hydrodynamic performance enhancement using stern ...

HYDRODYNAMIC PERFORMANCE ENHANCEMENT USING STERN WEDGES, STERN FLAPS AND INTERCEPTORS Shiju John, MD Kareem Khan, PC Praveen, Manu Korulla and PK Panigrahi Naval Science & Technological Laboratory, India SUMMARY Improving the hydrodynamic performance of ships is a major research area which has introduced various innovative concepts like stern wedges, stern flaps and interceptors. Latest studies include the relative merits of these devices in various permutations and combinations to improve the hydrodynamic benefits. The premise of operation of wedges, flaps and interceptors has been established through model tests and sea trial data. Experimental studies were carried out in High Speed Towing Tank at Naval Science & Technological Laboratory (NSTL) India, on a variety of hull models fitted with these devices. Model tests were performed on fast displacement hull forms, planing crafts and foil assisted catamarans to study the effects of these devices on hydrodynamic performance. The present paper outlines this work and also attempts to chalk out the importance of selecting the best configuration of performance enhancer devices without disregarding cost effectiveness. KEYWORDS: stern wedge-interceptor, stern wedge-flap, trim enhancement, economic feasibility NOMENCLATURE

∆ τ B Dint Dk N Nint PD 1.

Weight of ship (N) Trim angle (degree) Buoyancy (N) Induced drag force (N) Viscous drag force parallel to keel (N) Normal component of planing craft lift force (N) Induced lift force (N) Delivered Power (kW) INTRODUCTION

The last few decades have seen a lot of innovation in the form of energy saving devices to bring down the power required to propel ships. Among them the emergence of stern wedges, stern flaps and interceptors as hydrodynamic performance enhancers has been remarkable. These devices have proved effective in reducing the resistance of fast displacement hull forms, planing crafts and other advanced marine crafts. The stern wedge is a wedge faired into the stern form as shown in Figure 1(a) [1]. The stern flap represents an extension of the hull aft of the transom in the form of a flat plate. The flap is mounted to the transom at an angle relative to the centreline buttock of the ship as illustrated in Figure 1(b) [2]. Interceptor is a flat plate fitted vertically at the transom of a ship, covering a portion or full breadth of the transom and protruding a few centimetres below the transom as shown in Figure 1(d) [3]. These devices create a vertical lift force at the transom and cause the flow to slow down under the hull at a location extending from its position to a point generally forward of the propellers. This decreased flow velocity will cause an increase in pressure under the hull, which in turn, causes reduced resistance due to the reduced afterbody suction force (reduced form drag) [2].

Figure 1: (a) Stern wedge; (b) Stern flap; (c) Flow modification due to wedge/flap (d) Flow modification by interceptor

In designing energy saving devices, it is extremely important to anticipate the performance of the most optimised device in conjunction with the hull form. Case in point, as reference [1] explains the primary reasons for the effectiveness of these devices on small high speed crafts are significantly different from that on large ships such as destroyers. On small planing crafts, the vertical forces from a wedge, flap or interceptor may change the trim angle by about 4 to 5 degrees. Fixing an optimal trim angle for the most effective planing surface is the key to minimising resistance. Interestingly on a destroyer size ship, the action of these devices affects the trim by about 0.1 to 0.3 degrees. This has little measurable resistance effect. The major powering benefit, in fact, is credited to the induced change in the flow field around the hull. These flow field changes reduce the aft body drag and modify the wave resistance. Another observable benefit is that a stern flap or wedge cuts down the rooster tail significantly thereby reducing wake. In particular it is the pressure field change that improves propulsive performance of fast displacement ships. It is critical to identify these effects before recommending these devices on ships. Since computational efforts are not completely able to elucidate the performance of such devices when fitted to ship hulls, hydrodynamic model tests and full scale sea trials are still the most reliable option. Another important consideration is the speed range selected for optimisation. Reference [1] suggests that these devices are effective across medium and high speeds. They have a resistance penalty in the low speed ranges. For instance, the initial wedge design on DDG 51 decreased delivered power by 7% at high speed. At the same time, it had a powering penalty up to 4% at lower speeds [1]. If powering benefits are not achieved around the operational speed of the ship, the gain in the higher speeds may be easily offset. It again depends on whether the aim is to increase the top speed or to decrease delivered power in the operational speed range. In the former case, a low speed penalty does not matter much. However, the latter scenario is most prevalent. Therefore it is equally important to reduce low speed penalty when looking at powering benefits in the high speed zone. In ship-building industry, cost involved in any activity is a critical element. Reference [4] gives an idea regarding economics of stern flaps and wedges. An attempt is made to assess the optimisation process based on performance enhancement from economic perspective. This paper discusses experimental investigation at NSTL into optimisation of hydrodynamic performance enhancers on three different hull forms – fast displacement hull form, planing craft and foil assisted catamaran. The towing tank at NSTL has a length of 500 m, width 8 m, depth 8 m and a maximum carriage speed of 20 m/s. Measurement of resistance, heave and trim of the model at various steady state speeds is carried out using a Kempf and Remers Dynamometer.

