Stern suction

Stern suction is like when you blow on a straw, but instead of blowing out, you suck in. It's the same thing that happens when you suck on a popsicle stick or straw and it sticks to your tongue.

On a boat, stern suction happens when the boat is moving forward and the water is flowing past the back of the boat (the stern). This creates a sort of low-pressure area behind the boat. When you're swimming behind the boat, or when something is thrown into the water behind the boat, it gets pulled towards the boat because of this low-pressure area - like when you suck on the straw and the drink gets pulled towards your mouth.

This can be dangerous for people swimming behind the boat, as the suction can pull them towards the boat and potentially into the propeller. It's important to stay away from the back of the boat when it's moving, to avoid the risks associated with stern suction.