THE USE OF A VERTICAL BOW FIN FOR THE COMBINED ROLL AND YAW STABILIZATION OF A FAST PATROL BOAT

Transcription

1 THE USE OF A VERTICAL BOW FIN FOR THE COMBINED ROLL AND YAW STABILIZATION OF A FAST PATROL BOAT J Alexander Keuning, Shiphydromechanics Department, Delft University of Technology, Netherlands Guido L Visch, Shiphydromechanics Department, Delft University of Technology, Netherlands SUMMARY From many research projects it is known that for fast patrol boats the motion behavior in large stern quartering and following seas is often a limiting situation for its operability. The broaching tendency that may occur with most of the relatively small (shorter then 5 meters) and fast (more then 25 knots) patrol boats often implied that a significant change in forward speed or heading had to be made to prevent serious problems. The rudder action of the aft rudders in particular in stern quartering seas, required to keep the boat more or less on track, significantly aggravates the rolling motion of the ship and so the tendency towards a broach. A vertical fin at the bow however would have an opposite and thus positive effect on the roll motions in those conditions. By using this forward vertical fin (or bow rudder) to control the yaw motion of the ship in large waves, in addition to the rudders aft, due to the direction of the lift force and its phase the rolling motion is reduced instead of increased, contributing significantly to the resistance against broaching. The introduction of such a vertical fin on a conventional bow is difficult due to all kinds of practical reasons. The very shape of the hull according to the AXE Bow Concept, introduced by the author in earlier publications since 21, however makes it quite feasible to place such a vertical controllable fin at the foremost end of the ship. In the paper the mechanism and the physics involved of such a vertical bow fin in stabilizing the yaw and roll motions in waves will be described. In addition the results of an extensive series of experiments with an AXE Bow model fitted with various realizations of such bow fins will be presented. Finally a series of tests with a free running model fitted with such a bow fin has been carried out in the sea keeping tank of MARIN in stern quartering seas to check the principle behind the idea. A limited number of these results will be presented in the paper. NOMENCLATURE Lwl Bwl T V Vmax GMt k Fy Mz Mx 1. INTRODUCTION Length waterline Breadth waterline Draft amidship Displacement Maximum Speed Transverse Metacentric Height k-factor Side Force Yaw Moment Roll Moment The use of fast craft in a seaway has always posed many challenges to the comfort of those on board and the safety of the ship. Partly this is due to the fact that most applications of fast ships are restricted to the relatively smaller vessels. If we consider ships with speeds in excess of 25 knots as fast, their typical length is generally restricted to 5 meters over all. This implies that the waves they encounter tend to be relatively large compared to the ship size. Improvement of the sea keeping behavior of the ship may typically be found in increasing the pure size of the ship, but this comes at a cost. In the past decades considerable attention has been paid to improving the operability of fast ships in head waves because in those conditions severe damage to people on board as well as to the ship itself could be experienced. The emphasis was on the limitation of the vertical accelerations and in particular the big peaks, i.e. the slams. Typical improved hull forms have been developed and build, such as the Enlarged Ship Concept, Ref [1], [2] and the AXE Bow Concept, Ref [3] and [4]. Much has been achieved in this respect and the operability has been increased significantly. In the present study emphasis is placed on other restricting phenomena when sailing with fast ships in a seaway. One of these limiting phenomena is the tendency to broach when sailing at speed in following or stern quartering seas. 1.1 THE BROACHING PHENOMENON Broaching is a well known phenomenon and may be best described as a coupled roll-yaw and pitch motion of the ship. From full scale experience and systematic research it is known that this broaching behavior is often introduced through a combination of a lack of transverse stability of the ship (at speed) and insufficient directional stability. What generally happens can, in physical terms, best be described as follows: the ship is sailing at high speed in stern quartering seas. Through the high speed the encounter frequency of the ship with the waves is low. Let us now assume the waves come in from the port quarter. When a high wave reaches the stern of the ship the stern is lifted. Because more often then not the sterns 7

