The transverse strength of ferro-cemet boats which have typical transverse frames named “Steel frame” or “Truss frame” has been investigated experimentally and analytically. Three phases of critical state have been observed in the flexure tests of straight ferrocement boards. They are defined as follows ; i) proportional limit, ii) watertight limit, and iii) collapse. i) is the state where a crack of the mortar begins to propagate in the plate. ii) is the state where the crack penetrates the plate. The criteria of crack tip strain are set up for the states. The criteria are applied to the analysis of the strength of the boat models subjected to water pressure, and prove to have a good correlation with experimental results. The results show that the models have cracks even at full load draft, and are doubted if they keep watertight until the draft reaches deck height, but maintain much surplus strength until they collapse.