Note: There is an updated, expanded version of this article available here.

Montana class general overview:

Last and final battleship class designed for the United States Navy with design process starting in paralel to the BB-61 Iowa class after the escalator clauses of the 2nd London Treaty were triggered in 1938-39. Originally the concept started it’s life as an enlarged BB-57 South Dakota type ship and was considered as the continuation of the traditional, heavily armed and armored but slow USN style battleship line with BB-61 being an offspring fast type. As the design progressed during 1940 more and more protection was requested as the full potential of the Mark 7 16″ gun was being realized with the new, 2700# super-heavy shell – all this in addition to a heavier main and secondary armament. Several variations were tried with many combinations of main and secondary armaments, protection and speed ranging from lowly 14″ gunned variants to the huge 300+ meters long BB-65-8 that had best of all worlds.

Early variants were interesting in that the designers looked at quadruple 16″ turrets with twin 6″ DP guns as secondary batteries. The quad turret proved to be too heavy and the 6″ DP secondaries were simply not ready in time for the design – also the number that could be carried was only 12 guns, 6 per broadside (which is even with auto loading guns would provide somewhat low volume of fire.)

Later when the fresh war meant relaxation of all artificial (that is political) limits a full spectrum study was prepared: now with the main armament settled at 12-16″/50 guns in four triple turrets even the smallest ships come in at 51.000 tons and at the other end of the spectrum sat two monsters of 67.000 tons standard/80.000 tons fully loaded displacement with a length of 1050/1100 feet. These numbers are even more impressive if we take into consideration that these ships would have been built with a fully welded construction, meaning approx. 5% weight saving compared to riveted construction more commonly used by other navies.

The final series of designs returned to the 60-61.000 tons standard mark but even that was almost twice the size of only the previous generation ships (BB-55/57 classes) – and most of the extra weight was invested into extra protection while the main armament grew only a mere 30% in the number of barrels while keeping the same caliber! This factor alone demonstrated very well that gun technology was advancing faster than passive protection and if battleships are to be kept for the future a new standard of protection has to be reached – and quite probably the Montana class would have established the new standard battleship type of the USN, just like the Nevada class did at the outbrake of WWI.

Also interesting to note that this was the first USN design that had a wider beam than the Panama Canal’s existing locks – but a new set of locks was in planning just paralel with the design of this class.

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Preliminary Designs:

First series of designs were done in 1939, all were made to adhere to the 45.000 tons standard displacement limit, set by the 2nd London Treaty’s escalator clause. Another clause also permitted gun caliber to be raised to 16″ back from the previous 14″.

July 1939 (Please note that hull no. BB-65 was hoped to be the first ship built to this design at that date)

*ns= normal shell 2240 pounds; hs= heavy shell 2700 pounds; all designs were capable of 27.5 knots speed.

Reportedly the 16″/50 quadruple turret was not well liked, though it offered nice savings in weight and optimization in other characteristics. The cited reasons were too high rotating weight  (400t more vs the triple turret) and an extremely large cut in the strength deck – therefore requiring extra strengthening in structure and foundations, plus increased electrical capacity to supply the bigger motors needed for turning and elevation of the guns.

Very interesting is that the IZ was calculated in many cases against the 16″/56 Mark A gun, a prototype that was a linered down 18″/48 gun and was by far the best belt penetrator among any BB guns ever built and test fired. On the other hand the 16″/45 Mark 6 was taken as standard for deck protection as it’s ballistic characteristics were better suited for deck penetration then the later Mark 7 50-cal length weapon – the letter one being a middle ground.

Design A was essentially a BB-57 with one turret added and guns changed from Mark 6 to 7, while design B tried 6″/47 DP guns (which pushed the displ. well over 45.000 tons). The low number of secondary guns was not liked, but adding 4 extra guns would cost almost 500 tons direct weight plus more in armored volume. The C series tried the quadruple turret concepts and weight saved was invested in extra protection versus better guns and/or the heavy shell. Generally using the better guns meant a good 8 kiloyards shrinking of the IZ, leaving the versions protected on BB-61 scale (12.1″ belt with 19 degree inclination + 4.75″ main AD) with only 6.000 yards of IZ. If one adds the 2700 pound shells into the mix then these ships have no or even negative protection – BB65-C4 with increased belt restored this to a mere 4 kyard wide IZ, not too much, especially with a speed of only 27 knots. Why this is important is that it shows very well that no matter how large a battleship design grows the principle of armoring them against their own shells means that even these colossal ships are very tightly constructed.  Designs F-J were all in search of better protection by sacrificing some firepower, either one triple turret or caliber reduction to 14″ -non of them too attractive for fans of firepower (and what’s the point of a huge BB if not that? :)). Interestingly the designers of the time were thinking along the same lines and contemplated using the 18″/47 Mark A and the 16″/56 Mark 4 (physically the very same gun – pls follow the link for more info) as well but the idea was dropped for now as the gun proved too heavy (~190 tons per gun) to provide more than 6-7 guns on 45 ktons while keeping a good enough protection. What’s more the 16″/56 had a barrel life of only 120 shots which was barely above the usual allotment of 100 rounds per gun. Also as tests somewhat later proved the existing 16″/50 Mark 7 with the super-heavy shell was more than capable of penetrating anything afloat at that time (the USN had no firm idea in 1939-40 about the size and main armament of the Yamato class, though rumors circulated about the use of 18″ weapons).

