Katsumi KUROITA, based on an investigation conducted by Tokyo Imperial University's naval hydrographic department into the tidal currents on May 2, 1185, proved that the westward tidal current in the Kanmon straits started flowing eastward at 8:30 in the morning, reaching a speed of eight knots by around 11 AM, and returned to flowing westward at around 3 in the afternoon; asserting that the times given in the "Gyokuyo" for when the battle was fought (from noon to 4 PM) were correct and thus that the battle took place in the afternoon, he further stated that as long as the current was flowing east the Taira clan had the advantage, but after it switched back to flowing west, there was a reversal of fortunes and the Minamoto clan gained the upper hand.
黒板勝美は大日本帝国海軍水路部の元暦2年3月24日（ユリウス暦で5月2日）の関門海峡の潮流の調査を元に、午前8時30分に西への潮流が東へ反転して、午前11時頃に8ノットに達し、午後3時頃に潮流は再び西へ反転することを明らかにし、合戦が行われた時間帯は『玉葉』の午の刻（12時ごろ）から申の刻（16時ごろ）が正しく合戦は午後に行われたとして、潮流が東向きだった時間帯は平氏が優勢で、反転して西向きになって形勢が逆転して源氏が優勢になったとした。