Feature 8 | ProPulsors
and noise levels would not be higher and rolls-royce ABP, Adjustable bolted
the blades would still be free from erosive propeller.
cavitation but with an improved cavitation
inception speed. This performance was
expected despite some modifications that the main dimensions of the “final-design”
were made to the shaft line and the hull propeller are given in the table.
geometry due to structural reasons, which The propeller diameter has to be
affected the wake field negatively. adapted to a given shaft speed and is
normally selected slightly smaller than the
Wake field optimum diameter in order to avoid large
For such a complicated propeller design, efficiency drops at light sailing conditions
the hull, shaft line and appendage design such as start of life draught and following
is of major importance to the performance wind. If the propeller shaft speed is an
of the propeller. The inflow to the open parameter the propeller diameter,
propeller, the wake field, sets the level from an efficiency point of view, should
of the achievable propeller performance came closer to the propeller disc. The be selected as large as possible. A large
for the propeller design. Without good trim of the vessel was changed and a 2deg propeller diameter reduces the loading
design attention to the wake field, the flap was added at the stern in order to per unit area giving better efficiency, less
performance of the propeller can not reduce the resistance. These modifications sheet cavitation and reduced tip vortex
reach its maximum level. The ACA drew affected the inflow to the propeller. The strength. Larger propeller diameters
on a wide range of experience to develop wake field of the HY24 hull form is shown will, however, also give a smaller tip-hull
the HY23 hull and shaft line design, in the lower part of the figure, and it is clearance. With these contradictory
which was used for the initial tests of the seen that the maximum axial wake peak requirements, a compromise has to be
“near-design” propellers. The wake field of in the top position increased. The shadow made; select the largest possible propeller
the HY23 hullform is shown in the upper behind the bracket arms is somewhat design with due cognisance of pressure
part of the figures reproduced here, where more pronounced and the flow velocity pulse levels and noise.
the contours show axial wake levels and below the propeller hub was increased. The IEP System can deliver maximum
the arrows flow velocities in the transverse The increased maximum wake peak at the torque at a range of shaft speeds. This
direction. A ‘good’ twin-screw wake field 12 o’clock position means that the blade range was already defined giving only a
keeps the variations of flow velocity and loading at this position will be increased small window for the selection of propeller
flow directions to a minimum during resulting in higher pressure pulse as well diameter. The smallest possible hub size
the revolution of the propeller. From the as stronger tip vortex giving increased that meets the strength requirements was
upper part of the figure it can be seen noise levels. The increased flow velocity determined to be 179 size (hub diameter
that the wake has a typical twin-screw below the propeller hub gives increased of 1.79m), which went through as the
characteristic. Notable is that the wake on load variation and higher risk for blade final design.
the inner propeller radii is low, which is root cavitation. All these differences The direction of rotation is important
of importance on this type of vessel where had to be handled in producing the for propulsive efficiency as well as pressure
blade root cavitation can be an issue. “final-design” propeller. pulses and noise levels. Typically, outward
Between the “near-design” propeller rotation gives better efficiency, while
and the “final-design” some modifications Main dimensions inward rotation results in lower pressure
were made to the hull lines and the shaft At an early stage of a project the main pulses. The optimum solution depends
lines to produce the HY24 hull form. propeller dimensions as diameter, number on the transverse wake field, therefore
Most notably, following discussions with of blades and hub type have to be defined. it is very important to study efficiency,
Lloyd’s Register and modal analysis of The propeller diameter, number of blades pressure pulses and noise, before the
the A-brackets, the A-bracket arms were and the shaft speed are matched for direction of rotation is finally decided.
moved aft to increase the natural frequency optimum performance; the hub type and
of the A-bracket design beyond the shaft size are selected as small as possible whilst Blade design
speed range. Hence the A-bracket arms meeting strength requirements. Some of The propeller design point – for which
vessel speed, shaft speed and delivered
power the propeller is to be designed –
Kamewa twin ABP - 179 ABP/5-B-sl
is one of the first things to decide when
Propeller diameter D 6700mm starting with a propeller blade design.
Hub diameter d 1790mm
The design point determines the level
No. of blades Z 5
and amount of pitch and camber that will
be given to the blade. For a fixed-pitch
Direction of rotation Inward
propeller the variation in design point is
Propeller main dimensions.
limited to differences in draught, fouling
42 Warship Technology October 2009
p40-47_WT Propulsors-QE OCT09.indd 42 17/09/2009 17:03:04
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