A method and arrangement for reducing the weight and optimizing the longitudinal strength of a watercraft, which is especially suitable for transportation of liquefied natural gas (LNG) or other corresponding mediums and comprises a hull, which has a deck extending over at least the main part of the watercraft and a number of substantially spherical cargo tanks arranged successively in the longitudinal direction of the watercraft and a deckhouse, which extends substantially above the deck. The hull of the watercraft is provided with a continuous protective casing structure known as such and which is arranged on top of the cargo tanks. The deck of the watercraft is arranged on the hull so that the proportion of its height measured from the bottom of the watercraft to the height of the uppermost continuous portion of the protective casing structure on top of the cargo tanks is at most 0.55, preferably at most 0.5. In addition, the protective casing structure is fixed to the deck and/or other structures supported to the hull and is dimensioned together with the other parts of the hull so that together they constitute an essential part of the overall strength of the watercraft.