﻿ Propeller Law
 PSModel a look inside HOME Introduction Quick Start Data Input Data Output Main Menu Main Screen Shortcuts Testing: Test Sheet Examples Technical Horsepower Losses Hull Speed Kt Breakdown Propeller Law Wake Factors Glossary References Propeller Law: The Propeller Law is an idealized set of assumptions that can be used for boats operating in the displacement mode below the Hull Speed. These assumptions are: 1) boat resistance is proportional to boat speed squared, Resistance = C1 x MPH2 2) propeller thrust is proportional to boat speed squared, Thrust = C2 x MPH2 3) propeller RPM is proportional to boat speed, Propeller RPM = C3 x MPH 4) propeller torque is proportional to RPM squared, Propeller Torque = C4 x RPM2 5) horsepower absorbed by the propeller is proportional to RPM cubed, Propeller HP = C5 x RPM3 The constant, Cn, can be found for any of the equations above if the two other values in the equation are known.  For example, if PSModel predicts a propeller thrust of 50.0 lbs at a boat speed of 8.0 MPH then C2 can be determined as follows:  C2 = 50.0/8.02 = 0.78125 Once C2 is known, the entire curve representing MPH vs. Thrust can be plotted. 