Predicting the Inception
of Ventilation
A HydroComp Technical Report
Report 134
Overview
Propeller
ventilation is when surface air is drawn into the propeller disk.
This can occur when a propeller does not remain submerged dynamically
during motions or statically at light draft.
This report
introduces a method to predict the inception of ventilation, which
was derived from a publication from staff at Shanghai Jiao Tong
University, China. The published method has been revised to make
it suitable for general use.
Prediction
method
The inception
of ventilation correlates to thrust loading. Greater thrust means
more suction side "vacuum", which in turn, means a greater
likelihood of ventilation.
In the referenced
publication, the prediction of the inception of ventilation relied
on the relationship between pitch, RPM and advance velocity as
a measure of thrust loading. The implementation of the method
herein converts the relationship so that a limiting "critical"
speed of advance, V_{ACRIT}, is used as the indicator
of ventilation. Nonventilating performance is maintained when
V_{A} > V_{ACRIT}.
Variables
D = diameter [ft; m]
G = gravitational constant [32.2 ft/s^{2};
9.81 m/s^{2}]
H = hub immersion below WL [ft; m]
n = shaft speed [revs/s]
P = pitch [ft; m]
R = radius [ft; m]
V = ship speed [ft/s; m/s]
w = wake fraction
Formula
K = tip speed parameter
V_{A} = speed of advance [ft/s; m/s]
V_{ACRIT} = critical speed of advance [ft/s; m/s]
Range of applicability
The prediction
method is suitable for the following ranges of parameters.
H/R = 1.0 to 1.5
K = 0.05 to 0.44
Example
This example
checks if ventilation is predicted to occur if the propeller tip
should reach the surface during a particular operating condition.
D = 2.50 ft
G = 32.2 ft/s^{2}
H = 1.25 ft
n = 13.0 revs/s (shaft speed)
P = 2.50 ft
R = 1.25 ft
V = 20.3 ft/s (12 kts)
w = 0.06
H/R = 1.0
K = 0.076
V_{A} = 19.1 ft/s
V_{ACRIT} = 20.0 ft/s
Conclusion:
V_{A} is less than V_{ACRIT}, so ventilation
is predicted to occur.
References
Wang Guogiang,
et al., "Propeller Air Ventilation and Performance of Ventilated
Propeller", SHL Shanghai Jiao Tong University, China
