Notes and Reflections on Books and Media
by Hannah Leitheiser
World of Sound
"there is another way in which sounds may be made in the water which is very striking. The effect occurs whenever a body is moved so quickly through the water that it leaves a cavity behind it which the water from the sides has not time to fill up as fast as the body makes it. It is not filled with air, because the cavity is entirely under water. The rapidly revolving screw of a steamer often makes cavities of this kind. Their chief characteristic is that, when they are fully closed up again by the water crowding in from all round, there is no cushion of air to break the force of the blow when the sides meet each other. The blow has all the suddenness of an explosion. A sharp impulse given to the water in this way travels well and far and is, as we shall see later, the basis of the noise made by moving steamships. The effect is well known to naval engineers, because it often has a destructive effect on ship propellers ; the blows are as violent as if they had been struck by hammers." - World of Sound
Had just assumed propeller degradation was due to...I guess...erosion, but it seems they move fast enough to generate a vacuum, and so they develop impact damage.