A P R A C T I C A L APPROACH TO A FAST D I S P L A C E M E N T SHIP'S
S T A B I L I Z A T I O N IN HEAD SEAS
Victor Sokolov and Serge Sutulo
"Balttechnoprom" Co.
7/19-12 K I M Av., 199155 St. Petersburg, Russia
A B S T R A C T
Fast displacement and semi-planing ships are typically especially vulnerable in head seas at resonance and above-resonance conditions when large heave and pitch amplitudes, unacceptable acceleration levels and/or excessive slamming loads can occur. Veiy often, all those phenomena would force the ship's captain or master to reduce the speed deliberately thus partly impairing the natural advantage and the veiy sense of existence of such vessels. That is why, all technical solutions effectively leading to attenuating heave and pitch motions are of considerable practical interest.
In the proposed contribution, a group of new passive anti-pitch devices (above-M'ater small aspect ration fins with negative setting angle and the protruded ram-like stabilizing bow) patented by one of the authors is described. Given are also heuristic considerations which resulted in these inventions. Significant effectiveness of one of these devices M'as confirmed by means of a model experiment and through the time-domain mathematical modelling based on the strip theoiy and on a simple semi-empiric theoiy for the stabilizing forces. Both the experimental and numerical results combined M'ith the absence of significant expected side effects support the belief that the proposed stabilizing devices could be implemented on many fast catamarans and mono-hulls.
I N T R O D U C T I O N
Conventional fast displacement monohulls and, especially, catamarans have become lately a very popular mean of fast water hansportation successfully competing with more sophisticated craft such as the hydrofoils, SES and various hybrid vessels. When properly designed, these ships combme certain robustness and simplicity with low resistance and wash, and good seakeeping qualities. However, the latter have certain limitations. In the case of the fast displacement catamarans (FDC) those are mainly related to the wet-deck slamming which appears inevitably at a certain sea state bearing in mind that most FDC are relatively small semi-planing ships operating at Froude numbers rangmg from 0.5 to 1.2. Even when the ship's sfructures are designed to withstand the high hydrodynamic pressui'e and loads resulting from slams, the latter still w i l l cause shuctural vibrations impairing habitability of these ships mostly designed as fast passenger ferries. Another unpleasant phenomenon is the increased acceleration level resulting in a higher sea-sickness rate and in the degradation of the working conditions of the crew.
The both phenomena are mainly linked with longitudinal ship motions in head seas and any technical solution leading to a reduction of the ship's response to the wave excitation in such regunes must be considered with gi'cat attention. Reduction of the ship's sensitivity to the wave action not only extends
weather operational limits and the average speed in typical conditions but can also help to reduce the required hull sfrength and, hence, the weight of the ship sfructui-es that w i l l result in a better economic efficiency.
For any ship of some given architectural type, size and displacement, improvement of the seakeeping qualities can be achieved by means of one of the following approaches:
1. The hull shape optunization with the objective of reaching the best values of certain seakeeping criteria.
2. Use of special stabilizing devices reducing the ship's response to the waves.
3. Combination of the both previous approaches meaning that there are some stabilization devices but they rather have the character of certain local hull deformations and thus can be called integrated stabilizers. The first approach has been fried by various authors. One of the latest attempts was undertaken by Grigoropoulos (2004) who applied a local direct search method to minimize the weighted ship response to regular waves. The hull shape was supposed to be defined by a relatively small number of parameters subject to rather sensible consfraints. Application of this method to normal displacement ships, such as a 8600 tons desfroyer, resulted in 10¬ 20 percent reduction of relative bow motions and accelerations without deteriorating the propulsion perfonnance in still water. However, the same
International Conference on Fast Sea Transportation FAST'2005, June 2005, St. Petersburg, Russia
optimization applied to smaller and relatively faster semi-planing ships will likely resuh in more modest improvements due to a lesser certainty of resistance and seakeeping mathematical models at higher Froude numbers and especially for multihull configurations
In any case, dramatic unprovements of the ship's behaviour through the hull shape optimization are not very likely as any reasonable hull shape designed with account for accumulated shipbuildmg experience and haditions will be satisfactory ui most respects. O f course, lifting the conshaints could make the optunization more effective but then, very likely, the ultimately seakeeping-optimized configurations will just look like that of a SWATH.
