The distance from the propeller to the hull has a linear influence on the pressure impulses. A reduction of this clearance by 50% doubles the pressure impulses. To account for the ice class it is necessary to increase the thickness of blade sections. This increases the propeller induced pressure impulses. To avoid transferring vibration to the hull, the distance from the blade to the different part of the hull must be kept above certain minimum limits. Guidance figures for open propellers with moderate power density and no special noise requirements apply a tip clearance which is 20-25% of the propeller diameter for non-skew propeller or 15-20% of D for high skew propeller. The lower values can be used for ships with slender aft body and favourable inflow conditions whereas full after body ships with large variations in wake field require the upper values to be used.
For ferries and cruise liners the tip clearance is normally 25-30% of D.
Base line clearance minimum is 50-100mm. In twin-screw ships the blade tip may protrude below the base line.