The Screw Mobile (スクリューモービル,Sukuryū Mōbiru?) is a boss in Sonic the Hedgehog 3, that the player has to fought at the end of Hydrocity Zone Act 2. Screw Mobile takes eight hits to be defeated. The battle takes place in a flat arena with water just deep enough to cover Sonic, threatening to drown him.
In standalone Sonic the Hedgehog 3, Doctor Robotnik 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 spinning. 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 three times, dropping eight depth charges as it goes which explode after a delay of approximately two seconds. After this, the doctor repeats his attack pattern regularly.
The Screw Mobile can be hit whilst descending with the propeller turning. 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 depth charges, which by standing over one and jumping at the last moment, the explosion will propel the player's character into the air, enabling him or her to hit the boss again. Eight hits are required to destroy the machine and open the capsule at the right of the arena to clear the Zone.
In the locked-on version of Sonic the Hedgehog 3 & Knuckles, Knuckles fights this vehicle piloted by EggRobo after getting through the other pathway, while the arena itself isn't different. The attack patterns remains the same as with Sonic and Tails fight, but EggRobo is able now to create the vortex of water with the Screw Mobile without descending, presenting fewer opportunities for Knuckles to attack. Knuckles is also unable to reach the Screw Mobile from the descending water vortex to due his lower jump height, as he has to utilize the depth charges to hit the mobile eight times.
Like in the Big Shaker fight, if the player stays at the water enough until the drowning music starts playing, the mid-boss BGM from Sonic & Knuckles starts playing at the background instead of regular boss theme. This glitch also works in the locked-on version as well.
There is a glitch in locked-on version of Sonic 3 & Knuckles, whereby if Super Sonic is caught in the vortex, Super Sonic will jump and land on the top of the vortex as if it were not attached; due to Super Sonic being invincible, he doesn't take damage, allowing Robotnik to be defeated in a few seconds.