Anyway, be the hull form optimized or not, special ship stabilizing devices can flirther improve the ship's behavioui- in rough seas. The rest of the resent paper will be dedicated to these devices. The analysis w i l l , however, be limited to the longitudinal stabilization with the help of hydrodynamic stabilizers. Stabilizing tanks, very popular for the roll reduction, have never been applied successfully for heave and pitch attenuation although this still doesn't mean that this is theoretically unpossible and maybe in the ftihire some suitable technical solutions will be found and implemented.
2. D E V E L O P M E N T O F A N T I - P I T C H I N G FINS
Fhst, it must be noted that the traditional term "anti-pitching fms" is not quite accurate as such fins will affect not only the pitch but also the heave and, to a lesser extent, the surge. However, there is probably some rationale behind the hadition as, on one hand, pitching is certainly the most annoymg kind of ship motions in head seas and, on the other hand it is most subject to reduction.
Primarily, the anti-pitching fins were considered in application to normal moderate-speed monohulls. Conolly (1967) describes resuhs of full-scale hials of two identical Royal Navy desfroyers one of which was equipped with the fixed bow anti-phching fins. The fins were located near the ship's stem and the base Ime formmg a small-aspect ratio wing of a bi-frapezoidal shape and with a NACA profile. This wing was installed at a certain setting angle with respect to the base plane (the leading edge was higher than the hailing edge). This angle was chosen after the model tests from the condhion of minimum still-water resistance. However, these hials have in fact condemned the anfi-pitching fins, at least for classic displacement ships: although their effectiveness was confirmed, they also produced unacceptable sfructural vibration. An elementary linear mathematical model for such stabilizers was, anyway, created and validated against tank data (Vugts, 1970). The idea of pitch reduction gained new momentum with the catamarans. A natural anti-phching device is then a
hydrofoil linking the both hulls near the stems and the base plane (Catamaran stabilizmg fin, 1968). This hydrofoil was of a substantially large aspect ratio which, hydrodynamically, was even approxunately doubled as the hulls could be considered as endplates. The overall configuration was structurally very different from that on the monohulls and there were shong reasons to believe that the vibration will not threaten that much.
The catamaran fishing vessel Experiment-2 equipped with such a hydrofoil was built in Russia m the middle of 70s (Dubrovsky and Lyakhovitsky, 2002) and entered then the commercial service. Although consfruction of such catamarans was soon discontinued as they turned out economically mefficient, that relative success in full-scale longitudmal stabilisation inspired sttidies in hydrodynamic of stabilized muhihuU vessels (Sutulo
1975, 1976, 1981, 1982). The most interesting results of those investigations were:
1. A simple numerical algorithm for calculation of the hydrofoil's optunal setting angle was devised. The optknality criterion was the zero-lift condhion m still-water which was very close to the condition of minimum induced drag. The thus defmed setting angle depends mainly on the bow waterline enhance angle and is usually equal to 5-15 degi-ees with the raised leading edge.
2. A twisted stabilizing hydrofoil was proposed for complete suppression of the mduced drag. The local setting angle is then varying along the span in accordance with the flow sheamlines calculated with a potential flow code.
3. A fully nonlinear mathematical model for the hydrofoil stabilizing forces was developed and h was demonstrated that the linearized model error could become much greater than it had been believed. Namely, the exact kinemafic relations must be used and, as typical attack angles on moderate-speed ships can be very large, up to 50 degrees, the hydrofoil works the great part of the time m the stalled regime. A semi-empiric model of the dynamic stall phenomenon accounting for the hysterisis loop appearing in the lift curve.. 4. Developed was a seakeeping code supporting
estimation of the stabilized ship's responses in the frequency domain. The hydrofoil nonlinearities were taken into account by means of the harmonic and statistical linearization.