2 of these ships are broad and flat the ship is simultaneously heeled to starboard. Through this combined pitch and roll motion the bow is now more deeply submerged in the wave crest just in front of the boat. This deep submergence in combination with the roll angle introduces an asymmetry and so a considerable yawing moment on the ship to port. In addition the whole sequence of events leads to a considerable loss of directional stability. This is further aggravated by the fact that these ships in most cases have two rudders each at one of the ship of which the port (windward) rudder will now most likely be partly lifted out of the water. In order to keep the ship as much as possible on a straight track considerable rudder action is e required. The rudders are pulled over to starboard to correct for the course change and the yawing moment. The rudders, placed aft and underneath the hull, generate a lift force to port and so a counter balancing yawing moment to starboard. Simultaneously however they also generate a considerable rolling moment and in the particular situation under consideration to starboard, which leads to an increase in the undesirable roll motion. If all goes well control is maintained and the boat brought back to its original course with the roll- and the pitch angle at reasonable and manageable values. In the worst case the yaw motion gets out of control and the ship ends up in beam seas at excessive heel, sometimes even leading to a capsize. The photos in Figure 1 show the two phases of a moderate broach. Figure 1. Phases of an AXE hull model without bow fin broaching. It is known that the phenomenon is most eminent in waves in between 1.3 and 1.5 times the ship length and so for a 4 meter vessel this implies that the encounter frequency becomes almost zero with such waves (i.e. wavelength of 6-7 meters) at or around 2 knots. Solutions for preventing or reducing the broaching tendency of a ship in typical environmental conditions, such as the North Sea, can be found in: Increasing the length of the ship (design issue) Increasing or decreasing the speed of the ship considerably (operational issue) Changing the heading of the ship with respect to the waves (operational issue) 8

3 Increasing the transverse stability of the ship and so reduce the roll angle (Design and operational) Increasing the directional stability by the addition of skegs aft (design issue) Appling an additional vertical bow fin (bow rudder) fore. 1.2 POSSIBLE DESIGN SOLUTION The possible solution which will be evaluated further in this report, is the last one: i.e. an additional vertical fin at the bow. It is designed to reduce the tendency to broach. The very shape of the AXE Bow hull and fore body makes it possible to introduce such a vertical bow fin without much difficulty. The philosophy behind this is that the vertical bow fin forwards effectively generates the desired yawing moment to keep the ship on track because it is more immersed than emerged as is the case with the rudders aft while at the same time it produces a roll moment that reduces the prevailing roll angle. A typical vertical bow fin or bow rudder fitted on an AXE Bow could look like depicted in Figure 2. The presumed advantages of a vertical bow fin for yaw and roll motion control in stern quartering and following waves are: The rudder remains immersed on the most important moment, i.e. when the bow is pushed down and the stern is pushed up. It generates a large additional yaw moment It generates a considerable roll moment, In the coupled roll, yaw and pitch motion of a ship in following and stern quartering waves it has a positive contribution to the roll stabilization Possible disadvantages could be: Increased calm water resistance due to the transition between rudder and hull Increased construction weight at the bow 2. VALIDATION OF THE PRINCIPLE IDEA To check whether the principal idea works it was decided to carry out a dedicated model experiment with a model of an AXE Bow in the new seakeeping basin of MARIN at Wageningen. This test was carried out in conjunction with the FAST Project described in previous publications, Ref [4] and Ref [5]. The model used was the AXE Bow model of the FAST project, a 55 meter long patrol boat capable of speeds up to 5 knots. Main Particulars of the ship are: Length = 55. meter Beam WL = 8.46 meter Draft midship = 2.26 meter Displacement = 517 tons Speed max = 5 knots GMt = 2.5 meter Figure 2. Bow Fin fitted at an AXE hull model Figure 3. Linesplan of the used Aexebow model 9

