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5.3.1

Tests conducted

A set of calm water resistance measurements was made for both SWATH models, for speeds from 0.4m/s to 4.0m/s in steps of 0.2m/s, at the nominal displacement. These are compared in figure 5-3, in which the results have been scaled to a full size of 60m using the ITTC friction line with the form factors indicated. The corredponding model sinkage and trim are indicated by figure 5-4. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Froude number 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 resistance/displacement (kN/t) SWATH #2 (1+k=1.3) SWATH #1 (1+k=1.4)

Figure 5-3: Comparison of full scale total resistance of SWATH #1 and SWATH #2

The form factors used in figure 5-3 were obtained from Prohaska plots, which are based on the premise that the wave resistance coefficient,CW, is proportional toF r4 _{(at least at low Froude}

number), and that the total resistance coefficient is given by CT = CW + (1 +k)Cf, hence CT Cf =m ³ F r4 Cf ´

+ (1 +k), giving a straight line with vertical axis intercept (1 +k). (In fact it is not necessary thatCW ∝F r4_{, merely that limF r}

→0C_CfW = 0.) Application of the form factor

thus obtained to the resistance results relies additionally on the assumption thatkis a constant over the entire Froude number range, which is highly questionable for the SWATHs (both of

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Froude number -0.005 -0.004 -0.003 -0.002 -0.001 0.0 -0.025 -0.02 -0.015 -0.01 -0.005 0.0 SWATH #2 SWATH #1 SWATH #2 SWATH #1

steady pitch (right axis labels) steady heave/L (left axis labels)

Figure 5-4: Steady state heave and pitch (positive bow down) in calm water for SWATH models as a function of Froude number

which have submerged transoms) considering the different transom flow regimes at low and high Froude number. The inapplicability to other than slow vessels without immersed transoms is also noted by Couser et al. [18], who also cite numerous other methods of estimating form factors showing both a significant variation with Froude number and considerable disagreement of results for a given hull.

Estimation of form factors by this method can only be done with any certainty in the case of SWATH #2 since SWATH#1 has limited resistance data at low Froude number, although the sequence formed with the reference hull form factor of 1 +k= 1.052 _{is consistent with the}

ranking of the three hulls as members of a semi-SWATH family. Furthermore the sensitivity of the comparison to the choice ofkdoes not appear to be very great. Given the dubious nature of calculating form factors and the huge variation with different methods of calculation [18], it is impossible to tell on the basis of information at hand how indicative the results presented here are of the true resistance.

As might be expected because of its greater surface area, SWATH #1 has in general a slightly higher resistance, particularly at higher speeds where the frictional component of resistance dominates. In the near vicinity of a Froude number of about 0.4 the trend is reversed, however the difference is not particularly significant, and may even be due to experimental factors. It could alternatively be explained in terms of the hump in the wave making component of resistance, which might be smaller for the SWATH #1 because of its finer waterlines and more deeply submerged hull. The reference hull resistance results broadly follow the trend established by the two SWATH models, although the comparison should be made with caution because of its different test conditions3_.

2_{The reference hull was tested by MARIN, who used this value. It is not known how the value was obtained,}

but it could not have been obtained using Prohaska’s method from the resistance data presented by MARIN in their test report.

In addition to their larger wetted surface area the SWATHs may show higher resistance because of their large deeply submerged transom areas (contributing significantly to both form drag and wave-making drag). However it should be pointed out that no effort was made to optimise the SWATHs for resistance, and considerable scope is left for improvement.

In summary, although the SWATH models most likely had a significantly higher resistance, this should not imply that SWATHs in general are significantly worse. A well designed SWATH (or semi-SWATH) should have only slightly greater resistance than an equivalent catamaran, and this may be a worthwhile sacrifice if seakeeping properties are improved.

Regular seas

Seakeeping tests in regular waves were conducted for both models at their nominal draught (22.00kg) and at a draught deeper by 25mm (corresponding to 25.72kg for SWATH #1, or 28.50kg for SWATH #2) for various speeds wave heights, and frequencies. The ranges of fre- quencies chosen differed depending on speed, but in most cases were sufficiently extensive to clearly define the resonant peaks in the transfer functions. The only exceptions to this was at the higher speeds, and particularly for SWATH #1, which had a lower resonant frequency. In these cases the wave frequency corresponding to the encounter frequency that caused resonance was too low to be produced in the tank without major problems (these problems and others will be discussed in section 5.4).

Speeds and wave heights at which tests were conducted are shown in table 5.2 for the two models and two draughts used. The speeds cover the range up to a maximum of 3.5m/s, corresponding to about 37kt full scale. The effect of wave height was only investigated for three combinations of model and speed, and found to be fairly linear, so further investigation was not considered warranted. At greater wave heights there was also a practical operational problem with the first SWATH model of swamping near the resonant frequency, and the shape of the hull made it very difficult to remove the water. The second model was built with more freeboard, but near its resonant frequency it almost hit the carriage at larger wave heights.

speed (m/s) 1.0 1.5 2.0 2.5 3.0 3.5 SWATH #1, 22.00kg 40 40 40 40 40 20, 40 SWATH #1, 25.72kg 30 30 30 SWATH #2, 22.00kg 40 40 20, 40 40 20, 40, 60 40 SWATH #2, 28.50kg 40 40 40

Table 5.2: Showing wave heights (mm) for which tests in regular waves were conducted

Results of these tests will be presented in chapter 6.

Random seas

Some preliminary tests in irregular waves were conducted for SWATH #1, but, mainly because of the short tank length, and high speed and low natural frequency of the model, irregular wave testing was discontinued. More detail is given in section 5.4.

5.3.2

Conventional hull form results

It was originally envisaged that the semi-SWATH results be compared with comparable results for their reference hull, the Incat 74m wave piercing catamaran. However there were problems with access to the relevant data for commercial reasons. On the other hand, data relating to the seakeeping of a systematic series of conventional high speed hull forms was available through the Australian Maritime Engineering Cooperative Research Centre (AMECRC) ([63] and [10]). Two contrasting examples from the AMECRC systematic series were taken as representative of conventional hull forms. These are shown in figure 5-5, and principal particulars are shown in table 5.3. These formed the basis for comparison with the semi-SWATH results, and will

Model L/B B/T Cb LWL(m) LCB(m) RofG (m) WSA

¡

m2¢ _∆(kg)

#04 8.00 4.00 0.447 1.6 0.7136 0.400 0.3056 7.158 #05 4.00 4.00 0.395 1.6 0.7136 0.400 0.6297 25.344

Table 5.3: Relevant AMECRC systematic series model parameters

subsequently be referred to asAMECRC #04 andAMECRC #05.

Of these AMECRC #05 was closest in displacement and draught to the two semi-SWATH models, while AMECRC #04 was closest in beam.

DWL

(a) AMECRC #04

SCALE (cm) 0 5 10 15 20 25

DWL

(b) AMECRC #05