Correlating K_{T}/K_{Q}
for Symmetricsection Propellers
A HydroComp Technical Report
Report 132
Overview
Certain
"thrusterstyle" propellers are required to develop
equal thrust in both the "ahead" and "astern"
direction. These types of propellers do not have conventional
blade sections (e.g., foil, ogival). Rather they have sections
that are symmetrical in both the chordwise direction (no skew)
and in thickness (equal thickness distribution about the midchord
with no camber). See the accompanying graphic for an example of
this propeller type [Ref: Saunders, H.E., Hydrodynamics in
Ship Design, Vol. 2, SNAME, 1957].
Propeller
Performance
There is no
systematic series available for propellers of this type (that
we have found). However, in the reference cited above we have
found model test and geometric data for one symmetricsection
propeller. The propeller has the following specifications:
Blades 
EAR 
P/D 
BTF 
Rake 
Skew 
Hub/D 
3 
0.42 
0.96 
0.045 
None 
None 
0.16 
As the typical
range of interest for these propellers in the lowspeed, highthrust
regime (the socalled "bollard" condition), we limited
our investigation to the lowest Jvalues which were tested (0.20
was the lowest J). This information will allow us to derive correlation
multipliers that can be used in either NavCad (K_{T} and
K_{Q} multipliers) or PropExpert (T and P factors).
Correlation
to the Bseries
We looked
at all of the available openwheel propeller series and determined
that the Bseries gave the most consistent correlation. The following
table shows the results of this investigation at two Jvalues:
J 
Prop
K_{T} 
Bseries
K_{T} 
K_{T}
ratio 
Prop
K_{Q} 
Bseries
K_{Q} 
K_{Q}
ratio 
0.20 
0.290 
0.321 
0.903 
0.0429 
0.0448 
0.958 
0.30 
0.258 
0.292 
0.883 
0.0390 
0.0414 
0.942 
Summary
Based
on this information, a strategy for modeling symmetricsection
propellers at the lowspeed, highthrust condition is:
 Bseries
propeller
 Thrust
correlation factor = 0.90
NavCad: K_{T} multiplier, PropExpert: T factor
 Torque/power
correlation factor = 0.96
NavCad: K_{Q} multiplier, PropExpert: P factor
Please
remember that this analysis was performed for one propeller of
one particular geometry (e.g., 3 blades, 0.42 EAR). Therefore,
as with all design data, this information is subject to a designer's
own information or preferences.