2.

POWERING ENHANCEMENT OF FAST DISPLACEMENT HULL

The effectiveness of stern wedges, flaps and interceptors in improving the powering performance of a fast displacement hull form of 138m length was studied at NSTL. Extensive model tests were done on a 1:26.135 model, including resistance and propulsion tests, using these devices and their combinations. Figure 2 shows the ship model fitted with these devices. Significant work done in reference [1] shows that stern wedge-flap configuration performs better than stern wedges or flaps in isolation. “Cross-over” speed i.e. the speed at which resistance begins to reduce is an important factor here. The approach at NSTL was to first arrive upon an optimum wedge-flap configuration and then research on the possibility of interceptors. As a first step, the best stern-wedge configuration had to be identified. Wedge chord length

Wedge angle (deg)

1 % of LBP

8

10

12

14

15.5

1.5 % of LBP

8

10

12

14

15.5

Table 1: Wedge variants tested

Contour

Flap angle (deg)

Flap1: Radius = 1˚

10

12

14

Flap2: Along WL

10

12

-

Table 2: Flap variants tested Appended hull resistance tests were done to optimize for the best stern-wedge configuration. Stern wedges tested were of sharp V shape cross-section at constant angle chord-wise and faired edges along the hull span-wise. Variants of wedges tested are given in Table 1. The change in resistance may be attributed to the change in dynamic trim/sinkage and reduction of the effective wake width at the transom. The minimum effective wake width happens for the 14˚ wedge of 1% chord length. Decrease in effective wake width suggests lesser wastage of energy and thus lesser resistance. The rooster tail shifts further aft of transom at higher speeds corresponding to 28-30 knots which is an indication of lesser effective wake length. Lesser wake lengths improve the resistance performance of the vessel. The 10˚ wedge of 1% chord length gave the best resistance reduction from 22 knots to 32 knots. A resistance reduction of about 2.2% was achieved at 30 knots. The reduction in dynamic trim/sinkage is highest with this wedge configuration. This is indicative of better resistance performance of the ship. This wedge configuration was selected for further hydrodynamic studies with stern flaps.

Figure 2: Stern wedge, flap and interceptor fitted on Fast Displacement Hull

Figure 3: Transom flow pattern for selected configurations showing differences in "breakaway", "neckdown" and hydrodynamic lengths at full scale speed of 28 knots

Two stern flaps were designed for comparative study. The different configurations of stern flaps used during the resistance tests are given in Table 2. These flaps were tested along with the 10˚ wedge of 1% chord length. Flap2 of 10˚ was found to perform the best when fitted with the 10˚ wedge. This configuration reduces resistance from a speed of 24 knots to 32 knots. The resistance at 30 knots was reduced by 2%. However what is more significant is the reduction in effective wake behind the hull and the height of the rooster tail. Stern flap caused a considerable “neck down” of the transverse width of the stern wave pattern. “Neck down” is explained in reference [1]. However, it was felt that there is still scope for improving on the powering performance. Motivated by the hydrodynamic performance of interceptors, it was an intuitive decision to go for a combination of stern wedgeinterceptor at this stage. Interceptor of protrusion type was used with 1mm, 2mm and 4mm protrusion (at model scale) below the 10˚ wedge. The wedge – interceptor combination of 10˚ wedge and 1mm interceptor was seen to be the most effective among these variants. Interceptor

