A tender off a Maritime Self-Defense Force transport vessel due to be deployed to Iraq next month crashed into a breakwater at Kure port in Hiroshima Prefecture on Thursday evening.
All 13 crew members on board were injured, four of them seriously.
The 10.9 meter-long launch off the Kunisaki, based at the MSDF's Kure base, crashed into the middle of the breakwater at around 7:10 p.m., according to officials at the Japan Coast Guard Kure office and the MSDF Kure District Headquarters.
On Friday, MSDF officials told reporters that the accident was probably caused by the crew's misjudgment of the vessel's position.
Adm. Koji Kato of the Kure headquarters said there appeared to be nothing wrong with the tender itself.
He said one crewman had been positioned at the front of the boat to act as lookout.
Naoto Yoneda, captain of the Kunisaki, said the accident would probably not affect the dispatch of his ship.
"Once the order is given, we will carry out our mission," he said.
The four seriously injured crew members sustained skull fractures and other injuries, while the remaining members suffered minor injuries, the officials said.
The tender had been anchored about 8 km off Kure and is believed to have crashed into the breakwater, located about 50 meters offshore, on its way back to port.
The sea was rough due to strong winds at the time of the accident, the officials said.