Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec, 2014
the running trim and wetted area of the hull, resulting in a small decrease in resistance. The largest step resulted in full un-wetting of the afterbody, which caused the model to run at too large a trim angle and porpoise. The intermediate step height (2.85% of beam) resulted in trim near the optimum 4-degrees over the entire range of planing speeds, and no porpoising instability.
3.6 UNCERTAINTY ANALYSIS
Two types of measurements were made in this test, electronic measurements of resistance, trim and heave made during the experiments, and manual measurements of wetted surface area from photographs and static trim from a digital inclinometer. Uncertainty for electronic measurements was estimated by repeat runs of a single condition. Uncertainty for manual measurements was estimated from the precision of the instrumentation used.
Uncertainty analysis for repeatability was performed by making 10 repeat runs of the hull
in the unstepped
configuration. Standard deviation was computed for speed coefficient, R/W, Trim and Heave. The standard deviation is multipled by confidence interval.
2 to estimate the 95%
Table 2: Uncertainty Analysis of Repeatability Runs Cv R/W
Trim (deg)
3.9370 0.2029 3.9379 0.2014 3.9377 0.2013 3.9380 0.2046 3.9379 0.2000 3.9380 0.2047 3.9381 0.2064 3.9378 0.2018 3.9379 0.2041 3.9381 0.2009
Standard Deviation 0.0003 0.0021
3.3969 3.3536 3.3522 3.3532 3.3742 3.3796 3.3684 3.3741 3.3761 3.3676
0.0140 Heave
(beams) 0.1076 0.1041 0.1112 0.1075 0.1043 0.1055 0.1069 0.1065 0.1073 0.1074
0.0020
The trim reported in the data tables and figures is the sum of the measured change in trim and the initial static trim, measured manually from a digital inclinometer, with a precision of 0.1 degrees. The uncertainty in static trim measurement for this test is an order of magnitude higher than change in trim measured electronically.
The measurements of wetted length were made from observations of underwater photographs of the model, striped at 1-inch (25.4mm) increments, or 0.05 beams.
It
is assumed here that measurements are rounded to even increments of stripes on the model. Compounding the
B-94
Table 3: 95% Confidence in Measurements R/W
+/- 0.0042
Trim +/- 0.20 deg Heave S/b2
4. CONCLUSIONS
Special operations craft operate at lower volumetric Froude numbers than high-speed multi-step pleasure craft,
conventional patrol boats.
but higher volumetric Froude numbers than Studies have been undertaken
to determine the effects of stepped planing hulls for these craft. Lee [5] studied a variety of multi-step hulls, and these tests indicated that a promising configuration may be a large step located aft of the LCG. The current study explored this further.
An example was provided showing how patrol boat resistance is affected by large shifts in LCG. It was shown that while
high speed resistance can be
significantly reduced by moving LCG aft, the resistance in the hump speed region increases substantially. This is unacceptable for naval craft that have to operate at multiple speeds. To have low resistance in the hump speed regime, it is necessary to have an LCG somewhere around 40% of the length forward of the transom, much too far forward for high speed operations.
A stepped hull offers a good compromise between high and low speed performance because at high speeds the flow separates cleanly off the step, effectively reducing the hydrodynamic LCG, but at low speeds the afterbody contributes to the hydrostatic buoyancy needed to keep trim low throughout the hump-speed regime.
In the present study, a configuration of stepped planing hull featuring a transverse step located approximately 25% of LOA, forward of the transom was explored experimentally. This location is farther aft than typically seen in practice, but previous experiments with multi- step hulls [5], as well as studies of LCG effect indicated that
this may be a promising configuration for the
designated load condition and speed. The experiments showed that the step reduced the resistance of the model at speed coefficients above 3, while increasing resistance at speed coefficients near 2. These effects increased with increasing step height.
©2014: The Royal Institution of Naval Architects
+/- 0.0040 beams +/- 0.20
possible errors for the forward and aft steps results in an uncertainty of wetted length measurements of 0.1 beam.
Table 3 summarizes the 95% confidence limits for the measurements, including both manually read measurements and the repeatability analysis from the electronic instrumentation. The values in Table 3 are plotted above and below the data points in Figures 9-11 as error bars.
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