of 1mm alone was tested by removing the wedge and at this stage seemed to perform better than the wedgeinterceptor configurations. Wedge-interceptors could bring down resistance only after a speed of 24 knots while interceptor alone was slightly better at reducing resistance from 22 knots onwards. The reduction at 30 knots was of the order of 1.5%. The performances of these devices were also studied from propulsion point of view. Self-propulsion tests were conducted to evaluate the effect on delivered power and propulsive efficiency. Propulsion performance of the stern wedge/flap/interceptors gave a comprehensive insight into their effectiveness. The results are summarised in Table 3. The following are the major observations: •

Delivered Power (PD) is reduced by all the energy saving devices in a speed range of 22knots to 32knots.

•

The wedge – interceptor combination of 10˚ wedge and 1mm interceptor proved to be the

optimized configuration in terms of delivered power reduction. Thrust deduction fraction (t) and effective wake fraction (w) was considerably low for wedge-interceptor configuration. Lower thrust deduction fraction indicates increase in thrust power and lower effective wake fraction indicates accelerated flow onto the propeller which facilitates higher propeller thrust.

“breakaway” speed to 26 knots from 30 knots. “Breakaway” is explained in reference [1]. The interceptor variations indicate a clean breakaway at 28 knots. Also, the effective increase in hydrodynamic length is slightly better for the stern flap-wedge combination as seen in Figure 3. The lengthening effect is expected to give powering benefits at high speeds by reducing wave making resistance.

•

The wedge-interceptor combination improves the total propulsive efficiency at 30 knots to 0.68 from the actual value of 0.65. The least PD is for wedge-interceptor.

•

Stern wedge-flaps and wedge-interceptors caused the maximum reduction in width of the stern wave pattern. The total area of turbulence and whitewater is reduced. The stern wedge-flap combination reduces the transom flow

With a defined limited engine power, it was found that these energy saving devices will help to increase the top speed of the ship. The increase in top speeds for all the tested configurations is given in Table 4. The vessel with the wedge-interceptor would attain a speed 32.60 knots; that is an increase in top speed of about 0.60 knots. Thus, wedge – interceptor configuration was found to give maximum powering benefit for the present hull form in the speed range of 22 to 32 knots.

Vs (knots)

Wedge 1%,10˚

Wedge 1%,10˚, Flap2 10˚

Wedge 1%,10˚, Interceptor 1mm

Interceptor 1mm

16

17

17

13

23

18

14

17

10

17

20

15

17

11

7

22

0

-1

-2

-2

24

-1

-2

-2

-5

26

-1

-5

-7

-7

28

-2

-4

-9

-6

30

-1

-4

-6

-4

32

-7

-10

-10

-9

Table 3: Percentage change in Pd (kW) w.r.t. base hull

Configuration

Increase in top speeds

Wedge 1%,10˚

0.10

Wedge 1%,10˚, Flap2 10˚

0.41

Wedge 1%,10˚, Interceptor 1mm

0.60

Interceptor 1mm

0.34

Table 4: Increase in top speed in knots wrt base hull at 32 knots

3. TRIM ENHANCEMENT OF PLANING CRAFTS The running trim is a critical point in the dynamics of a planing craft. Planing crafts generally operate at an equilibrium trim at any speed. Equilibrium trim is that trim at which the weight of the craft, the propeller thrust, the viscous drag and dynamic lift generated are in equilibrium. The equilibrium condition of the planing craft is represented by equations 29, 30 and 31 in reference [5].

