Ship's hull under construction is supported by blocks and/or supports. During hydro-static test, a huge number of additional supports are required to withstand water-weight of thousands of tons. On the other hand, since a ship may be considered to be an elastic body on elastic foundation, the effect of load shoud be extended beyond a tank flooded. From this it follows that the amount of supports under the tank may be reduced. Furthermore, if both block and support are used together, each allowable stress for estimation should be modified. In this paper, based on the experimental result and theoretical investigation, a method to find the most favourable number of support will be proposed.