Heavy running occurs when engines reach the maximum continuous rating before they reach full rev/min. FP propellers can suffer heavy running due to propeller/hull fouling, engine ageing, too high pitch on the propeller, change in normal operating conditions or modifications to the ship. The propeller is usually designed according to the trial curve, i.e., at 100% rev/min the absorbed engine power should be from 85% to 90% MCR. Ships in service operate according to the service curve which is significantly above the trial curve.