Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec
The tank has a manned carriage on which is installed a dynamometer for measuring model
total 3.2
INSTRUMENTATION AND MEASUREMENTS
The resistance of the model was measured with a force block dynamometer mounted between the fitting in the model and the tow post. Dynamic sinkage at the centre of gravity was measured
with a rotary potentiometer
attached by a gear to a track on the free to heave tow post. The tow post was mounted at the longitudinal centre of gravity of the model. The trim angle was measured with a rotary potentiometer in the tow fitting. The longitudinal acceleration of the towing carriage was measured using a piezoresistive accelerometer [CFX USCA-TX, Range 10g] mounted on the carriage. This enabled the constant speed run section to be detected during the analysis in order to maximise the run length, as illustrated in figure 3.
All data signals were acquired using a high speed data logger [IOTECH DaqLab 2001] at a sample rate of 5000Hz and
stored on a laptop PC. Four pole
Butterworth anti-aliasing filters with a cut off frequency of 2000Hz for the accelerations and 200Hz for all other signals were employed. The sample rate and anti-aliasing filter frequencies were selected for the subsequent seakeeping experiments, based on nominal full
scale
requirements [15]. The time base was scaled from full scale to model scale for a nominal scale factor of λ=5.435, with the resulting factor being rounded to two for convenience.
3.3 TEST PROCEDURES
The models were tested in calm water at speeds from 4 to 12m/s. Measurements of model dynamic sinkage, trim angle and resistance were made. In addition photographs and video of the run were used to identify the dynamic wetted surface area. In accordance
with ITTC
Recommended Procedures on boundary layer turbulence stimulation [16], no stimulation was applied to the model as all but the lowest speed tested (4 m/s) resulted in a Reynolds' number higher number of 5x106.
than the critical Reynolds'
Each run commenced with the recording of zero levels for every transducer. The carriage was then accelerated down the tank to the required speed. The carriage speed was determined from the time taken to pass through a 15.24m (50ft) section of the tank with automatic timing triggers at the beginning and end. At the end of the run beaches at the side of the tank were automatically lowered to calm the tank. Adequate time for the waves in the tank to settle was left between runs. This averaged out at a time of 12 minutes between runs.
4 RESULTS resistance
together with various computer and instrumentation facilities for automated data acquisition.
Calm water resistance is presented graphically (figures 5 to 10) in the non-dimensional form of resistance divided
by displacement weight in Newtons (RT/Δ). The dynamic sinkage is non-dimensionalised by the cube root of
displacement volume (ZV/∇1/3) and presented in figures 11 to 16. Dynamic trim angle is presented (figures 17 to 22) in degrees and the dynamic wetted surface area is non-dimensionalised by displacement volume to the power of two thirds (SV/∇1/3) and shown in figures 23 to 28. All values are plotted against volumetric Froude number. In order to present these data in a format useful for designers and to retain maximum accuracy, tables 4 to 10 present all of the dimensional experimental data for models A,B,C,D,C1 and C2, respectively.
An uncertainty analysis has been conducted using the method described in [17] and using a 95% confidence limit. The results are presented as percentage uncertainty in tables 4 to 10. Some of the data in these tables are presented without uncertainty due to loss of the data files, which prevented an analysis, but it is expected that the uncertainty would be similar to the other conditions since the setup was identical.
Recommended ITTC resistance coefficients are
determined in order to illustrate a scaling procedure implementing Savitsky's formulation for whisker drag [18].
4.1 RESISTANCE
The calm water resistance of models A,B,C,D,C1 and C2 is presented in figures 5 to 10, respectively. These
illustrate that the RT/Δ is approximately the same for models A to D. This is reinforced in figure 29 which shows the influence of L/∇1/3 on resistance, although in general resistance is decreased with decreasing L/∇1/3. The total resistance of models C1 and C2 is reduced as a result of the reduced wetted surface area caused by the transverse steps.
This influence of the transverse steps on resistance compared with the parent hull is shown in Figure 33. This shows a significant reduction in resistance as speed increases for the stepped hull models. Interestingly, there is no significant difference between the single step and double step. It is worth noting that model C1 was the only model to show signs of porpoising (at a speed of 10m/s). The reason for this is unclear, since the run condition lies outside the expected porpoising limits as depicted in Savitsky [19].
The influence of increasing the load coefficient can be seen in figure 37. Model C has a load coefficient of 0.25 and model C+ represents an increased displacement, with a load coefficient of 0.30. This indicates that there is an inverse relationship between load coefficient and RT/Δ.
©2010: The Royal Institution of Naval Architects
B-57
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