air is then forced into the bands and escapes through machined perforations to create a barrier of air bubbles in the sea about the hull, thus trapping machinery noise within the hull where it is dissipated. Prairie Prairie (acronym: propeller air-induced emission ) on the other hand, is fitted either near to or on the ships propellers and compressed air is pumped through small holes in the edges of the propeller. As might be expected, keeping a set of small holes clear of fouling under the surface of a ship or submarine can present a problem. On submarine installations this was prevented by running a trickle of fresh water through the system when the ship was in port. Most of the creatures that foul an ocean-going ship's hull cannot survive in fresh water. A similar system called Agouti was fitted to Royal Navy frigates and such vessels as . Submarines In submarine use the large volume of air needed to operate the system needs to come from the surface, and therefore Prairie-Masker can only be used when the diesel submarine is snorkeling. This is not a serious limitation since the major noise source that needs quieting are the diesel engines, which are used only when snorkeling. When used by an antisubmarine warship, it is so effective in hiding its acoustic signature that often they are requested not to use the system when exercising with submarines. This is because of the possibility of a collision with the submarine which might come at or near the surface in close proximity to the warship without hearing it. See also Anechoic tile References External links Prairie/Masker system description at FAS Picture of propeller testing Prairie installation in drydock Equipment of the United States Navy