA weapon. Heck, some of the mounts only allowed elevation to 40 degrees.

I'm just curious why the 5"/50 had such a heavy mount compared to the 3.9"/65. It would seem to be the other way around. I think even the gun of the 5"/50 is 33% or so heavier.

According to Campbell -
Max mount elevation 90deg; elevation @ 16deg/sec; train @ 12-16deg/sec; RoF 15-21 rpmpg.
MV 3314 ft/sec; with a fixed round 44in in length. Wow.

Yeah Byron, the 3.9"/65 could truly compete with the US 5"/38 as far as the best DP weapon for AA purposes. It just had somewhat smaller shells, no VT fuse and Japanese fire control

A good weapon, just limited by other issues.

A separate question, anyone know how effective it was in surface combat? I don't think they even made a common round for it, just HE which would seem to indicate they did not see much use for it in surface engagements. Seem like it would not be too bad though, high enough velocity to penetrate light (destroyer, maybe CL) armor, again just a smaller projectile.

The IJN 3.9in was designed for all angle loading with power ramming and a loading tray. The 5in/50 used fixed angle loading, with no loading tray and was hand rammed.

Are perhaps these are the issues that added a fair amount of weight of the mount perhaps Dunmunro?

Garyt wrote: Yeah Byron, the 3.9"/65 could truly compete with the US 5"/38 as far as the best DP weapon for AA purposes. It just had somewhat smaller shells, no VT fuse and Japanese fire control
A good weapon, just limited by other issues.

..... I don't recall making a comparison with the 5in/38 The "wow" comment was just a reaction to the 44in length of the single-loading cartridge and the very high MV of the Japanese gun.

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Re the heavier 3.9in/L65 and the lighter 5in/L51 disparity mount weight, a close reading of Campbell's "Naval Weapons of World War Two" suggests some possible reasons -

[1] Service experience with the 5in/L51 had led to opinion within the IJN that the mount had been too lightly built.

[2] The 3.9in/L65 mount incorporated complex all-elevation power-ramming and auto fuze-setting features which the 5in/L51 lacked.

[3] The 3.9in/L56 mount possessed much higher training and elevation rates than the 5in/L51, which suggests incorporation of more powerful (hence heavier) power units.

FWIW.

B

I don't recall making a comparison with the 5in/38 The "wow" comment was just a reaction to the 44in length of the single-loading cartridge and the very high MV of the Japanese gun.

Actually Byron, taking out the fire control issues and only looking at the "gun" itself, I think it compares well with the 5"38.

The explosive effects are roughly equal to the square root of the TNT equivalent. This gives on a comparative rating system a 1.82 per round for the 5", and a .97 for the 3.9", pretty close to a 2:1 for the 5"/38.

Which is not bad. The mount of a dual 3.9" weighs only 44-60% that of a 5"38, again roughly a 2:1 ratio, this time in favor of the 3.9".

As the 3.9" has a nominally higher rof, and a higher velocity round, I'd give the 3.9" a slightly more effective rating in AA ability, not counting again fire control or VTS fusing.

For surface to surface actions, taking weight of the mount into account, I'm not sure how they compare. But you are looking at being able to put a little more than 2 rounds to the 5"/38's one round based upon weapon weight.

Most Japanese 5" weapons had a ROF of 8-10, which pales into the ROF of the 5"38 ROF of about 18-19. However, when we look at the fact that you can mount 2 or better Japanese 5" weapons for one US 5" from a weight perspective, the Japanese 5"/50 comes out a bit better, with better range and velocity. Of course this is surface to surface only, the Japanese 5"/50 was remarkably ineffective as an AA weapon.

Now when you start looking at the lighter single mounts carried by destroyers, some were not the enclosed base ring type and the often lacked an internal hoist, which combined to give them a greatly reduced ROF compared to the full blown dual pedestal type.

[1] Service experience with the 5in/L51 had led to opinion within the IJN that the mount had been too lightly built.

[2] The 3.9in/L65 mount incorporated complex all-elevation power-ramming and auto fuze-setting features which the 5in/L51 lacked.

[3] The 3.9in/L56 mount possessed much higher training and elevation rates than the 5in/L51, which suggests incorporation of more powerful (hence heavier) power units.

All this makes sense.

I think I recall reading that the 5"/50 had a bit more dispersion than expected and this was often attributed to not having a robust enough mount. Am I remembering correctly here, or is this another gun I was thinking of?

GaryT - Campbell did make that connection with respect to the Japanese 5/50: light construction > flex > excessive dispersion.

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