The state election watchdog said Sunday that shipboard voting for the 21st presidential election will run Monday through Thursday.

According to the National Election Commission, voting will be held for three-thousand-51 sailors aboard 454 ships captained by South Koreans, including deep-sea fishing vessels. 

Shipboard voting, introduced in the 2012 presidential election, takes place during presidential elections and National Assembly elections. 

The date and location are decided by the captain of the vessel during the voting period, and at least one impartial South Korean national must be appointed as an observer.

Shipboard voters must vote in the presence of the observers at the polling stations on their ships, and the ballot papers must be sent directly to the relevant election commissions by fax.