Now that there are "sinkable" ships in Simple Planes, I thought it was high time we had something to sink them with. This is a simple "dumb" torpedo that can be launched from the underbelly of your aircraft. I say "dumb" because while it's self propelled, it can only travel in a straight line.
To attach it to any aircraft download the file and save just the torpedo as a new "sub assembly." Then open the aircraft of your choice and attach the torpedo to the plane using pylons or a detacher. Make sure the pylon/detachter attaches to the body of the torpedo not the warhead itself. Otherwise when you go to release the torpedo, the warhead will simply fall away leaving the torpedo body and engine still attached to the plane. Finally set the pylon/detacher and the torpedo's engine to the same activation group, this way when you go to press 1, 2, or etc the engine will start at the same time the torpedo releases.
You'll need to be low and level to the water when you release the torpedo. Coming in too steep, too high, or at a weird angle, could cause the torpedo to dive deep and get off course when it comes back up to the surface. Worse yet the impact could cause the warhead to separate from the body.

*Note: Because the XML for the prop has been modded, watch for a glitch when setting the engine's activation group. Occasionally it'll pop back to the game's default minimum diameter setting. To fix delete the glitched torpedo and drop in a new one