A laboratory study was performed to measure the pressures caused by fluid acceleration beneath a floating parallel-piped block. Dynamic fluid pressure was measured at discrete points beneath the block for various fluid velocities, block angles of attack and block thickness-to-depth ratios. Some of these pressures tended to stabilize the block while others tended to underturn it. The measured pressures were used to calculate block underturning moments and a hydrostatic analysis was used to calculate a block righting moment. From this, a densimetric Froude underturning criteria is presented.