speed-cone

from The Century Dictionary.

• noun A contrivance for varying and adjusting the velocity-ratio communicated between a pair of parallel shafts by means of a belt.
• noun A cone-shaped form made of light canvas painted black displayed by each vessel when war-ships are steaming in fleet or squadron, to indicate to all other vessels the speed and direction of motion of its propelling engines. The cone is suspended from the yard-arm by signal-halyards in such a way that it may be made to point upward or downward to indicate motion ahead or astern respectively. The height to which the cone is hoisted between the ship's rail and the yard-arm is proportionate to the speed of the engine. In a twin-screw vessel a cone on each side of the vessel may be used to give indication of the motions of the starboard and port engines respectively.