Shortening Salah on a ship anchored in the middle of the sea

A: If a passenger ship stops out at sea in one place for some reason, for more than four days, those on board must perform all the Salah in full not shorten them, and each Salah should be performed in its due time. However, if they stay there for four days or less, or those on board do not know when the ship will leave that place, it is permissible to shorten and combine Salah in congregation.