Noise testing during 1996 and 1997 found that the hydrodynamic noise signature—the noise made by a submarine passing through the water—was excessive, particularly at high speed. The shape of the hull was the main cause: although a scale model of the design had been tested during the funded study and was found to have a minimal signature, the hull shape was changed after the contract was signed, primarily by a lengthening of the submarine and a redesign of the bow dome to accommodate the larger-than-expected main sonar and reduce its blind spot (the baffles). The design had not been retested, as who would pay for this could not be agreed on. Propeller cavitation, caused by water flow over control surfaces onto the propeller at certain speeds, was the other main noisemaker. Cavitation had not been a problem with earlier Swedish submarine designs or during early testing of the Type 471 design, but the propeller had to be redesigned late in the process to provide more power, and like the redesigned hull, was not retested.