龙首战舰是十二世纪丹麦舰队的主力战舰。
Dragon Boats are the command ships of12th century Viking fleets .
潜艇作为海军作战的两大主力战舰之一，其下潜深度的提高一直是人们追求的目标。
As one of the two most important battleships of navy , the diving depth increase of submarine is always the pursuit .
主力战舰