Doctor Robotnik’s Waterspout and Depth Charge Machine (スクリューモービル,Sukuryū Mōbiru?, lit. "Screw Mobile"), alternatively spelled as Doctor Robotnik’s Waterspout & Depth Charge Machine, is a boss in Sonic the Hedgehog 3 that the player encounters at the end of Hydrocity Zone Act 2. It takes eight hits to be destroyed. The battle takes place in a flat arena with water just deep enough to cover Sonic, threatening to drown him.
The Waterspout & Depth Charge Machine is piloted by Dr. Robotnik (or by EggRobo, if the player is using Knuckles in the locked-on game), which is a version of the Egg Mobile equipped with an orange propeller spinning attachment and depth charge dispenser behind it, which drops small pipe bombs.
In the standalone version of Sonic the Hedgehog 3, Doctor Robotnik will descend into the water while spinning the propeller, which will whip the water into a vortex. The player must keep jumping away from this vortex 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. The Waterspout & Depth Charge Machine will then slowly fly from side to side three times, dropping eight pipe bombs as it goes which explode after a delay of approximately two seconds. After this, the doctor repeats this attack pattern regularly.
The Waterspout & Depth Charge Machine can be hit while 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 pipe bombs, which by standing over one and jumping at the last moment, can be used to propel the player's character into the air, enabling them to hit the boss again. Eight hits are required to destroy the machine and open the capsule to the right of the arena in order to clear the Zone.
In Sonic the Hedgehog 3 & Knuckles, Knuckles fights this vehicle piloted by EggRobo after entering the arena through a different pathway. The attack patterns remain the same as the previous version of the fight, but EggRobo is able now to create the vortex of water with the Waterspout & Depth Charge Machine without descending, presenting fewer opportunities for Knuckles to attack. Knuckles is also unable to reach the Waterspout & Depth Charge Machine from the descending water vortex due to his lower jump height, so he must utilize the bombs to hit the mobile eight times.
Like in the Whirlpool Machine fight, if the player stays in the water long enough until the drowning music starts playing, the mid-boss BGM from Sonic & Knuckles will start playing at the background instead of regular boss theme. This glitch also works in the locked-on version of the game.
There is a glitch in Sonic 3 & Knuckles, where 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 within a few seconds.
If caught in Robotnik's vortex, Sonic, Tails, or Knuckles enter their death animation while being sucked up towards the ship's propeller.