4 The model was equipped with two water jets with steerable nozzles. The maximum deflection angle of the nozzles was restricted to 23 degrees either side. At the aft end also two fixed skegs were fitted to the hull. The tests were carried out with the free running model, solely propelled by the two waterjets. The unique SMB facility of MARIN allows the model to run completely free of the towing carriage in irregular waves from any direction. The course of the model is controlled by an autopilot. In the tests with the bow fin there was a direct 1:1 mechanical link between the steering adjustment of the waterjets and the bow rudder. Only the direction of the deflection of the bow fin was reversed with respect to the aft rudders to yield a similar yaw moment resulting from the bow fin as was established with the steering nozzles aft. The tests were carried out in one typical North Sea spectrum, which, according to the available wave scatter diagrams of that area, is only exceeded 5% of the time all year round. The main particulars of this spectrum are : a significant wave height Hs equal to 2.5 meter, a peak period Tp equal to 6.75 sec and a energy distribution over the frequency range according to the normalized Jonswap spectrum. Considering the wavelengths in the spectrum a forward speed of around 2 knots was chosen because this posed the largest likelihood of broaching in the situation chosen, i.e. a wave incidence angle of 315 degrees (i.e. port stern quartering). In the spectrum realization a considerable number of tests was carried out to obtain a test run duration of circa 2 hours at full scale. The tests were carried out both with the AXE Bow model without vertical bow fin and with the model fitted with the vertical bow fin. The main particulars of the bow fin used are those depicted in Figure 2. The results are presented in the following figures: in Figure 5 the results for the conventional AXE Bow and in Figure 6 the results for the AXE Bow fitted with the vertical bow fin. The results are presented as plots of the probability of exceedance (in percentage of the total number in the entire time trace) of the peaks and the through of the time signal under consideration. The horizontal scale is sized to fit the Rayleigh distribution, which comes out as a straight line. The extremes of the peaks and troughs are found at the far right side of the plots, i.e. with the low probability of exceedance. over the entire track is some.5 degrees to starboard, which is understandable with the waves coming from the port stern quarter. Roll [deg] Yaw [deg] Nozzle [deg] 15 5 crests troughs Rayleigh Plots without bow fin / / /5 Signal unit Mean stdev Min Max Roll deg,38 2,21-8,41 9,88 Yaw deg, 2,97 -,66 16,4 Aft-Rudder deg -4,6 11,88-23,57 23, Figure 4. Rayleigh Plots and Statistics without bow fin As may be seen from these results the effect of the application of the vertical bow fin in these conditions is quite significant: The significant roll amplitudes are reduced by some 3% and the maximum roll amplitude encountered during the 2 hours even by some 4%. For the ship without bow fin the maximum roll angle to starboard is slightly larger than the maximum roll to port. The average roll angle 1

5 Roll [deg] Yaw [deg] Nozzle & Bow Fin [deg] 15 5 crests troughs Rayleigh Plots with bow fin / / /5 Signal unit Mean stdev Min Max Roll deg,55 1,56-4,8 6,82 Yaw deg, 1,49-5,39 6,55 Nozzle & Bow Fin deg -2,79 8, -25,86 23,23 Figure 6. Rayleigh Plots and Statistics with bow fin It is also of interest to note that with the application of the bow fin the reduction of the roll amplitudes to port is considerable larger than the reduction to starboard. This may be partly explained by the fact that the autopilot used to keep the ship on track is controlling the nozzles for the ship without bow fin and both the nozzles and the fin for the model with bow fin. This autopilot has as only input signal the yaw angle (course of the ship) and not the roll motion. The average offset in the course due to the wave action from port quarters shows up as an average nozzle angle of circa 2.5 degrees. This yields the differences in the distribution of peaks and troughs in roll for the model with vertical bow fin. Because the bow fin introduces a significant roll moment and this is not the case with the waterjet nozzles which are placed much closer to the vertical center of gravity of the ship. An autopilot which controls the combination of both, i.e. controlling yaw and roll simultaneously, in a way similar to the already existing rudder-roll stabilizers, may possibly overcome this phenomenon. For the time being the average between the distributions of the peaks and the troughs could be considered for the sake of comparison. The roll angle reduction in that situation with the bow fin added increases then even further and well to over 5%! A similar trend may be seen with the yaw motion:with the bow fin added the yaw motion is significantly smaller than without. Here the reduction in both the significant and the maximum amplitudes is also in the order of 5%. In particular the reduction in the extreme values of yaw and roll are of interest because these may be the introduction of a broach. From the registration of the rudder angles during the tests it may be seen that much less rudder action (i.e. smaller angles) is necessary to keep the ship on track for the model with bow fin. This is understandable because the amount of control (surfaces) has been increased significantly. In the situation without bow fin the maximum nozzle angle is reached more often than not. In the situation with bow fin this is hardly the case, which leaves much more room to control the ship in those conditions. The general conclusion that may be drawn from this experiment is that the application of the vertical bow fin in stern quartering seas is very effective indeed in reducing both the roll and the yaw motion. 3. THE VARIOUS CONCEPTS OF THE BOW FIN Now the validity of the concept has been demonstrated, the actual design of the fin and the design of a controller had to be assessed. The first step in this process was to establish the effectiveness of various bow fin designs in generating side force, yaw moment and rolling moment with respect to the one used during the tests at MARIN. The aim of the series of experiments was to determine the minimum size rudder that is adequate for the job. The reason behind this aim is found in some structural and interior layout limitations and the possible negative effect of the bow fin on the calm water resistance because the fin will not be used for a certain amount of time and should therefore generate as little disturbance as possible in those conditions. This aim was to be achieved by measuring a number of the hydrodynamic derivatives necessary for inclusion in the mathematical model available at the Ship hydrodynamic Department (FASTSHIP) for all configurations considered feasible as vertical bow fin on the AXE Bow. The following six different configurations have been examined: 111

