Experimental Study of the effect of spray rails for a V-shaped high speed Planing hull
Dae-Hyuk Kim, Kye-pyo Rhee, Nakwan Kim, Jin-HyungAhn
*Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, 151-744, Korea
†Agency for Defense Development, Changwon, 645-798, Korea
e-mail: , , ,
ABSTRACT:This paper presents an experimental study of spray rails fora V-shaped high speed planning hull. Spray rails appended at hull bottom reduce the spray generation. It changes fluid flow pattern around hull. Generally, it is used for the resistance reduction. Some model tests for the investigation of spray rail effectiveness are conducted by Clement (1964a, 1964b) and Muller-Graf (1991). But hull forms in papers are planing hull and semi-displacement hull respectively, so the effectiveness of spray rails for other hull forms cannot be guaranteed. The hull form used in this paper is V-shaped high speed planning hull which has very thin and shape forepart that is very similar with axe-bow hull which is defined by Keuning (2001). In calm water tests, the resistance and running attitude (trim angle, rise of C.G.) are measured and the spray generation change is evaluated from video capture. Model tests are conducted by using high speed towing carriage and measurement system at Seoul National University. From the test results, it can be known that the spray rail is very an effective appendage to reduce the spray generation.
INTRODUCTION
The hull form of commercial ships have developed to be suitable for the slow speed and large size in order to transport a large amount ocean freight. Because of the increasing maritime freight volume by ships, most of research contents of naval architecture area have been focused on the commercial ships (Bulk carrier, Oil carrier, Container ship, … ) which operate for the only transport purpose.The high speed vessel has the very high operation speed, so not only hull forms but the fluid flow around hulls are very different with commercial ships. It is very important reason that one cannot use theoretical and experimental methods which have been developed and validated for the commercial ships. Because of this, International Towing Tank Conference (ITTC) first established High Speed Marine Vehicle (HSMV) special committee in 1981. Until now ITTC have tried to study the global research trend of high speed vessels and to help research groups to exchange their research results.
Among high speed vessels, planing hullsare most commonly adopted for the high speed vessel hull. The planing hull is defined as the specific hull form which has small frictional drag by “planing”. During planing, the weight of the vessel is mainly supported by hydrodynamic pressure loads, with buoyancy having less importance (Faltinsen, 2005). Due to the special hull form and high speed, there are some interesting phenomenon which are mostly negligible at the commercial ships. This paper is an experimental study of “spray” phenomenon. Planing hulls have very distinct stagnation area where the hydrodynamic pressure is strongly concentrated. A spray sheet is generated at this stagnation area and scattered hull rear side (Clement, 1964a) which is shown in Fig. 1.Some researchers categorized the spray as one of the components of the wave making resistance in the view of the energy loss.But general wave mechanics cannot explain spray phenomenon, so it is generally dealt with an independent phenomenon. An excessive spray is one reason of the increasing resistance and obstruct stable maneuvers, so it must be reduced some extent.
Fig.1 An example of spray of high speed hull (Clement, 1964a)
Previous research is composed of three parts which are the investigation of spray phenomenon, the estimation method of spray resistance and the increase of total resistance by spray rail.
Savitsky, et al. (1958) shows that the spray sheet starts at the intersection points of stagnation line and chine line and presents that the spray height and speed are affected by the trim angle and deadrise angle from theoretical and experimental methods. Payne (1986) studied that the thickness and progress angle of the spray sheet of wedge-type hull by using the physical fact that the pressure drag is induced by the loss of momentum. Latorre, et al. (1989) presents that Weber number is similarity number of spray phenomenon based on previous experiment results (Savitsky, el al., 1958). Ha, et al. (1994) estimated the pressure drag component induced by spray by using momentum conservation theory from the spray scattering direction and speed measured at model tests. Savitsky (1964) proposes the calculation method for the estimation of lift and drag of planing hull. This method includes the formula which calculates the area and location of spray generation. Latorre (1983) proposes the estimation method of the spray thickness and uses the method to divide the total resistance into pressure component and friction component. Savitsky, el al. (2007) compares the resistance estimation results calculated by hull, air, spray components with model test results. Above researches are all focused on the hydrodynamic phenomenon or resistance estimation of the spray generation. Different these researches, there have been efforts to reduce the resistance and to control running attitude (trim angle, rise of C.G.) are by spray rail practically. The spray rail is very thin and long appendage which is attached at the hull bottom. Clement (1964a, 1964b) investigated the effect of long and short spray rail to reduce the total resistance without the change of the trim angle and rise of C.G. by the model tests of a planing hull.Muller-Graf (1991)also conducted model tests by using spray rails, but he uses a semi-displacement not planing hull. Lee, et al. (2010) and Park, et al. (2011) carried out similar experiments to study the effects of spray rails for a planing hull.
In the initial design stage of high speed vessel, previous experiment results (Clement, 1964a, 1964b; Muller-Graf, 1991) can be good reference results which contain a large amount of experiment which are shown in Fig. 2 and Fig. 3. But results are about only some planing hulls and semi-displacement hulls. It means that nobody guarantees the performance of the spray rails for a new different shaped high speed hull form. In this paper, the effect of spray rails on anV-shaped high speed planning hull is investigated by using model tests in towing tank. The hull is different with conventional planing hulls which have constant deadrise angle and with semi-displacement hulls which have rounded bilge hull bottom shape. The paper is focused on the running attitude change by long spray rails which have various breadths and attachment locations. And the total resistance change is investigated from the running attitude change.