Two problems which are related to planing craft trim are discussed here. In the first case discussed in section 3.1 the focus is purely on trim reduction. On the other hand, the second problem stated in section 3.2 is on resistance reduction by trim enhancement. It is interesting to see how the solutions evolve depending on the application. The effectiveness of stern flaps, wedges and interceptors are utilized in both the scenarios. One can see that these devices are highly effective in modifying hydrodynamic performance of planing crafts. 3.1

However, there is an optimum trim angle for every speed at which the resistance of the planing craft is minimal. More often the equilibrium trim is different from the optimum trim. Thus, the planing craft generally runs at the equilibrium trim thereby leaving room for resistance reduction. The equation is quite simple. If we can force the boat to operate at the optimum trim, resistance can be reduced by a great deal. This is particularly important in the hump zone and take-off speeds in the fully planing zone. Optimising trim can be effectively done by the use of trim reducing devices such as stern flaps, wedges and interceptors. These devices control the trim of the planing boat by providing an additional lift component at the aft which also contributes in reducing the required thrust. Refer Figure 4. The increased pressure at the aft is also found to improve propulsive efficiency [1]. Also interceptor increases the exit velocity at transom resulting in clean separation at transom. The transom hollow and rooster tail is thus reduced reducing wake [3].

TRIM CONTROL FOR OPERATIONAL REQUIREMENTS

A survey boat with a planing hull was reportedly facing problems in that its hydrographic equipments which are placed on the hull bottom were surfacing in the preplaning zone. As a result, the survey operations were hindered even at low speeds such as 10 knots. Model tests were done at NSTL on a 1:4.6 scale model and it was found that the operational speed of 10 to 11 knots was close to the hump trim. Waterlines on the boat model at these speeds suggested that even though the hull bottom did not come out of water it was too close to surface. It could be that a combination of excessive trim and heave was hindering its functionality by exceeding the minimum depth of operation.

Figure 4: Force decomposition on planing craft The planing craft has a hump trim which corresponds to the hump resistance. The optimum trim usually happens to be less than the hump trim. Hence, in the hump zone resistance can be reduced by reducing trim. The planing craft in the fully planing zone operates in the post hump trim zone. As the speed increases in the fully planing zone trim decreases. At some point it falls below the optimum trim. Beyond this point the trim reduction devices are of no use. Here, mechanisms to enhance the trim by providing lifting surfaces ahead of the LCG are effective. Modified spray rails which provide an additional component of lift are an option. Figure 5: Trim and heave reduction at model scale

Trim controlling devices were designed and fitted to the model. Model tests were done and the results are depicted in Figure 5. It can be seen that wedgeinterceptor is the best option in reducing trim and heave at the operating speeds. But at fully planing speeds, the trim reduction is too high that it causes the resistance to shoot up. It is suggested that at high speeds the interceptor be withdrawn. At 10 knots, the trim was reduced by 1.2 degrees and at 12 knots by 2.2 degrees. Heave was reduced by 4.7 mm at model scale. Retrofits with either interceptor or to be more effective wedge-interceptors are suggested as solutions. Since the present planing craft has a Z-drive, practically putting a stern-wedge may be difficult because of space constraints at the transom. In that case, an interceptor is a much simpler and feasible option. 3.3

TRIM CONTROL FOR RESISTANCE REDUCTION

In the following example, the resistance of a planing boat was required to be reduced in the speed range of 40 to 45 knots. The manufacturer was not able to meet the contractual condition of achieving these speeds with the installed engine power due to an increase in hull weight during construction. Hydrodynamic model tests were done at NSTL on a 1:13 model using stern interceptors,

wedges and wedge-interceptors to find out a possible solution. The optimum trim of the boat at these speeds was identified as 4 degrees from model tests. It was found that at the required speeds of 40 and 45 knots the running trim was higher than the optimum trim. It was anticipated that the resistance may come down by augmenting the trim and lift of the craft. A combination of optimum trim and heave was achieved by the use of interceptors. Further research was done by testing with wedges and a combination of wedge-interceptors. In all cases it was found that a trim of 4 degrees and a heave up of 45 mm to 50 mm at model scale gave the least resistance. This is clearly visible in Figure 6. A resistance reduction of 3.3 % was achieved at 40 knots by an interceptor protrusion of 1.5 mm. The boat was already fitted with conventional spray strips [6]. Further, additional spray strips were designed so as to cut off the remaining spray downwards and thereby provide an additional component of lift at the forward. As a result, the resistance reduced by 4.2% at 40 knots. At 45 knots, the situation was slightly different. Trim reducing devices would not help at 45 knots. The interceptor was not effective in bringing down the resistance at 45 knots even at a trim of 4 degrees and a heave of 50mm. By putting the additional spray strips, the resistance could be brought down by 1% at 45 knots.

