Early 1950s Plans for Defensively Equipped Merchant Ships

[A Tentative Fleet Plan]

Stumbled across some early 1950s NATO documents concerning the defensive arming and equipping of Merchant Ships. SGS.1 and the 4" Mk 25 are briefly mentioned, although unsurprisingly the vast majority of equipment intended for DEMS was supposed to come from US stocks.

There are more documents than I've uploaded here, and content tends to overlap with the first I've uploaded.

 

[TinWing]

Stumbled across some early 1950s NATO documents concerning the defensive arming and equipping of Merchant Ships. SGS.1 and the 4" Mk 25 are briefly mentioned, although unsurprisingly the vast majority of equipment intended for DEMS was supposed to come from US stocks.

There are more documents than I've uploaded here, and content tends to overlap with the first I've uploaded.

I’m not sure what the document meant by the 4” Mk 25? As far as I can recollect, the Mark numbering sequence for Royal Navy 4 inch guns ends with the 4” L33 Mark XXIII for the A-class submarines. Oddly, I think it might use different shells and casings than the more common 4” L40, so ammunition availability is a question mark? With a short barrel and lightweight mounting it would have been very appropriate for merchant vessels. I think a few found their way onto export patrol vessels. The only other 4” gun I can think of is the 62 caliber Mark Q exported to Chile, which is obviously inappropriate.

 

[A Tentative Fleet Plan]

I’m not sure what the document meant by the 4” Mk 25? As far as I can recollect, the Mark numbering sequence for Royal Navy 4 inch guns ends with the 4” L33 Mark XXIII for the A-class submarines. Oddly, I think it might use different shells and casings than the more common 4” L40, so ammunition availability is a question mark? With a short barrel and lightweight mounting it would have been very appropriate for merchant vessels. I think a few found their way onto export patrol vessels. The only other 4” gun I can think of is the 62 caliber Mark Q exported to Chile, which is obviously inappropriate.

Mark 25.was intended as a DEMS mounting for the Type 42 Frigate (considered alongside others).and Defensively Equipped Merchant Ships, @JFC Fuller discusses it in further detail in this reply:

4" QF Mk.21 on 4" single Mk.25 RP.50.

The 4" QF Mk.21 was a variant, lighter weight, of the common 4" QF Mk.16. I don't have details on the mounting but I would guess it was designed as a replacement for the Mk.23 mounting with the Mk.19 4"/40 gun that had been used on DEMS and small warships during the war. Something that could be controlled by a Simple Tachymetric Director (STD) or Small-ship Gunnery System Mk.1 (SGS1), using an already in-service gun.

It wasn't for the Type 42, it was considered as an option for that design alongside the Vickers Mark Q and the 4" QF Mk.16* on 4" twin Mk.19* RP.51. The latter was chosen, then grandfathered on to the general purpose sloop that became the Type 81. Ultimately the 4" QF Mk.16* on 4" twin Mk.19* RP.51 was replaced in that design with the 4.5" on single Mk.5 mountings, primarily because DNO wanted to standardise on the 4.5" gun.

 

[Roodbaard1958!]

I found some drawings for merchant ships in the 1950-1960 with preparations made for mounting guns. For example the drawing for a 10,000 tons deadweight cargo ship dated 8 December 1955 used by the Kon.Mij. De Schelde, Vlissingen, Netherlands for yard number 285 the Melbury Beacon, url: https://hdl.handle.net/21.12113/FAE585F302D64934AB24B58F94509EE8
She was to be armed in times of war with Bofors, Oerlikons and at least a single 4" gun at the stern