Tinclads and cheats – the heavy cruisers of the Washington Treaty

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Matthew Wright

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I'm a New Zealand historian with a life-long interest in matters military, particularly naval engineering. My books have principally been published by Penguin Random House. I'm a Fellow of the Royal Historical Society at University College, London.
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The ‘Washington Treaty’ of February 1922 limited national warship tonnages by ship type, displacement, and gun calibre among other things. It was an unprecedented step, the first effective arms-control arrangement of its kind, made possible largely on the back of financial and human exhaustion following the First World War. It also served to heavily influence ship design for over a decade. The heavy cruisers of the Washington Treaty were some of the most varied concepts to arise.

Cruiser limits were negotiated around Britain’s new Hawkins class, and allowed up to 8-inch guns on 10,000 tons standard displacement. There was little debate; both Japanese and US designers had been working on 8-inch designs of similar scale to Hawkins, and this seemed ample. However, a ten-year ‘battleship holiday’ swiftly threw the focus to cruisers, and a ‘capability race’ erupted in which everybody faced the same challenge: squeezing enough military characteristics into 10,000 tons.

heavy cruisers of the washington treaty

IJN Takao on trials in 1932. Public domain, via Wikipedia.

Japan took the lead, developing ‘Treaty’ cruisers that were exceptionally fast, well armoured and heavily armed – a combination that raised eyebrows with British and US naval designers who were discovering that the limit let them have any two of the three. Publicly, Japanese engineers had found efficiencies, including an undulating sheer-line that theoretically allowed them to reduce structural weight without compromising hull girder strength. This was true; the IJN did everything they could to maximise combat capability for displacement, to the point where they eventually hit the limits of metallurgy and stability. The Mogami class, particularly, initially suffered from inadequate structural strength and too much top-weight.

heavy cruisers of the washington treaty

IJN Myoko after surrender at Singapore. Public domain.

But the main reason why Japan could squeeze ten eight-inch guns, 34-plus knots and around a 100 mm armour belt or similar into their Treaty cruisers was simpler. They cheated. The four Myokos – Japan’s first full-scale Treaty cruisers – displaced around 11,630 tons standard and at full load topped 14,980. Their successors were similarly over-weight.

The issue became particularly obvious with the light cruiser Mogami and her three sister ships, meant to displace no more than 8,000 tons and carry 155-mm (6.1-inch) guns by terms of the Second London Naval Treaty of 1936, the second successor agreement to the Washington Treaty. The Japanese government admitted to 8,500 tons. However, Britain’s Director of Naval Construction analysed the figures and suggested that Japan could indeed squeeze everything into that displacement – providing the hull was made of cardboard. Although sources vary, it seems Mogami was originally designed for 9,500 tons standard, re-designed during construction to displace 11,200 – and that rose to at least 12,400 tons standard when all four of the class were re-built with hull bulges and ten 200 mm 3rd Year Type guns, making them heavy cruisers. By this time, however, Japan had formally repudiated all treaty limits.

heavy cruisers of the washington treaty

The Italian heavy cruisers Trento and Trieste around 1938. Public domain, via Wikipedia.

Other nations cheated too. The Italians compromised on range – not needing it in Mediterranean waters – for higher speeds, but their ‘Treaty’ cruisers still ran 10-15 percent over limit. The Germans ignored the issue completely. The Kriegsmarine was permitted to build cruisers to Treaty limits via the London Treaty of 1935, but the Admiral Hipper class were around 15,900 tons standard.

heavy cruisers of the washington treaty

HMS Cornwall, one of the Kent class heavy cruisers generically known as ‘Counties’. Public domain.

Only the British, French and Americans stuck to the rules. Britain’s various ‘County’ classes mounted eight 8-inch on large and fast but very lightly armoured hulls. Financial restrictions prompted smaller ‘B’ type heavy cruisers with just six guns, albeit better armoured – of which only two of a planned seven were built. The US Navy – after trying a 10-gun design – standardised on a 9-gun arrangement, again with excellent speed but minimal armour, prompting the nickname ‘tinclads’.

It was only after Treaty restrictions were lifted at the end of the 1930s that things changed. Britain’s construction plans focused on 6-inch cruisers. However, at the urging of Winston Churchill, sketch designs for 8-inch heavy cruisers were drawn up in the early war years, typified by a nine-gun ship, properly armoured against 8-inch fire, with a displacement of about 15,500 tons. Churchill wanted even larger vessels, but the Admiralty calculated that two Vanguard-class battleships could be built for the same cost as three of Churchill’s mega-cruisers, and could enter service sooner. In any case, Britain’s available industry had to be focused where it was needed – and neither new heavy cruisers nor multiple Vanguard-class battleships could be justified.

heavy cruisers of the washington treaty

USS Bremerton in September 1945 – a Baltimore class heavy cruiser. Public domain.

Things were different across the Atlantic, where superior United States industrial capacity made it possible to build multiple heavy cruiser classes alongside other war requirements. USS Wichita – an 8-inch development of the Treaty 6-inch cruiser Brooklyn – set the pattern. What followed was a series of outstanding cruisers – the Baltimore and Oregon City classes – properly balanced at around 13,900 tons standard and about 17,600 tons full load. These were followed by the even better Des Moines class of around 16,900 tons standard, completed after the Second World War, arguably the best heavy cruisers ever built.

It is clear, then, that the 10,000 ton limit agreed in 1922 was too low for a militarily balanced 8-inch cruiser. But the intent of that Treaty was to limit the scale and number of warships; and in that sense, it was a great success.

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