▁days ▁of ▁W W II . ▁Y RC - 5 ▁was ▁ab o ard ▁the ▁USS ▁W idge on ▁during ▁the ▁attack ▁on ▁Pear l ▁Har bor ). ▁In ▁ 1 9 3 1 , ▁a ▁one - fif th ▁scale ▁model ▁of ▁a ▁div ing ▁bell ▁for ▁sub marine ▁rescue ▁work ▁was ▁built ▁and ▁tested . ▁Design ▁called ▁for ▁the ▁bell ▁to ▁with stand ▁the ▁external ▁pressure ▁encountered ▁at ▁a ▁depth ▁of ▁at ▁least ▁ ▁of ▁water , ▁and ▁the ▁test ▁showed ▁the ▁model ▁ful filled ▁this ▁requirement ▁with ▁a ▁factor ▁of ▁safety ▁of ▁about ▁ 3 . 5 . ▁The ▁vessel ▁was ▁tested ▁under ▁external ▁pressure , ▁failure ▁occurr ing ▁in ▁the ▁shell ▁at ▁a ▁pressure ▁of ▁. ▁Since ▁the ▁head ▁of ▁the ▁vessel ▁remained ▁int act , ▁it ▁was ▁decided ▁to ▁make ▁a ▁test ▁of ▁the ▁head ▁itself ▁in ▁order ▁to ▁determine ▁its ▁strength ▁relative ▁to ▁that ▁of ▁the ▁shell , ▁and ▁if ▁possible ▁to ▁obtain ▁some ▁measure ▁of ▁the ▁str esses ▁occurr ing ▁under ▁load . ▁The ▁head ▁col lapsed ▁at ▁a ▁pressure ▁of ▁, ▁indicating ▁its ▁strength ▁under ▁external ▁pressure ▁was ▁about ▁ 1 0 % ▁in ▁excess ▁of ▁that ▁of ▁the ▁shell . ▁ ▁The ▁re vised ▁Sub marine ▁Res cue ▁Chamber ▁had ▁improvements ▁including ▁a ▁soft ▁se al ▁g asket ▁for ▁se aling ▁the ▁sub marine / bell ▁interface ▁sk ir ting , ▁and ▁a ▁floor ▁installed ▁to ▁maintain ▁air - space ▁in ▁the ▁bell ▁during ▁raising ▁and ▁lower ing . ▁M oms en ▁in ▁his