An installation vessel has upper and lower decks (40, 50) that are vertically spaced about 3 meters apart, with most of the lower deck covered by the upper one, but with a rear portion (60) of the lower deck being uncovered. The upper deck is used primarily for storage, while the lower deck is the one used for installation of anchor chains, wires, ropes, etc. that pass from a winch (81) to a stern roller (84) at the rear of the lower deck, so dangerous conditions of chains, wires, ropes, etc. moving along a deck during installation are confined to the lower deck. A main crane (72) that lies on one side of the vessel, lies forward of the rear end of the upper deck at a location wherein the crane can reach all portions of the uncovered lower deck portion and most of the upper deck.