Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
In terms of the freewheeling propellers, drag coefficients range from a low of ~0.2 for the B-Series 2-blade, 10- inch-pitch propeller to ~1 for the Sailer 2 and Sailer 3 (6- inch-pitch) propellers. The B-Series 2- and 3-blade, 6- inch-pitch propeller drag coefficients were significantly below those for the Sailer series, and ranged between 0.5 and
0.7. For all freewheeling propellers, the drag
coefficients showed a slight monotonic decrease with increasing speed. This indicates that the drag scales with velocity to a power slightly less than 2.
The above computed drag coefficients for locked propellers may be compared with estimates given in Larsson and Eliasson [3] as 1.2
This value is quite
similar to the values shown in Figure 4 which is not unexpected as values just above unity are typical for sharp-edged blunt bodies at high Reynolds numbers.
The freewheeling results appear more interesting. Recall that MacKenzie and Forrester [1] reported a drag reduction of approximately 50% for a freewheeling propeller relative to a locked propeller, particularly at higher speeds.
drag
between the finest grid result, and the medium and coarse grid results, respectively. The results indicate that the variations are smallest with respect to the locked configurations, but small enough in all cases to not alter the conclusions in terms of drag reduction.
However, Larsson and Eliasson [3]
estimate a 75% reduction in drag. Warren [9] also provides an example with a reduction in drag of approximately 50%. This corresponds to
coefficients in the range of 0.3 to 0.6. However, contrary advice, that is freewheeling propellers produce greater drag than locked propellers, is given in the textbook by Kinney [4], but
without clear justification.
Consequently, there is little consensus regarding the levels of drag reduction to be expected for freewheeling vs. locked configurations. The present results in terms of percentage drag reduction are summarized in Figure 6. The reduction for the Sailer 2 propeller ranges from 8% at 1 m/s to 13% at 4 m/s. For the Sailer 3 propeller, the percentage reduction ranges from 21% at 1 m/s to 25% at 4 m/s. Reductions for the B-Series 2-blade, 6-inch-pitch propeller range from 43% at 1 m/s to 49% at 4 m/s. The reductions
for the B-Series 3-blade, 6-inch-pitch
propeller range from 57% at 1 m/s to 62% at 4 m/s. Finally, the B-Series 2-blade, 10-inch-pitch reductions range from 80% at 1 m/s to 83% at 4 m/s. The results show a small increase in percentage drag reduction as speed increases, consistent
MacKenzie and Forrester [1]. The results clearly indicate a significant
range
with the of benefit
findings of in terms of
permitting a propeller to freewheel, which is strongly dependent on the individual propeller characteristics.
The results presented in Figures 2-6 were obtained on fine-level mesh densities. Additional calculations using two additional lesser mesh densities were performed on each of the propellers at velocities of 4 m/s to assess the influence of truncation error. The 4 m/s velocity was chosen as this is expected, due to higher flow field velocity gradients, to be most demanding in terms of grid resolution requirements. A summary of these results is presented in Table 1. In particular, the results shown in the percent variations column indicate the difference
Figure 6. Percent
drag A primary
CONCLUSIONS conclusion is
reduction for
freewheeling
configurations relative to locked configurations. 5.
that drag reductions for
freewheeling vs. locked propeller configurations are highly dependent on the individual propeller design. In particular, the
locked configurations all have drag
coefficients in the range of ~1.1 to ~1.35 consistent with what one would expect for a thin plate oriented normal to the flow direction. The drag coefficient for the 10-inch- pitch propeller was slightly lower than those for the 6- inch-pitch propellers. However, the drag coefficients for the freewheeling propellers varied widely.
The drag
coefficient decreased significantly when the pitch of the freewheeling B-Series propeller was increased from 6 inches to 10 inches. The drag coefficients for the Sailer 2 and Sailer 3 propellers were only slightly below those for the locked configurations. This is a result of the thick, flat leading edge which incurs significant drag penalty due to a high (nearly stagnation) pressure acting over a significant area. difficult
to arrive at general
As a consequence, it appears conclusions regarding
percentage drag reduction for locked vs. freewheeling configurations.
Finally, we note that the freewheeling propeller results represent an ideal case, with no imposed torque on the propeller shaft. Under actual conditions, the freewheeling propellers experience a torque leading to lower RPM and increased
drag, the magnitude of which may be
significant (c.f., Ref. [9, 10]). In fact, results presented in Ref. [10] show that, as expected, the maximum drag levels occur at RPM levels intermediate between fixed and freewheeling configurations.
results were obtained with a uniform approach velocity, B-84 ©2014: The Royal Institution of Naval Architects In addition, the present
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