Comparison pics of the 18″/47 Mark A and the 16″/50 Mark 7 and a VW Beatle (this exhibit is still in existence today – so this 18″ weapon is the largest surviving naval artillery piece!)

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Jan-February 1940:

With the Second World War progressing international treaties were abandoned and this opened up the political gates even for naval designers, which was exactly the case for the Montana class developers. Not only that but in February 1940 the Secretary of the Navy (thanks in no small part to the agitation of said developers) ordered a third set of locks built for the Panama-canal, with a lock-width of 140 feet, thus removing the age old limitation of 108 foot beams for US Navy ships. These two factors basically meant that sanity (ie operational practicalities, time and money) remained the only limiting factor for new battleship construction. So came the Construction and Repair division of the USN who was responsible for ship preliminary designs at that time and produced a spectrum study of  no less than 12 new designs that still bore the BB-65 designation as Congress only ordered battleships up to BB-64, and those still to the BB-61 Iowa design. Since the previous range of designs showed that a main armament of 12-16″/50s in four triple turrets is the most desired layout all the new ships had this main feature.

First they went back to a stretched South Dakota variant (BB65-1) that was capable of 27.5 knots with the same powerplant (130.000 SHP) as said ship and was protected between 20-30kyards against it’s own guns (15.3″ belt and 5.5″ deck). This variant came out at a fairly good 51.300 tons, though it only had the same secondary battery. A newly developed 5″ gun, the 5″/54 Mark 16 was suggested by BuOrd in place of the 6″/47 DP Mark 16 as a good compromise between gun performance and weight. So design BB-65-2 was the same as BB-65-1 but secondaries were replaced with the 5″/54 guns and the ship stretched by another 20 feet to accommodate this. Faster variants were looked at as well that utilized the Iowa type powerplant (212.000 SHP) which could drive a 1000 foot long by 115 foot wide 58.000 ton hull at a speed of 31.8 knots (BB-65-6) – a hoped for 33 knots speed would require a calculated 318.000 SHP (and probably 6 shafts), this was BB-Y1. Alternatively a somewhat smaller, 61.000 ton hull could be driven at 34 knots+ by the same plant. Very important to note that these latter ships had a 14.2″ belt internally mounted just like in BB-57 and BB-61 at a slope of 19 degrees (deck was 5.5″). At this time questions arose about the validity of the internal belt (used earlier as a weight saving measure) as the hull was prone to flooding outside the belt and also battle damage repair was much more difficult for internal armor plates.

March-July 1940:

Final preliminary design series, ordered by the General Board in February 1940. This series contained 13 designs, but a few were only slightly modified, refined from the previous series. New were BB65-1 and BB65-2 (the ones from the previous series were refined into BB65-3 and -4). The new BB65-1 was a slightly modified BB-61 with extra protection but less speed (31 knots) where as BB65-2 was an enlarged BB-61 (980 feet length and 111 feet beam) keeping the 33 knots speed but gaining the heavier set of armor as well – both had 15.75″ belts and a 5.5″ deck. This meant a nice 18-30 kyards IZ versus the heavy shell fired from the Mark 7 16″/50. The larger ship came out at 53.500 tons as a nice compromise. With this however a return was made to 12 gun designs and -3 and -4 were simply the Jan-Feb series -1 and -2 buffed to 28 knots speed (-2 also had it’s SHP increased to 150.000). Finally came the very large design starting with -5 which had it’s IZ extended to 32.000 yards, requiring 6.2″ on the deck with an  57.500 tons , 930 feet long hull plus 150.000 SHP to maintain 28 knots. -6 was again a refinement of the January design and only gained in length and beam (making it 64.500 tons). Speed remained at 31 knots. At last the -7 and -8 were getting everything, high speed, heavy protection (18-30 kyards for -7 and 18-32 kyards for -8), 12 main guns – all this came at a staggering price as -7 weighed in at 65.000 tons with a 320.000 SHP propulsion plant and -8 was 67.000 tons on 366.000 SHP! Clearly these latter two would have required turbo electric drives and at least 6 shafts to transmit that kind of power to the water. At about this time in mid-March the decision was also taken to employ external belts though keeping the sloping at 19 degrees, similar in concept that was used in the old BB-55 design at a lesser slope. At this point no consideration was given to buff torpedo protection, however returning to the external belt clearly meant losing the advantages of the earlier tapered down belt’s secondary function as main torpedo bulkhead – and it’s primary value as protection versus diving shells. Two more designs were produced in June, -9 and -10, the former simply as a BB65-3 with extra belt protection but thinner decks (IZ moved to 18-30 kyards) and -10 as a nine gun variant which came in at only 48.000 tons.

