Seems an odd arrangement, nine boilers serving four sets of turbines, so the only way to develop full power would be to have them all operating as one unit. This of course raises the possibility of a hit or rupture anywhere in the system venting steam from the entire plant. In most large ships of the era, the number of boilers was a multiple of the number of engines, to allow split-plant operation. Or one could have an arrangement like the French Richelieu, six boilers and four shafts, so they could at least operate as two independent units. Even with all the boilers concentrated forward of the engine rooms, steam lines could be split by closing the appropriate valves. One might argue that with nine boilers, there would often be one down for routine maintenance, but on the day of battle one should be able to use all available power. In the WWI era and earlier, boilers were smaller, ships had as many as 42 and someone will probably cite an example of more; but by the 1930s new or reconstructed capital ships or cruisers had 4-12, serving 2, 3, or 4 shafts. I can't think of another apparent mismatch like the Yorktowns - anyone? - or a good reason for the arrangement. p.s. Many destroyers in various navies had three boilers for two shafts.