6 The first three are all vertical bow fins incorporated in the bow profile of the AXE Bow model as presented above. In principle it is a change the rudder area established by keeping the height of the rudder as in the original design used in the MARIN tests and reducing the chord length in two steps yielding the original or large rudder, the medium rudder and the small rudder. The principal dimensions of these rudders are depicted in the Figure 7a, 7b and 7c below. Figure 7 Small bow fin Figure 7a. Large bow fin Figure 7b. Medium bow fin The reduction of the bow fin area by reducing the chord length of the fins implies an effective increase in the aspect ratio of the fins. This had the additional beneficial effect that the beam of the cross section just after the aperture in which the fin was fitted became smaller also. This makes the transition or blending of the vertical bow fin shape, with its typical foil type cross section, into the hull more streamlined. Another possible realization of the bow fin is found in the use of a so called Magnus Rotor in the most forward part of the bow section. The other three configurations investigated were all based on the use of a Magnus rotor. The Magnus rotor works to the effect that a rotating cylinder placed in a flow generates a lift force perpendicular to the incoming flow. The lift force generated is proportional to the velocity of the incoming flow, which is the speed of the ship Vs in m/sec, the rotation angular velocity or ω in rad/sec of the cylinder and the radius of the cylinder in m squared. From earlier tests it is known that the Magnus rotor is a very efficient lift generating device. The very shape of the AXE Bow with its rounded sections lends itself very well for the application of such a rotor. Without extruding from the hull shape as is a rotor with a diameter of.35 meter can be placed at the bow. The rotor is extended in length till the design water line of the ship. The biggest challenge lies in the incorporation of the rotor in the hull shape and the design of the hull shape just abaft and in the vicinity of the rotor. No results in the literature were known about the effect of this on the lift generating capabilities of the Magnus rotor. Three different configurations have been tested: Configuration 1 with the hull of the ship faired around the aft half of the rotor. This configuration yields almost no deviation from the original bow design 112