5. Comparative calculations have confumed that the setting angle of the hydrofoil practically doesn't influence its stabilizing effect even when the asymmetric dynamic stall is taken into account. Of all these results, probably, those related to the dynamic stall were the most conhoversial: although in general the nonlinear model unproved the agreement between the predicted and measured hydrofoil effectiveness, often the best results were obtained without account for the stall but with full
account for the kmematic non-lmearity and no dhect confirmation of the stall's presence was obtained (Sutulo 1982). However, recently mdependent studies dealing with this phenomenon on anti-roll fms have appeared (Gaillarde 2003, Perez and Goodwin 2003). Anyway, the development and conshuction of the stabilized moderate-speed catamarans were discontinued and all the results outlmed above did not fmd extensive applications. Probably, very few of them can be applied to the hybrid or dynamically supported catamarans (Kang et al. 1993, Davis and Hollway 2003, Sclavounos and Borgen 2004) where a shnilar hydrofoil is supposed to be loaded hi order to produce lift reducmg the ship's draught, wetted surface and resistance. Likely, hydrodynamics of these lifting surfaces must be much closer to those of the hydrofoil ships on which linear models are in most cases quite adequate and sufficient.
3. NEW A N T I - P I T C H I N G D E V I C E S F O R F A S T CATAiVIARANS
Traditionally, the ship's conhactual speed in still-water conditions is a veiy important parameter whose non-fulfilment can bring serious financial damage to the builder. The ship's optknization from the resistance viewpoint presumes a smooth hull with as little number of appendages as possible. This requhement, however, can come in confradiction with the deshe to improve the seakeeping qualities with f m stabilizers as in most cases they mcrease the still-water drag of the ship. To resolve this conhadiction, a new kind o f stabilizing fms was suggested: the above-water small aspect ratio stabilizing fms (Fig. 1). The main idea behind this device is that it has no contact with the surface when the ship runs in still water but starts to enter the water even on relatively low waves reducing the longitudinal ship motions. The first Russian seagomg fast displacement catamaran Captain Korsak was buik by the Russian
Marine Technical Company, predecessor of the
present Balttechnoprom, m 1995 and entered the service. Initially, however, a series of at least two sister-ships was supposed to be constructed and the second vessel was to be equipped with the described anti-pitching fms. Unfortunately, due to the altered economic situation, the order was later cancelled and this device remained ununplemented up to present. But rather extensive studies concerning hydrodynamics of the stabilizing fms were carried out by Sokolov and Sutulo (1996). These sttidies mcluded experiments on regular waves in the towmg tank and also the development of a semi-empiric mathematical model for this kind of stabilizer. The latter had to be done practically from scratch as the configuration and the flow specifics were very different fi'om those observed on fraditional anti-pitchmg fms.
Fig. 1: Low aspect ratio above-water anti-pitching
fins: 1—ship's hull; 2—fin; 3—waterline; the setting angle a is negative.
The principal resuhs of those studies (more detailed infonnation can be found in the referenced paper) are: 1. Significant effectiveness of the proposed fins was
confirmed experhnentally.
2. Observations of the fin enhance process in head waves permitted to establish distmction between 3 different regimes: normal flow around a fully submerged hydrofoil, short planing followed by a hansition to the first reghne, and penehation with the formation of a jet on the upper surface and of a cavity on the lower surface. There were no cases of hard chock entrance with the fin surface approximately parallel to the water surface at the moment of contact.
3. The f m normal force was supposed to constittite of two mdependent parts: the potential force esthnated as on a flat plate m the accelerated flow and the viscous part esthnated by means of the combination of the approximate low aspect-ratio wing theory with the cross-flow drag theory. The viscous force was estimated withm the framework of the quasi-steadiness concept which is rather accurate for small aspect-ratio wings. A few adjustable parameters were infroduced to account for transient reghnes with partially and slightly submerged fms and later tuned after the experunental data.
4. A sunple time-domain seakeeping simulation code including the mathematical model of the fins was developed. Using the fi-equency-domam formulation was impossible due to the substantially nonlinear character of the system including the entering and suifacing wings. Shictly speaking, the algorithm should have included memory functions or some equivalent tool but in the existing simplified version the ship-method complex algebraic equations of heave and pitch were fonnally re-written as ordinary differential equations with constant coefficients depending on the encounter fi'equency. For a non-stabilized ship this is fully consistent but when non-linear forces
International Conference on Fast Sea Transportation FAST'2005, June 2005, St. Petersburg, Russia
from fms are added, this model remains approximately valid. In some sense, this approxunation is equivalent to the harmonic Imearization and one can expect that the amplitude values w i l l be estimated with sufficient certainty. 5. Systematic calculations have given results which
agreed with the experunental ones but also made possible estknation of the heave reduction (practically none) and also attenuation of acceleration in the passenger cabin and of the bow relahve displacements (less significant).
