Thrust is a propulsion force exerted on a craft that enables the craft to sail. The thrust deduction fraction is a conventional definition, which relates the hull resistance to the propulsor thrust. This fraction always has positive values for conventional propellers but for waterjets this fraction becomes negative in some specific speed range.
Arash Eslamdoost has developed two techniques for understanding the physics behind the waterjet/hull interaction and specifically the negative thrust deduction fraction. The first technique is based on a potential flow code and the other is based on a Reynolds-Averaged Navier Stokes solver.
The research demonstrates that the thrust deduction fraction is dependent both on the hull resistance increment and the waterjet system influence on the flow around the hull. It is revealed that the contribution of each of these effects to the thrust deduction fraction varies a lot depending on the operating speed range. In the intermediate and the high-speed range, it is almost just the resistance increment, which contributes to the thrust deduction fraction, but in the low speed range both the jet system and the hull resistance increment contribute to this fraction.
Arash research indicates that the difference between the net thrust and the gross thrust of the waterjet system is the reason for the negative thrust deduction. This is mostly due to the non-atmospheric pressure distribution at the nozzle exit which can be due to the nozzle geometry or the submergence of the nozzle in the transom wave.