The Screw Mobile (スクリューモービル) is the second boss boss of Sonic the Hedgehog 3, fought at the end of Hydrocity Zone Act 2.
This boss is similar to the Big Shaker mini-boss. As the player enters the arena, the water level drops to the floor and Dr. Eggman/Eggrobo flies in, his craft fitted with an impeller underneath and depth charges at the back. He flies from right to left above the player, where they cannot reach him by jumping, but periodically lowers himself to the water (except for with Knuckles), revving up his impeller. During this time, it is possible to jump and hit the front of his machine, but when he hits the water, the player will want to be as far away as possible.
Upon touching the water, he begins to draw it toward him, while slowly rising. This draws the water into a plume beneath him. If the player does not move to the edge of the screen and hold this direction, they will be drawn into it. Being drawn into this plume will immobilize the player and may cause damage, should they make contact with the impeller. When Eggman releases the water, he moves off and the plume begins to recede gradually. The top of the plume is solid and can be jumped on to reach Eggman's machine, particularly as it drops to the water's surface. The player must be quick though, for the plume will settle and disappear about half a second after it stops on the water. As he flies across the top, he will drop some depth charges which, while damaging in the water, create a explosion that can launch the player high into the air, allowing an attack on Eggman. After eight hits, his craft explodes.