US Navy 18" gun battleship design: BB61 "slow" scheme from April 1938

icyplanetnhc (Steve)

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The discussions about the Montanas reminded me of the "slow" BB61 proposal from April 1938 that had 18"/48 cal rifles. I'll share what I've posted in other forums about this design.

So, for a quick background refresher, in early 1938, when it became more apparent that Japan would not sign the Second London Naval Treay, the US Navy began looking into battleship designs with standard displacement of 45,000 long tons, the planned limit of the "Escalator Clause". Compared to the preceding 35,000-ton BB57 South Dakotas, the 45,000-ton BB61 design schemes included both "slow" 27.5-knot schemes that increased firepower and/or protection as well as "fast" 33-knot schemes. The "slow" designs considered a variety of main battery options, including twelve 16" guns or nine 18" guns. Ultimately, the "fast" BB61 designs were prioritized, which would eventually become the Iowa class. The "slow" designs would be early precursors to the BB65/67 Montana class design.

My comments

The powerplant for this scheme has the same output as the South Dakota (BB57) class which has a 666 ft waterline length. Thus, assuming the same propulsion machinery arrangement, it appears that this design scheme's hull has 134 ft more waterline length to work with to accommodate the 18”/48 triple turrets and increase the fineness ratio.

Based on the hull dimensions and displacement values, the ship would have a block coefficient of 0.637 at max load, better than North Carolina (BB55) and South Dakota (BB57) but not as good as Iowa (BB61). During New Jersey's sea trials in 1943, she made 27.92 knots with 126,400 SHP, and for Iowa's sea trials in 1985, she made 28.08 knots with 109,700 SHP. Given that the April 1938 design scheme's fineness ratio is much closer to Iowa's than the South Dakota's, I'm inclined to believe the power and speed figures when the hull is clean (i.e. out of drydock with no bottom fouling).

Due to the inclination of 19 degrees, I would certainly expect that the belt to be mounted internally similar to arrangement on the SoDak/Iowa designs, and due to the thickness, likely on 35# or even 40# STS, which is a potential area for weight increase. Similarly, due to the additional length and bending moment, I also wouldn't be surprised if the outer hull side strakes by the belt were increased to 60# STS, and the main armor deck combination is more akin to the Iowa (4.75" Class B on 1.25" STS strength deck) than the SoDak (5-5.3" Class B on 0.75" STS strength deck).

There could be some concern about the hull form given that the three-gun 18"/48 turret has a barbette diameter of 41' 0", which is nearly 4 feet wider than the barbette of the three-gun 16"/45 Mark 7 turret which has a diameter of 37' 3". I think the barbette diameter could be an issue at the #1 turret, while #2 and #3 turrets may be fine; looking at the Booklet of General Plans for the SoDaks, there seems to be ample room to play with for the #2 and #3 turret. Even then, with the same machinery arrangement as the SoDaks and 134 ft more waterline length, they may be able to get adequate space and clearance around the #1 turret by making the tapering of the hull near the bow more gradual, though it would no doubt still be a tight fit as on the Iowas.

Had this ship actually been built, I would expect that the max load displacement of the ship would creep up by 3,000 tons to the 59,000-60,000 ton range, perhaps even slightly higher, in order to accommodate for potential unforeseen strengthening and also the additional AA guns and electronics that would be mounted. Incidentally, assuming that draft is linearly proportional to displacement (outside of edge conditions), a displacement of ~59,400 long tons would increase the design max draft to 37.8 ft, which is almost exactly the same as Iowa's actual max draft during World War 2.

In any case, the US Navy ultimately deemed that the 16"/50 rifle was well suited for their needs and chose it over the 18"/48; all of the design schemes for BB65/67 Montana were planned around the 16"/50, with the exception of a few whacky ones that considered 14"/50 rifles.

Visual appearance

With regards to how this ship would visually look like, one can reasonably postulate that it would look like a 1939 SoDak with a stretched bow and stern, and I can explain the rationale below.

For one, the machinery output is 130,000 SHP, the same as the 1939 South Dakota class. As such, the ship may be able to duplicate the machinery used on the SoDaks, thus resulting in the design's superstructure also being largely identical (given that battleship superstructure generally spans the length of the machinery spaces).

On the other hand, the design's waterline length is 800 ft compared to SoDak's 666 ft, so the design has 134 additional ft to play with to increase fineness ratio and accommodate the bigger turrets. The beam remains the same at 108.2 ft. Thus, if the design duplicates SoDak's machinery layout, then this additional length would be in the bow and stern sections from the machinery traverse bulkheads onward.

41dfdal.jpg


References:
  • Norman Friedman, U.S. Battleships, an Illustrated Design History, page 308
  • William Garzke & Robert Dulin, United States Battleships, 1935-1992, page 111.
CharacteristicValue
Waterline length800 ft
Beam108 ft 3 in
Draft35.96 ft
Max displacement56,595 long tons
Battle displacement54,495 long tons
Standard displacement45,495 long tons
Speed27.5 knots
SHP130,000
Range (15 knots)15,000 nmi
Main battery9 x 18” (457mm)/48
Secondary battery20 x 5” (127mm)/38
Belt (19 degrees on 30# STS)14.75” (375mm)
Heavy deck (on 30# STS)5.1” + 0.75" (130mm + 19mm)
Bomb deck1.5” (38mm)
Splinter deck0.63” (16mm)
Barbettes, conning tower21” (533mm)
Turret face20” (508mm)
Turret, CT roof10” (254mm)
Traverse bulkheads16.75” (425mm)
Splinter protection2.5” (64mm)

Weight (from Friedman, p. 308.)
WeightTons
Hull2,115 (almost certainly a typo, likely 21,115 based on difference between design full load and all other components)
Hull Fittings1,697
Protection13,037
Engineering (Wet)3,500
Armament3,464
Ammunition1,638
Equip&Outfit476
Complement192
Stores & Fresh Water322
Aeronautics54
Reserve Feed Water650
Fuel Oil8,000
1/3 Stores & F.W.350
Designed Full Load54,495
Belt6,226
Heavy Deck4,010
Bomb Deck1,344
Splinter Deck454
Torpedo Bulkheads3,310
 
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icyplanetnhc (Steve)

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I decided to do some quick math to check the hull weight by subtracting all the other components from the design full load, and got 21,115 tons for hull weight. As such, the 2,115 ton figure given in the source is probably a typo or transcription error.
 

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