Figure 6: Plots of model resistance against heave and trim corresponding to full scale speed of 40 knots and 45 knots (Dark blue indicates the least resistance and dark red indicates highest resistance)

3. PERFORMANCE ENHANCEMENT OF FOIL ASSISTED CATAMARAN (FOILCAT) Hydrodynamic performance evaluation of a twin hull asymmetric catamaran, supported by foils, was performed at the High Speed Towing Tank at the Naval Science and Technological Laboratory (NSTL), Visakhapatnam. An asymmetric catamaran supported by two NACA 66 airfoil sections fitted at the bottom of the hull was chosen as the basic form. See Figure 7. The cross deck of the catamaran was designed to give a planing lift to aid take-off at lower speeds. Approximately 80 sets of calm water resistance tests were conducted for two hull forms to optimise the hull – foil configuration and to study the effectiveness of spray rails and stern wedges in reducing the hump drag. Details of the development of the foil assisted catamaran hull form may be obtained from reference [7].

Lwl) at the after end of the cross-deck and triangular spray rails whose length and location were chosen so as to reduce the resistance mainly near the hump speed. Finally, the cross-deck was modified by adding a wavepiercing form at the bow for better sea-keeping performance [7]. A triangular stern wedge of length 100 mm, was introduced as an integral part of the cross-deck at the aft is shown in Figure 7. Three configurations of the wedges, viz. θ = 3°, 6° and 8° were tested at displacements corresponding to 90% and 100% of the all-up weight. A comparison of the behaviour of the stern wedges is given in Figure 8. The 6˚ stern wedge gave the most encouraging result. The hump drag of the optimised hull – foil configuration was reduced by 12%. The spray rails reduced the hump drag by about 3% [8]. The resistance reduction and trim reduction in the hump zone is clearly seen in Figure 8.

A number of features were introduced in the hull form so at to reduce the resistance hump. These included a 6 degree deep ‘V’ shape stern wedge (about 6 percent of

Figure 7: Linesplan of foilcat with wave piercing hull (left); 1:6.5 model at NSTL (centre) and details of wedge (right)

Figure 8: Resistance & Trim plots of foilcat with wedge variants

4

ECONOMIC FEASIBILITY

Going by the powering performance improvement exercise done on the fast displacement craft, it may be seen that the low speed penalties are making the whole scheme uneconomical. Reference [9] discusses scale effects on the performance of stern flaps and wedges. It compares among fast displacement ships the model test results and the sea trial data of USS RAMAGE (DDG 61). The 1:20 model-scale stern flap experiments indicated maximum reduction of 6.4% and an increase in top speed of 0.4 knots. Also, a low speed penalty below 12 knots was predicted. However at full scale, stern-flap reduced ship power by 5.6% to 15.4% and increased top speed by 0.9 knots. There was no penalty at any speed. The model scale tests under-predicted the stern flap performance in the range of 12% at speeds of 14 to 18 knots, but only about 2% when approaching top speed. Further, geosim studies indicated that “cross-over” speed decreased with increasing model size. Also, the resistance reduction improved with increasing model size. The stern flap scaling multiplier technique proposed in reference [9] is applied to the present model at NSTL. The corrected estimation is that the full scale performance with the best configuration gives a resistance reduction of 12.6% at high speeds. The low speed penalties are also nullified by applying this correction. The annual savings in fuel consumption is regarded as 4% as suggested by similar studies in reference [4]. The fitment of a stern flap or wedge or even an interceptor can cause very little effect on the building cost of the ship roughly estimated to be of the order of 1%. This is expected to be easily offset by the annual fuel savings itself considering the powering benefits. Besides, huge savings are expected to follow. The planing craft in section 3.1 is currently operational. Hence, the energy saving devices have to be retrofitted. The cost of retrofit, if assumed to be of the order of 50 lakh Indian rupees, can be evened out by fuel savings in 1 year. Besides, the operational requirements can be effectively met. For the planing craft mentioned in section 3.2, the requirement of the owner is to achieve the contractual condition failing which will invite huge penalties. Therefore, the use of energy saving devices is unavoidable. The installation cost of interceptors is expected to be less than 5% of the total cost. It is understood that differential interceptors are also improving the turning ability of the vessel by enhancing the heel. The stern wedge designed on the foil assisted catamaran has already been fitted by bending the aluminium plating on the 10m technology demonstrator vessel. The drag reduction at hump speed gave remarkable improvement in performance. The cost of wedge fitment was absolutely minimal. To complement this, the fuel savings in the hump zone was remarkable.