Fig. 2: VieM' of a fast naval combatant equipped M'ith
the "tusk" integrated stabilizer
It must be emphasized that both the experhnent and computations indicated to the advantage of a negative f m settmg angle. This propeily is very different from what is observed for a fully submerged hydrofoil and can be explamed by the specific substantial nonlinearity. On the other hand, the inhial idea behmd this stabilizer was to airange it in such a way that it would cut o f f the peaks of the exciting force in the bow region and the observations left an impression that just this was happening. In fact, however, the real effect is not so obvious as the stabilizing mechanism is substantially more complicated.
Natural evolution of these stabilizers may result in an integrated stabilizer looking like a specially shaped forebody (Fig. 2) and named "tusk". The authors expect that this hull modification will resuh in even better behaviour of a ship in head seas but so far no direct confimiations of this can be presented.
C O N C L U S I O N S
A review of passive hydrodynamic stabilizers for longihidinal motions of catamarans and other fast displacement ships is presented and two promishig variants of robust passive stabilizers are suggested. It can be stated that these low-cost, reliable and simple devices can compete with much more complicated, although maybe somewhat more efficient active stabilizers like T-foils and confrolled flaps.
R E F E R E N C E S
1. Catamaran stabilismg fm, (1968), Shipbuilding and
Shipping Record, 112, p. 370.
2. Conolly J.E., 1970, Sea hials of anti-phchmg fms, Trora. T^TV^, 112, pp. 87-100.
3. Davis M . , Hollway D., , Effect of sea, ride conhols, hull form and spacing on motions and sickness mcidence for high-speed catamarans, Proc.
FAST'03, Session F, pp. 1-10.
4. Dubrovsky V., Lyakhovitsky A., 2002, Multi-Hull
Ships. Fair Lawn, NJ, Backbone Publishing Co.
5. Gaillarde G., 2003, Dynamic stall and cavitation of stabiliser fins and theh influence on the ship behaviour, Proc. FAST'03, Session E.
6. Kang Ch.-G, Hong S.-Y., Suh S.-H., Lee M.-Ch.,
Kun Y.-G., Gong l.-Y., 1993, Attihide control system for a high-speed catamaran with hydrofoils in waves,
Proc FAST'93.
7. Perez T., Goodwin G.C., 2003, Consfrained control to prevent dynamic stall of ship fin stabilizers,
Proceedings 6th Conference on Manoein'ring and Control of Marine Craft, Girona, Spain.
8. Sclavounos P.D., Borgen H., 2004, Seakeeping analysis of a high-speed monohull with a motions-conhol bow hydrofoil, J. Ship Research, 48, pp. 77¬ 117.
9. Sokolov V.P., Suttilo S.V., 1996, Shidy of the seakeepmg of a fast displacement catamaran equipped with above-water bow anti-pitching fins,
Trans, of the Third International Conference in Commemoration of the 300th Anniversaiy of Creation of Russian Fleet by Peter The Great (CRF-96). St Petersburg (Russia), pp. 487-514.
10. Sutulo S., 1975, Approxhnate solution to the problem of the hydrodynamic interaction between the bow stabilizing f m and the ship hull Trans. Leningrad Shipbuilding Institute, 96, pp. 102-108 (m Russian).
11. Sutulo S., 1981, On the mfluence of wave damping and of the non-linearhies of kinematical relations on the predicted stabilizing properties of anti-pitching fins. Trans. Leningrad
Shipbuilding Institute: Hydrodynamics in Ocean
Technology, pp. 105-114 (in Russian).
12. Suhilo S., 1982, Application of the harmonic linearisation technique for prediction of heave and pitch of stabilized vessels. Trans. Leningrad
Shipbuilding Institute: Ocean Technology, pp. 99¬
111 (in Russian).
13. Vugts J.H., 1967, Pitch and heave with fixed and controlled bow fins. Int. Shipbuild. Progi'., 14, pp. 191-215.
