This luck became a problem for Japan later on. The Japanese losses in 1942 operations were quite low as to destroyers and cruisers.
When the Unryu class was decided on in 1942 destroyer and cruiser escorts, which are necessary, were not given priority as
not a lot had been lost.
By the end of 1943 destroyer losses were 55. At the same time, destroyer production to the end of 1943 is listed as 23 but may
be a little higher.
1944 / 45 saw another 55 destroyers lost and around 26 replacements announced with only 5 being fleet types . the rest were Matsu
type which was smaller at 1,200+ tons and was mainly armed for AA and ASW work while still carrying four torpedoes (many of this
type were used to help escort convoys).
Light cruisers to the end of the war numbered about 5 only with one heavy cruiser laid down.
The successes of the initial aggressive expansion plus the carrier losses in 1942 combined to give priority to carrier production
while fleet destroyer and cruiser production got little. By 1944 carriers were becoming more susceptible to submarine and
surface attack as the combined fleet types were not able to be fielded in the required mix.
There are 2 problems for the Japanese in the 1930s for cruisers & destroyers.
1. The Treaty limits set in the 1930 London Treaty
2. The size of Japanese shipbuilding industry and its ability to react to its leaving the Treaty system
Treaty system
Under the 1930 Treaty, Japan was only allowed:-
12x8" cruisers total 108,400 tons being the Furataka, Aoba, Myoko & Takao classes at their declared Washington tonnages which couldn't be replaced before 1946; and
100,450 tons of cruisers with up to 6.1" guns. It had to remove from service everything up to and including the Kuma class by the end of 1936 in order to have the tonnage to build the 4xMogami & 2xTone that it planned for its 1931 & 1934 Programmes (all at their initially planned 8,500 ton declared displacements). None of the other light cruisers were replaceable under the Treaty before 1942.
Japan left the Treaty system at the end of 1936, having given due notice in Dec 1934. But until that point it couldn't breach the Treaty too blatantly.
Given the various problems it had with its ships in the early 1930s it couldn't rush into designing new ships. The actions it did take included:-
1. Upgunning the Mogamis from 15x6.1" to 10x*8"along with a host of other modifications between 1937 & 1940.
2. Upgunning the Tone class to 8x8" while under construction
3. Retaining the 2xTenryu & 5xKuma class in the fleet.
Under the 1930 Treaty Japan would have been allowed to retain 3xKuma class as training ships along with the battleship Hiei. Retaining these in the active fleet probably accounts for the building of the much smaller & cheaper Katori class in subsequent programmes.
Japan left the Treaty system because the other Powers wouldn't give it what it wanted. It asked for parity with the USA & Britain, but would have settled for about 80%. Instead it was being restricted to about 60%.
Subsequent Building Programmes
Japan's planning was done on the basis of multi year Programmes. And as war approached and changes got made, they become ever more confusing.
The 1937 Programme (intended to cover the next 6 years construction) included 2 training cruisers of the Katori class amongst the 66 ships listed (based around the first 2 Yamatos & the 2 Shokakus).
The 1939 Programme (based around a second pair of Yamatos & Taiho) included a Katori class training cruiser, plus 4 Agano class (design started Oct 1937) & 2 Oyodo class cruisers.
The 1940 Supplementary Programme included a 4th Katori class training cruiser which was suspended while under construction in Nov 1941 and was later scrapped.
A 1941 Programme planned 2 Ibuki class 8" cruisers (and the first Unryu class carrier)
That was the sum of the pre-war cruiser plans. A further 6 light cruisers were included in the various 1942 Programmes at different times.
But longer term planning by the IJN in Nov 1938 called for another 25 cruisers to be built in 2 phases through to completion in 1950. That would have included the 2 Ibukis, and 8 new 8" cruisers. The rest would have consisted of 4 modified (faster to accompany the planned Shimakaze class destroyer flotillas) Aganos, 8 new design AA cruisers and 3 cruiser flagships for the sub flotillas. This plan was then modiified over the coming couple of years. But with the exception of the Ibukis no steel hit the slipways.
Capacity of Japanese industry
And this is where the problems arise. There does not seem to have been a large enough number of suitably sized slips/dry docks to build everything planned quickly enough.
The Aganos were intended as replacements for the old light cruisers as leaders of the destroyer flotillas. Initially 2 were to be built at Kure Navy Yard, 1 at Yokosuka Navy Yard and 1 At Mitusbishi Nagasaki. Because of the pressure on these yards from other projects only the Yokosuka ship was built as planned. So 3 of the 4 Agano class were allocated to the Sasebo Naval Yard. But they had to be build consecutively on the same slip. So construction stretched from the laying down of the first ship in June 1940 to the launch of the last in April 1944 for completion that Nov. Build times of the 3 ships did not vary significantly (28,25 & 24 months respectively).
Oyodo was ordered in 1939 and was supposed to be laid down in June 1940 at Kure Navy Yard. But pressure of work delayed that until Feb 1941. The second ship was to be laid down on the same slip in Sept 1941. Delays to Oyodo delayed the sister Niyodo, and in Nov 1941 her construction was postponed and then cancelled altogether in Aug 1942.
The laying down of the Ibukis was delayed, as the Aganos had priority for the limited capacity available. Warship 300 / Ibuki was laid down on 24 April 1942 at Kure Navy Yard. Construction was stopped at the end of June only to be restarted at the end of July to clear the slip for aircraft carriers. She was launched in May 1943 and work suspended in July while consideration was given to converting her to a fast oiler or a carrier. Aug 1943 it was decided to convert her to a carrier and she was towed to Sasebo in Dec for conversion work to commence. Her sister, Warship 301, was laid down at MItsubishi Nagasaki on 1 June 1942. Construction stopped on 30 June and, as she was less advanced that Ibuki, she was scrapped to allow the carrier Amagi to be laid down in Oct.
This is one version of Japanese shipyard capacity in terms of slip size that I have seen on the net.
150m = 492ft
175m = 574ft
200m = 656ft
225m = 738ft
250m =820ft
275m = 902ft
300m = 984ft
An Agano cruiser was 174m (571ft) long
Oyodo was 192m (629ft) long
Ibuki class cruiser was 200.6m (658ft) long
An Unryu class carrier was 227m (746ft) long
While the Japanese had big carrier plans in 1942 (lead ship + 15 Unryu class, 5 improved Taiho) they prioritised the Unryus and knew full well that the modified Taihos couldn't possibly be completed until 1947/48.
Destroyers
Similar Treaty problems arose with the destroyers as there were also limits on overall tonnage. But Japan prioritised them once it exited the Treaty system.