Propeller Characteristics for Torsional Analysis

A HydroComp Technical Report
Report 107

Product link: NavCad, SwiftCraft, PropCad, PropExpert

This report comments on various characteristics of screw propellers that are used to perform a torsional analysis. Torsional analyses are typically performed by engine manufacturers for critical installations.

Moment of inertia

Propellers in operation exhibit a given moment of inertia. In addition to the spinning mass of the propeller, the figure used for torsional analysis must be increased due to water that follows the blades.

Mass moment

The value used to represent the material (mass) moment of inertia is known as "W-R-squared" (wr2). About 90% of the mass inertia comes from the blades and 10% from the hub. Typically supplied by the manufacturer from tests, this figure can easily be estimated by the following formula if unavailable:

wr2 = wd2/k where, w = propeller weight (mass)
d = propeller diameter
k = 28 high-skew naval propellers
      23 conventional merchant ship propellers
      19-25 sample of small commercial propellers

Entrained water margin

Due to water that follows the propeller blades as they spin, the "effective" moment of inertia is somewhat higher than the mass moment of inertia. The effect of this entrained water can be estimated by adding 25% to 40% to the above material moment of inertia. Propellers with small blade area ratio or pitch will be on the lower end of the range, while high blade area or high pitch propellers will be on the upper end.