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 "WRsquared"
(wr^{2}). 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:
wr^{2}
= wd^{2}/k
 where,
 w = propeller weight
(mass) 

 d = propeller diameter 

 k = 28 highskew naval propellers
23 conventional merchant ship
propellers
1925 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.