Fig.2An example of the effect by short bottom spray rail spray rail (Clement, 1964a)
Fig.3Geometry of spray rails (Muller-Graf, 1991)
MODEL AND TEST SETUP
V-shpaedhigh speed planning hull used in this paper is designed Seoul National University (SNU). The hull shape is very similar with the axe-bow hull. The introductory background and characteristics of the axe-bow hull are explained well by Keuning et al. (2001) andGelling (2006). The axe-bow hull can is introduced to reduce the motion in waves by the mechanism that the bow cuts through the water, and is less affected by passing through waves than a bow with more flare, making this bow type much less susceptible to pitching motion. It has been highlighted as a kind of alternative hull form of planing hulls. The body plan used in this paper is shown Fig. 4 and some main dimensions are listed in Table 1.
Fig.4 Body plan of SNU V-shaped high speed planninghull
Main dimension / ValueLength /Breadth / 5.6797
Length / Draft / 20.1884
Block coefficient (Cb) / 0.3131
LCGfrom midship (%) / 11.40
VCG from baseline (cm) / 6.80
Table 1 – Main dimensions of SNU V-shaped high speed planning hull
Model tests are conducted at towing tank of 0Seoul National University. High speed towing carriage (speed : 0.1 m/s ~ 10m/s) designed and used in previous researches (Kim et al., 2012; Park et al., 2011) are a main test equipment. Fig. 5 is a photo of the high speed towing carriage Fig. 6 is schematic diagram of the high speed towing carriage system. For the high speed, the weight of the high speed carriage is about 600 kg and towed by wire-drum type. Controller generates the constant torque command to make the constant towing speed. The measurement system is composed of the strain gauge and potentiometer. The strain gauge is for the resistance measurement and the potentiometer is for the measurement of the running attitude. For the measurement of trim angle and rise of C.G. in steady state, free to pitch and heave system is constructed shown in Fig. 7
Fig.5High-speed towing carriage in Seoul National University
Fig.6 Plan view of high-speed towing carriage in Seoul National University (Kim, 2013)
Fig.7 Test setup for free to heave and pitch system (Kim, 2013)
SPRAY RAIL
Basically, there are some geometrical parameters of spray rails, for example, length, breadth, section shape, size, appended location and so on. According to previous study (Muller-Graf, 1991), spray rails have triangular section shape to reduce spray generation and break off angle (an exterior angle of section outside) have to be larger than 90 degree inshown in Fig. 8. In this paper, 4 kinds of section size (0.0114B, 0.0189B, 0.0303B, 0.0379B) with a right-angled triangle are used, the length of rails is all 2/3L. They are attached from the transom to 2/3 longitudinal location. Fig. 9 shows that a spray attachment example case (90% location of hull breadth and 0.0379B section size).
Fig.8Cross section shape of spray rails used for model tests
Fig.9An example figure of hull with spray rail (0.0379B)
LocationSize / 50% / 70% / 90% / 50%, 90% / 50%, 70%, 90%
0.0114B / o
0.0189B / o / o / o / o
0.0303B / o
0.0379B / o / o / o
Table 2 – Test conditions of spray rail
TEST RESULTS
In this paper, model tests are conducted with various section sizes at 90% location of hull breadth. 4 kinds of section sizes are all used. To investigate the effect of attachment locations, model tests are conducted with 50%, 70%, 90% location of hull breadth (section size 0.0189B, 0.0379B). All test conditions are listed in Table 2. The towing speed is 7 kinds including zero speed. Fig. 10-12 show that the results of the resistance and running attitude with various spray rail section size. Although spray rail section size is changed, resistance and rise of C.G. are very similar and only trim angle is changed.
Fig. 10Resistance results with various spray rail section size
Fig. 11Trim angle results with various spray rail section size
Fig. 12rise of C.G. results with various spray rail section size
If the effect is not quite different with spray rail section size, small sized rail is more effective. So smallest one is used for the model tests to investigate the effect of the attachment location. Fig. 13-15 show that the results of the resistance and running attitude with various spray rail location. Except 50% transverse location case, the resistance results are all similar. From the trim angles, it can be known that more outside attached spray rails are more effective to reduce the trim angle. Rise of C.G. results are all similar.
Fig. 12Resistance results with various spray rail location
Fig. 13Trim angle results with various spray rail location
Fig. 14Rise of C.G. results with various spray rail location
To investigate the decreasing amount of the spray generation quantitatively, spray scattering angle is measured by test photos shown in Fig. 15.Regardless of rail size and location, the spray scattering angle is decreased about 10~20 degree. The results are listed in Table 3.
Fig.15 An example of photos for the comparison with various spray rail location at Fn = 1.80 (w/ spray rail 0.0379B, 90% location)
LocationSize / 50% / 70% / 90% / 50%, 90% / 50%, 70%, 90%
0.0114B / 12.5 o
0.0189B / 20.8o / 12.5o / 9.1o / 10.4o
0.0303B / 10.6o
0.0379B / 14.9o / 10.0o / 9.2o
Bare hull / 31.6o
Table 3- Spray scattering angles (deg.)
CONCLUSION
In this paper, the effect of the spray rails on the resistance and running attitude for SNU V-shaped high speed planning hull is investigated experimentally. Model tests are conducted with various rail size and attachment locations. The result from model tests can be summarized by:
1)Spray rails at 90% location of hull breadth are all effective to spray decrease regardless of the rail section size (0.0114B ~ 0.0379B).
2)Spray rails are more effective to reduce spray and trim angle at more outsize location.
3)The number of attached rails show very small different results.
4)Spray rails at 50% location of hull breadth increase the total resistance.
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