This is a http://en.wikipedia.org/wiki/Kort_nozzle for a boat or submersible propeller. The purpose of this is to increase the thrust at low speeds. These are commonly used in commercial ROVs.The profile is Maritime Research Institute Netherlands (MARIN) No. 37, which sacrifices some forwards thrust for better backwards thrust.The inner radius of the nozzle is 10mm, can be scaled to fit any propeller size. The wider end should point to the front.The profile curve was traced by hand from a GIF image, so it's not hyper accurate, but should be pretty close to the original profile.In practice one would also add mounting points, but since those depend on what this would be mounted on, I didn't include any (yet).Edit: added a version with 62mm inner diameter and a mounting bracket with two 5mm boltholes. The bracket is 20mm by 50mm and the boltholes are 62mm away from the center of the nozzle. This version is suitable for a ROV vertical thruster with 60mm propeller.