7 Configuration 2 with a gap just behind the rotor in length equal to the diameter of the rotor, which is then rotating in a sort of gap. This gap will have some influence on the calm water resistance when the rotor is not in use. Configuration 3 with a Magnus rotor extending below the bow. In real life this would be a retractable rotor. The shape of the AXE Bow lends itself very well to such a set up. It yields an unobstructed hull when not in use and a most likely very effective rotor when used. In addition the shape of the AXE Bow places this rotor at a considerable distance below the center of gravity generating large roll moment. The principal dimensions of the three configurations are depicted in the Figure 8a, 8b and 8c. Figure 8c. Retractable Rotor An extensive series of experiments have been carried out using the same model of the AXE Bow as used in the previous MARIN free sailing experiments described above. In the present tests however the model was not fitted with the waterjets but with two rudders aft. This was done because for the sake of comparison the rudders produced much more repeatable results than the waterjets, with their flow dependent steering properties. The dimensions of these conventional aft rudders are presented in Figure 9. Figure 8a. Faired in Rotor Figure 9. Conventional rudders All configurations of a vertical bow fin as mentioned above have been fitted to the model and consequently been tested in the tank. Figure 8c. Free Rotor The new series of tests have been carried out in the towing tanks of the Delft University of Technology. The tank is 142 meters long, 4.25 meters wide and has a maximum water depth of 2.5 meters. The towing carriage is capable of achieving speeds up to 8. meters per second. During the tests the model was rigidly connected to the towing carriage by means of a six component dynamometer and the six degrees of freedom oscillator called Hexamove which was used in this measurement setup as a model position and attitude manipulator. Forces and moments have only been measured on the 113

8 model as a whole, no forces on the rudders or rotors have been measured separately. The test layout is depicted in Figure. the bow fins and in Figure 12e for the rotors. Here too, the results for the aft rudders are presented in the rudders figure. Finally in Figure 12c the roll moment is presented for the bow fins and in Figure 12f the results for the rotors. Direction Aft Rudder Angle Bow Fin Angle Bow Rotor Positive Trailing Edge Starboard Trailing Edge Starboard Anticlockwise From above Figure. Hexamove setup The tests have been carried out with the model in the calm water trim and sinkage corresponding to the forward speed under consideration. The following parameters and all their possible combinations have been varied during the tests: Forward speed of the model at 15, 25 and 35 knots full scale for the bow rudders and at 15 and 2 knots for the rotors, due to limitations imposed by the available facilities at that time. The fin angle between minus 2 and plus 2 degrees Three different yaw angles, i.e. and plus and minus 5 degrees. In the case of the Magnus rotors different relations between forward and rotational velocity of the rotor expressed in the k factor, i.e. k =. The tests generated a large amount of results for use in the mathematical model. In the context of the present paper only a limited amount of the results can be presented. These results are primarily aimed at facilitating the comparison between the various configurations. In Figure 12a and 12d the side force on the ship is presented at 15 knots. This speed has been chosen because it makes a comparison between the configurations possible since it is used with all configurations. In Figure 12a the results for the bow fins are presented and in Figure 12d the results for the rotors. For the sake of comparison the same results for the conventional rudders aft are presented in the rudders figure. In Figure 12b and 12e the yaw moments of the various configurations is presented, once again in Figure 12b for Figure 11. Defenitions Fy [kn] CONVENTIONAL SMALL MEDIUM LARGE FY Rudders Figure 12a. Side Forces Rudders MZ [Kn*m] CONVENTIONAL SMALL MEDIUM LARGE Rudder Angle [deg] MZ Rudders Figure 12b. Yaw Moments Rudders Rudder Angle [deg] 114