5

SUMMARY

The following points are outlined for future works:

7.

•

In fast displacement ships, the performance of wedge-flap combination is much better than flap or wedge in isolation.

•

The novel concept of wedge-interceptors on fast displacement ships may become an even better option for powering reduction subject to full scale results.

•

The reduction in effective wake width and wake wash is more in wedge-interceptor and wedgeflap combinations.

•

Breakaway speed is reduced by 4 knots at model scale by stern wedge-flap combination. The interceptor combinations gave a reduction of 2 knots.

•

Effective increase in hydrodynamic length is best for stern wedge-flap.

•

Planing craft trim is highly reduced by wedgeinterceptor combination. Reduction in trim by interceptors is better than stern wedges.

•

The powering performance of a planing craft is better optimised by interceptors than a wedge.

•

On high speed planing crafts, the importance of spray rails is elucidated. Spray rails may be designed in such a way to cut off spray and provide an additional lift.

•

Energy saving devices are equally effective on hybrid crafts like foil assisted catamarans.

•

The use of energy saving devices listed here seems to be economically feasible.

ACKNOWLEDGEMENTS

We would like to thank Prof. O.P. Shah, IIT Kharaghpur and Shri VBS Ayyengar, Scientist, NSTL for their guidance in pursuing this work. We also thank Sunny Verma, Scientist, NSTL and the entire testing team of HSTT, NSTL for helping with the study. 8.

REFERENCES

1. KARAFIATH, G. et al, ‘Stern Wedges and Stern Flaps for Improved Powering – US Navy Experience’, SNAME Transactions, Vol. 107, 1999

2. YAAKOB, O., et al, 'Stern flap for resistance reduction of planing hull craft: A case study with a fast crew boat', Jurnal Teknologi, 41(A) Dis. 2004: 43–52 3. PRAVEEN, PC and MD. KAREEM KHAN, ‘Interceptor for better Hydrodynamic Performance of a Planing Hull’, International Workshop Conference & Expo in Engineering and Marine Applications, 2010 4. CUSANELLI, D.S. et al, ‘Effect of Stern Flaps on Powering Performance of the FFG-7 Flaps’, NSWC Carderock Division, Technical Report 5. SAVISTKY, D., ‘Hydrodynamic Design of Planing Hulls’, Marine Technology, Vol.8, No.4, Oct. 1964 6. SAVISTKY, D., DeLORME, M.F. and DATLA, R., ‘Inclusion of Whisker Spray Drag in Performance Prediction Method for High-Speed Planing Hulls’, Marine Technology, Vol.44, No.1, Jan. 2007 7. SRINIVASAN, VB et al, ‘Experimental Investigation on the performance of a High Speed Foil Assisted Catamaran’, International Conference in Marine Hydrodynamics, 2006 8. SRINIVASAN, VB et al, ‘An Investigation into the performance of Hybrid High Performance Vehicles’, 5th International Conference on High Performance Marine Vehicles, Australia, 2006 9. CUSANELLI, D.S., ‘Scaling Effects on Stern flap Powering Progress Report’, NSWC Carderock Division, Technical Report, September 2009

9.

AUTHORS BIOGRAPHY

The authors are scientists at the Hydrodynamic Research Wing of the Naval Science & Technological Laboratory, India. Their areas of work include improving hydrodynamic performance of marine vehicles through ship model experiments and computational fluid dynamics, development of hybrid hulls and propulsors and full-scale sea trials.