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PSModel
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19' Ski Boat

34' Sailboat

41' Utility Boat

Technical

Blade Area
Ratio

Calculating
the Cavitation
Number

Horsepower
Losses

Hull Speed

Kt Breakdown

Propeller
Geometry

Propeller Law

Wake Factors

Glossary

References

Blade Area Ratio:

Blade area in a Propeller Series is expressed by one of two ratios: Expanded Area Ratio (EAR) or Developed Area Ratio (DAR).  The difference between EAR and DAR is usually very small, and is due to the way blade area is measured in a propeller drawing.  Both EAR and DAR are ratios of total blade area divided by the propeller disc area.  Since the difference between EAR and DAR is usually small, a generic term called Blade Area Ratio (BAR) is used in PSModel to refer to these area ratios. 

The following example shows how to calculate the BAR for a 3 bladed, 12 inch diameter propeller with each blade face having a face area of 15 square inches:

Total blade area = 3 × 15.0 = 45.0 square inches

Propeller disc area = 3.142 × (R2) = 3.142 × (62) = 113.11 square inches

EAR = 45.0/113.11 = 0.398

DAR is related to EAR via the following equation:

EAR = 0.34 × (2.75 + DAR/Z) × DAR

or

EAR = 0.935 × DAR + 0.34 × DAR2/Z

where Z is the number of blades

An estimate of the total blade area for a small propeller can be obtained as follows:

1) Place a sheet of graph paper on the side of a table (the graph paper should have a minimum of 2 grid lines per inch horizontally and vertically).

2) Place the leading edge of a propeller blade on the graph paper with the propeller hub pressed against the side of the table.

3) Roll the blade from the leading edge to the trailing edge, and as you roll, have a friend trace the blade outline with a pencil.  Alternatively, place the propeller on the table and lay the graph paper on top of a blade and then trace the blade outline.

4) Count how many squares on the graph paper equal one square inch, then count the number of squares within the blade outline just traced.

5) blade area = (number of squares within the blade outline) / ( number of squares that make one square inch on the graph paper). 

6) Total blade area = (blade area) × (number of blades)