Blade Area Ratio Defined
A HydroComp Technical Report
Report 135
Overview
Blade area
ratio, or BAR, is a parameter used to relate the size of a propeller
blade to its diameter. It is critical to the control of cavitation
and changes to BAR affect its efficiency and thrustmaking performance.
The generic
term or "BAR", however, does not sufficiently describe
the blade area ratio. In fact, there are three types of BAR 
Projected, Developed and Expanded.
Calculation
of area ratio
For all three
types, the appropriate total blade area outside of the hub is
divided by the propeller disk area (e.g., pR2) to derive the area
ratio.
Definitions
The three
graphics shown below illustrate the three different types of BAR
for a common propeller.
Projected
Area Ratio (PAR)
The Projected
view is the one you see when you look down on the propeller. The
Projected area is the area of the outline as projected onto a
surface below. Projected area ratio is the smallest of the three.
Developed
Area Ratio (DAR)
Developed
area is the area of the blade outline if it could be untwisted
(i.e., as if the whole blade were unattached from the hub and
brought to zero pitch).
Expanded
Area Ratio (EAR)
Expanded area
is what if found if the Developed area could be flexibly unwrapped
on a flat surface so that all sections were parallel. Expanded
area is what is important to propeller designers, to treat the
propeller blade like a wing. In other words, the Expanded view
converts the propeller from its helix to a flat plane.
Expanded area
ratio is typically close in magnitude to Developed area ratio,
and is often used interchangeably.



Projected 
Developed 
Expanded 
Conversion
You can use
the the following formula to find approximate conversions between
the three types of BAR.
D = diameter
P = pitch
Z = number of blades
