Jet thrust deduction

Jet thrust deduction is the loss of net thrust a waterjet installation suffers from interaction with the hull, the analogue of the screw-propeller thrust deduction t. The jet’s intake ingests the hull boundary layer, the inlet ramp adds drag and changes trim, and the transom flow field is modified, so the gross jet thrust is not all available to drive the ship. It is expressed as a thrust-deduction-like factor (1 - t) on gross jet thrust, with t often near zero or slightly negative because intake suction can recover momentum, unlike a screw whose race lowers stern pressure. The ITTC waterjet procedure splits intake, pump, and interaction terms rather than a single lumped t.