 Devonport Dockyard is the UK 's only naval base that refits nuclear submarines and the Navy estimates that the Dockyard generates about 10 % of Plymouth 's income . Plymouth has the largest cluster of marine and maritime businesses in the south west with 270 firms operating within the sector . Other substantial employers include the university with almost 3 @,@ 000 staff , as well as the Tamar Science Park employing 500 people in 50 companies . Several employers have chosen to locate their headquarters in Plymouth , including Hemsley Fraser . 