9 MX [kn*m] MX Rudders 2 CONVENTIONAL SMALL 15 MEDIUM LARGE Rudder Angle [deg] Figure 12c. Roll Moments Rudders Fy [kn] FY Rotors 25 FAIRED IN 2 FREE RETRACTABLE K-factor [-] Figure 12d. Side Forces Rotors MZ [KN*m] MZ Rotors 7 FAIRED IN 6 5 FREE RETRACTABLE K-factor [-] Figure 12e. Yaw Moments Rotors MX [KN*m] MX Rotors 9 FAIRED IN 8 FREE 7 RETRACTABLE K-factor [-] Figure 12f. Roll Moments Rotors What may be concluded from these results is that the small fin at the bow generates a maximum side force of circa kn, the medium fin a maximum of 3 kn and the large fin a maximum of 4 kn. So the larger size bow fin is certainly the largest lift generator, although it is not proportional to size. However they all compare relatively low in efficiency with the conventional rudders, which generates a maximum lift force of around 12 kn. It should be noted however that this is generated by two conventional rudders aft. The total area of the conventional rudders aft added together is still almost half the area of the large vertical bow fin fore. Because they operate underneath the hull there efficiency is greatly enhanced by the end plate effect of the hull. This reduced efficiency of the bow fins may, amongst others be attributed to the rather complicated flow around the interception of the trailing edge of the fin with the hull geometry and also to ventilation effects. It was noted during the tests that serious ventilation could occur in the more heavily loaded conditions of the foils. This could be remedied by placing the top chord of the fins lower in the water guaranteeing a larger distance to the free surface or by the use of fences at the top. None of these have in the present study been investigated. In the near to broaching condition however this difference in efficiency could be quite different because at least one of the aft rudders may certainly be lifted partly out of the water as can be seen on the photographs in Figure 1. This will yield a serious reduction in efficiency due to loss of submerged rudder area and also ventilation effects. When the yaw moments of the three bow fins are compared the similar trend may be observed: the large fin produces roughly 2 knm, the medium fin 18 knm and the small fin 5 knm. As may be observed in the generated side force as well the maximum moment is reached at 15 degrees fin angle and not at 2, except with the small fin.. The maximum yaw moment with the conventional rudders is 3 knm and also reached at a 15 degrees rudder angle. The difference in side force production is larger between rudder and fins as the differences in yaw moment. The generated roll moments of the three bow fins are also significantly smaller than those generated with the conventional rudders, i.e maximum 14 knm, 28 knm and 6 knm compared to some 186 knm for the aft rudders. Although not shown here all forces and moments are strongly dependent on the forward speed. In most cases the increment with speed is rather more then quadratic. The results for the rotor show in general that the faired in rotor design is hardly more effective then the smallest fin in all modes, ie. for side force, yaw moment and roll moment. The rotor with the gap behind it, i.e. ( configuration 2 ) is far more effective and approaches the large bow fin in characteristics. By far the most effective is the (retractable) bow rotor in configura-tion 3. Although the rotor used in the tests is only half the span of the other two rotors it out performs 115

10 all the others. This can of course be explained by the fact that it is completely undisturbed by any other part of the structure. In addition combined with the AXE Bow hull it is so deeply submerged that it is entirely free from ventilation effects in any of the conditions tested. The biggest advantage may be however found in the relatively enormous roll moment it generates when compared with all the others, fins and rotors and in particular also with the conventional rudders aft. The retractable rotor outperforms the aft rudders in this respect with a factor of around 4. For the rotor in configuration 3 it is also obvious that the maximum lift is achieved at lower values of k, implying lower number of revolutions. The relative differences in calm water resistance of all the configurations is compared in Figure 13. From these results it is obvious that the faired in rotor has the least resistance increase Induced Bowadd. Resistance [%] References [1] J A Keuning and J Pinkster Optimization of the seakeeping behavior of a fast monohull FAST Conference Proceedings, Southampton UK 1995 [2] J A Keuning and J Pinkster Further design and seakeeping investigations into the Enlarged Ship Concept FAST Conference Proceedings, Sydney Australia 1997 [3] J A Keuning, J Pinkster and F van Walree Further inverstigations in the hydrodynamic performance of the AXE Bow Concept WEGEMT Conference on High Performance Vehicles September 22, Ischia Italy [4] J A Keuning, S Toxopeus and J Pinkster The effect of the bow shape on the seakeeping Performance of a fast monohull FAST Conference Proceedings Southampton September 21 [5] FAST Project Seakeeping Model tests for two patrol vessels MARIN SMB Small Medium Large Build In Free Telescope Figure 13. Comparison Rudders and Rotors The retractable rotor has the largest resistance increase when deployed, which will obviously be the case in beam seas to following seas, in which conditions the resistance increase is less of an issue. 4. CONCLUSIONS From the results of these experiments it may be concluded that a vertical bow fin will have a beneficial effect on the controllability of a fast ship in following and stern quartering seas. The configuration most suited, when combined with an AXE Bow hull shape is the retractable rotor underneath the bow. Second best is the medium to large bow fin. There is a great opportunity for a combined yaw-roll autopilot under these circumstances. 116