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But WHY is it a maximum speed for displacement boats in quiet waters, responding to this old formula? It is because the speed increases, the water displaced forms waves. At slow speed there are several along the hull. At fast speed there are only two: one at the bow and another an the stern. If the boat tries to go faster, the stern wave would go more farther but the hull would lose sustentation. It better explained with a picture. Look for boats going fast. You will see only two waves.