The cross girder system with large centre girder and bottom transverses, has been one of the most common type of bottom structure in centre cargo oil tanks of tankers. It is not sure, however, that this construction system is suitable one for tankers with long tanks. Calculating the reaction forces acting between the centre girder and bottom transverses, the following conclusion is obtained : Judging from the weight of material required, the bottom structure constructed only with transverses is the optimum one for cases of long tanks having aspect ratio over 1.0. As it is required for the centre girders to have sufficient strength to resist loads transmitted from keel-blocks in a dock or on a building berth, it is rather difficult to employ this type of construction to actual ships. In this paper, the loading condtion is also analyzed, and it is clarified that bottom structures which are simlar to the one without the centre girdre may be used for actual ships, because the minimum scantling of the centre girder required to support the load is very light.
