The 'Tench' class marked the ultimate refinement in the basic design whose ancestry could be traced back to the 'P' class. Externally they were virtually identical with the 'Balaos', and so closely related was the design that some of the later Balao contracts were converted to 'Tenches'. Though 25 boats had been completed by the end of hostilities, most were still working up in home waters; not a dozen, therefore, managed to see operational duty and none of these was lost. Total production was 33 boats between 1944 and 1946, with another 101 cancelled or scrapped incomplete.
Differences, though not obvious, were important. The first concerned machinery. In the 'Balao' class the four diesels each ran a direct-coupled generator, which served both to charge batteries and power the electric propulsion motors when surfaced. Each shaft had two motors, coupled to it via reduction gearing. Both the highspeed motors and the reduction gear were noisy (to the extent where it was fortunate that Japanese ASW techniques and equipment were so backward). Reduction gears were also expensive, temperamental, easy to damage and, traditionally, a slow delivery item in the USA (as was the turboelectric propulsion in battleships), It made sense, therefore, to develop a large and slow-turning motor that could be direct-coupled. Two of these larger units, with no associated gear housings, could be accommodated without the earlier awkward crank in the hull, but illustrates some of the problems facing submarine designers.
Fuel and ballast tanks were better organized, firstly to obviate the need to lead the vents of the ballast tanks through the pressure hull (where they constituted a flooding hazard) and, secondly, the better to compensate for the considerable change in weight and trim as stores were consumed during a long patrol. Even a further four torpedo reloads were squeezed in, and this, combined with radar and efficient mechanical fire-control computers, put the 'Tenches' far ahead of the opposition. In order to improve on the average, but slow, diving times of 55-60 seconds, the casings were pierced with many more lightening holes