I think it's the fact the little boat has a MARIN flag, sort of like a "Hey, look we are doing things!".

I'm no expert, but after some searching, I found this paper (I hope I got the link right, I'm not an expert at them either): www.mdpi.com/2077-1312/6/2/34/pdf Anyway, it says: "Single-screw ship wake fields are usually characterized by a strongly non-uniform distribution of velocities with a wake peak at the 12 o’clock position, where the axial velocities are particularly low. This means that the blade sections of a propeller operating behind the ship experience strong variations in angle of attack. As the hydrostatic pressure acting on the blade reaches its minimum at the same time as the blade experiences high angles of attack while passing through the wake peak, this region of the wake field is particularly critical in terms of cavitation." Note, by "wake field", they are not talking about the waves the ship produces. They are talking about the water that the ship drags with it as it moves. Let me rephrase the above as best I can: There are two reasons cavitation is worst at the top of a ship propeller. 1) The water is moving slower (relative to the ship/propeller) at this location. 2) Being closer to the surface, the water pressure is lower at the top of the propeller. Expanding on (1): When a ship moves through water, it drags some of the water with it. By the time the water gets to the propeller, the water near the top of the propeller has been pulled along with the ship more than the water near the bottom of the propeller. I'll just make up some numbers here. Maybe the ship is going 18 knots. The bottom of the propeller is a long way away from the hull. So the hull doesn't effect it too much. The water coming into the bottom of the propeller might hit the propeller at 17 knots. But, near the top of the propeller, the ship's hull has really been in the way of the water. The water might hit the top of propeller at only 13 knots. Thus the top blades have to push harder to get the water up to speed, and this makes cavitation more likely. NOTE: I DON"T KNOW IF THE WATER IN THE VIDEO IS APPROACHING AT DIFFERENT SPEEDS Now about (2): Cavitation happens when the local pressure gets too low. As you go deeper in water, the surrounding pressure gets higher. Thus, the deeper a propeller is run, the less likely it is to cavitate. On a large ship propeller, there could be a significant difference in pressure between the top and bottom of the propeller.

@@andrewsnow7386 Thank you for the explanation!

500m max depth