Modeling Tractorstyle
Azimuthing Podded Drives
A HydroComp Technical Report
Report 127
Overview
Tractorstyle
azimuthing podded drives (APDs) have recently become an attractive
propulsor option. By careful consideration of the underlying elements
of the APD pod style of propulsion, it is possible to model their
performance in parametric performance prediction software, such
as NavCad. By choosing proper correlation coefficients (i.e.,
KT and KQ multipliers), propeller series and propulsive coefficients,
you can reliably model an APD with standard propeller series data.
Propeller Performance
One might
expect that the efficiency of propellers for APDs would be higher
than for a standard series propeller (e.g., Bseries, Gawn). In
fact, this is not generally true. APD propellers have a large
hub (>30%) and often have design features to help mitigate noise
and vibration concerns, which degrade the efficiency a few percent.
These features typically include variable pitch distribution to
"offload" the tip and root areas, and a forward leading rake
to increase the distance from the propeller to the pod structure
immediately aft.
We have found that a
consistent numerical model can be constructed by using the Gawn
propeller as the basic propeller series. To account for the reduction
in efficiency versus the Gawn, we can choose KT and KQ multipliers
that reflect this. Based on published data, typical values in
the normal design range would be KTmult=1.02 and KQmult=1.06.
Resistance
Model testing of hulls
fitted with APDs have shown that the drag of a vessel with pods
is often a bit less than the sum of the barehull vessel drag
plus the independent APD pod drag. In other words, the "whole
is better than the sum of its parts". It is surmised that the
pods act like small stern bulbs and introduce a beneficial change
in the wave system  in the same fashion as we see with bulbous
bows. This notion is compatible with visual observations of the
wave system.
It is our suggestion
that the appendage drag of ADP pods be modeled and added to the
bare hull as for any appendage. Any actual resulting improvement
in the resistance of the podded hull can then be held as a design
margin.
Propulsive Coefficients
The amount of published
information for selfpropulsion tests of APD models is very limited.
From the literature, however, we can make a few recommendations
for typical values  wake fraction (W) is 0.050.08, thrust deduction
(TD) is 0.040.06, and relativerotative (RR) efficiency is 1.041.07.
It is important to correlate
W and TD, so that values in the low range of W match values in
the low range of TD, and vice versa. The intent of this is to
keep the hull efficiency relatively consistent at 1.011.02.
System Improvements
The high
RR is one of the principal reasons for the high efficiency of
APD systems. Placing a propeller in front of the pod strut induces
the recovery of rotational energy. In other words, the strut aft
of the propeller acts like a stator in the flow stream.
Even though
the propeller itself may be a bit less efficient due to the large
hub and design features used to minimize noise and vibration,
the amount of improvement of the APD system versus a conventional
propeller can be significant  on the order of 2%4%.
Summary
Based on
this information, a strategy for modeling APD propulsors is:
 Model and add APD pod
appendage drag.
 W=0.050.08, TD=0.040.06
(or, =0.8*W), RR=1.041.07.
 Gawn AEW propeller.
 KTmult=1.02 and KQmult=1.06.
As with
all design data, this information is subject to a designer's own
information or preferences.
