▁difficult ▁to ▁maintain , ▁and ▁they ▁t ended ▁to ▁leak ▁oil ▁into ▁their ▁gener ators . ▁ ▁In ▁ 1 9 5 6 , ▁the ▁Navy ▁decided ▁to ▁replace ▁the ▁pan ca ke ▁engines ▁with ▁three ▁ten - cy l inder ▁Fair b anks - Mor se ▁opposed - p ist on ▁ 3 8 D ▁ 8 - 1 / 8 ▁dies els . ▁These ▁were ▁similar ▁to ▁those ▁of ▁late - war ▁World ▁War ▁II ▁boats , ▁but ▁u pr ated ▁from ▁ ▁to ▁ ▁each . ▁To ▁accommod ate ▁the ▁larger ▁engines , ▁the ▁boats ▁had ▁to ▁be ▁length ened ▁some ▁nine ▁feet ▁in ▁the ▁engine ▁room ▁( three ▁additional ▁frames ▁between ▁frames ▁ 6 9 ▁and ▁ 7 0 ). ▁ ▁According ly , ▁in ▁ 1 9 5 7 ▁and ▁ 1 9 5 8 , ▁the ▁first ▁four ▁T angs ▁were ▁length ened , ▁while ▁G ud geon ▁and ▁Hard er , ▁still ▁on ▁the ▁ways , ▁were ▁built ▁to ▁the ▁new ▁length ▁with ▁the ▁new ▁engines . ▁This ▁prop ulsion ▁plant ▁was ▁used ▁for ▁almost ▁all ▁subsequent ▁US ▁conventional ▁sub mar ines . ▁ ▁The ▁tor ped o ▁tub es ▁were ▁also ▁red es igned . ▁The ▁six ▁forward ▁tub es ▁now ▁used ▁air - power ed ▁p ist on ▁e jection ▁p umps , ▁which ▁forced ▁a ▁sl ug ▁of ▁water ▁through ▁a ▁slide ▁val ve ▁behind ▁the ▁tor ped o ▁to ▁push ▁it ▁out , ▁rather ▁than ▁the ▁pul se ▁of ▁air ▁used ▁in