Mean Width Ratio & Disc Area Ratio

Formulae

• MWR = average blade width / diameter
• DAR = ( Pi x (diameter squared)) / 4

Our example vessel with 14 inch diameter prop and standard 33% DAR prop gives us 100% DAR of Pi x prop radius squared, which we divide by 3 for 33% DAR and find that 51 square inches of projected blade area are required.

MWR is therefore DAR / (0.51 x number of blades) = 0.22 MWR

Multiply MWR of 0.22 by Prop dia of 14 to arrive at average blade width of 3 inches.

Projected Blade Area & Developed Blade Area

Formulae

• Ap/Ad = 1.0125 - ( 0.1 x Pitch ratio ) - ( 0.0625 x ( Pitch ratio squared )

We take the 51 square inches of required blade area and note it down. This is Projected Blade area.

Pitch Ratio = Pitch / Diameter. To us this is 7/14 =0.5, however the spreadsheet uses the actual pitch / diameter figures and not the “rounded down for human consumption” answers and comes up with a Pitch Ratio of 0.477

We can now calculate that the relationship between Projected Blade Area and Developed Blade Area is 0.951.

We divide 51 required blade inches by 0.951 to determine that we require 53 square inches of Developed Blade Area.