Is this true? As far as I know sea ships are designed for a specific speed. Especially when they have a bulbous bow. Those bows are very inefficient at lower or

Is this true? As far as I know sea ships are designed for a specific speed. Especially when they have a bulbous bow. Those bows are very inefficient at lower or higher speeds. Edit: what I mean: I can imagine a speed reduction reduces emission by 20%. But when travel times will be over 20% longer nothing is gained.