Right after the war began, the former bakufu fleet led by ENOMOTO was anchored offshore near Mt. Tempo in Osaka, but when the army of former bakufu was defeated in the Battle of Toba-Fushimi, Yoshinobu and his followers, who had been in Osaka Castle, boarded and commanded the flagship ‛Kaiyo' to withdraw to Edo without notifying the shogun's retainers of the war party (in order to distinguish a warship from a troopship, "Maru" should be put only on a troopship; therefore, ‛Kaiyo Maru Warship' is inaccurate in this case).
開戦直後、榎本の率いる旧幕府艦隊は大坂の天保山沖に停泊していたが、鳥羽・伏見の戦いで旧幕府軍が敗北すると、大坂城にいた慶喜らは、主戦派の幕臣に無断で旗艦「開陽」に座乗し江戸へ引き揚げた（軍艦と輸送船を区別するため"丸"を付すのは輸送船のみとされており「開陽丸」は誤りである）。