1940 July marked the fall of France and this was a major event in USN planning as Congress was about to pass the “Two-Ocean Navy” act that would authorize the building of insane amount of ships – and the design for the crown jewel, a heavy battleship was badly needed. There was a good chance that Great Britain will fall as well then the United States had to fight Germany and Japan alone on both oceans. Huge number of ships also meant that existing designs will be mass-produced instead of delaying construction waiting for a better design (that is why for example the CL-55 Cleveland type was built in huge numbers although it proved to be a problematic design). Also now the very large fast designs fell out of favor as they were both expensive, took longer to build and surely unable to transit the existing Panama Canal locks (the 3rd set of locks was cancelled due to the war as well) and use existing docks and ports. Also 33 knots was deemed unnecessary for heavy BBs when most existing cruisers couldn’t do much more than that. The intermediate 30-31 knots speed was preferred (it has to be noted here that while 1-2 knots of difference in speed does not seem to be a big deal, technically it means that the powerplant can be reduced by 25-30%!) with designs -5 and -6 being favored as good middle grounds. Also some members of the General Board questioned the value of a 4th triple turret, requiring 10% extra displacement – so Preliminary Designs prepared 7 more versions:

– BB-65A (a -5 modified for more power but reduced length)

– two uncompleted 10 gun (3-2-2-3) variants; they were not competitive with the 12 guns schemes

Focus now shifted for 9 gun versions again to have the smallest possible, but fully protected battleship (BB65-11, -11A,-12,-13). BB65-13 was the most interesting as a fully protected BB-61 with only 28 knots speed. But the General Board made it’s decision for the BB-65A which had the 212.000 SHP powerplant of the Iowa class and was fully protected against the heavy shell from 18-32.000 yards. Clearly this one seemed to be the best compromise between speed, protection and firepower. Also this design featured first a secondary, lower, inner armour belt as a protection against diving shells – it was a continuous strip of heavy armour placed on the innermost, main torpedo bulkhead.

On the 19th July Congress passed the Two-Ocean Navy act, authorizing construction of 385.000 tons of battleships, ordering BB-65 and BB-66 as Iowa class ships (2* 45.000 tons) and BB-67-71 as the Montana class (5*59.000 tons) to a yet to be finalized design.

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Final designs (November 1940-January 1941):

After the authorization a fresh series were begun, this time with the BB-67 nomenclature and only 4 new versions were born. They stemmed from BB65-5 and -5A and were all just refinenments. First BB-67-1 was a 890 feet version of -5 (vs. 930 feet) and it is speculated that this was in order to comply with some docking restrictions. BB-67-2 was an answer to the further refinement of the Mark 7 16″ guns, as it’s newer mods had the muzzle velocity increased to 2500 feet/sec, meaning that the belt had to be thickened from 15.75″ to 16.1″ to maintain the IZ. At the same time the deck thickness could be relaxed somewhat to 5.8″ from 6.2″ thanks to the flatter trajectory due to the higher speeds. This change cost only 200 tons in displacement, but the after belt, protecting the steering gear box’s connections was eliminated , so standard displacement could be kept at 59.700 tons. Also the underwater protection against diving shells was finalized around this time with a 7.74″ inner belt added onto the 30 pounds (0.75″) thick torpedo bulkhead. Later this was redistributed to 8.4″ tapering to 1.5″ over magazines and 7.1″ tapering to 1″ over machinery spaces. BB-67-3 was a bit trimmed down version of -2, as some weight saving measures saved a few hundred tons as it turned out that the 212.000 SHP Iowa powerplant was more than what was needed for 28 knots (with it the ship was good for 29). So a completely new powerplant with an arrangement reminiscent of the Lexington class BCs and with 172.000 SHP was adopted instead. This also relaxed machinery space length so it can be kept at a level that was desired by deck space needs. Other changes were relatively minor, like the addition of a bomb deck aft. Finally BB-67-4 was the same only it added one more feet of armored freeboard (9 feet) and with this the finalized displacement came in at 60.500 tons standard. This was the design chosen for production and as such members of the fleet have been asked about it.

Generally people were surprised how little the addition of another 15.000 tons over the BB-61 design bought (and it was the same feeling for the Iowa‘s extra 10 ktons over the South Dakotas). Especially critics were picking on the same number of secondary battery barrels where comparison to foreign designs showed a lot more barrels on much more limited displacement – Bismarck and Littorio are given as examples (in fairness all those ships had a split battery with both high angle AA guns and single purpose low angle guns so only the relevant weapons could fire at a given target whereas in case of the Montana all guns could fire on both ships and planes).

Sources:

Norman Friedman: US Battleships, An Illustrated Design History

Norman Friedman: Battleship Design and Development

Dulin & Garzke: US Battleships

+ various internet sources