Doctor Robotnik's Waterspout and Depth Charge Machine

The Screw Mobile is a boss from Sonic The Hedgehog 3. It is fought at the end of Hydrocity Zone Act 2. The battle takes place in a flat arena with water just deep enough to cover Sonic.

Playing as Sonic or Tails
Eggman will descend into the water with the propeller spinning which will whip up the water into a vortex. The player must keep jumping to the side of the arena to avoid being sucked in. The propeller itself can also hurt the player if touched while turning. After a few seconds, the propeller stops, causing the vortex to subside. Then the Screw Mobile will slowly fly from side to side of the arena, dropping eight bombs as it goes which explode after a delay of approximately two seconds.

Playing as Knuckles (in Sonic 3 & Knuckles)
The attack pattern remains the same as with Sonic and Tails, however the Screw Mobile can now create the vortex of water without descending, presenting fewer opportunities to attack.

Defeating the Screw Mobile
The Screw Mobile can be hit whilst descending with the propeller turning (unless playing as Knuckles, as mentioned above). However to make the fight go faster, the player has two other options. The subsiding vortex actually acts as a temporary platform which the player can use to reach the mobile and score another hit or two. There are also the mines. By standing over one and jumping at the last moment, the explosion will propel the character into the air, enabling him or her to hit the boss again. Eight hits are required to destroy the machine.