TRIMARAN HULL DESIGN FOR FAST FERRY APPLICATIONS Stefano Brizzolara, Marco Capasso, Marco Ferrando, Carlo Podenzana Bonvino Dept. of Naval Architecture and Marine Technologies, Univ. of Genova, Italy; Antonio Cardo, Alberto Francescutto Dept. of Naval Architecture, Ocean and Environmental Engineering, Univ. of Trieste. ABSTRACT: Research studies about hydrodynamic performance of trimaran hull configurations and their application to real ship design, are relatively recent, having generally appeared in the last two decades of the last century, without relevant follow up in the shipbuilding world. The proposed paper aims to describe the main features of the hydrodynamic design of a trimaran ship as an alternative solution to contemporary mono hull or catamaran fast ferries (36-40 knots of service speed). This work has been developed in the frame of a three year joint research project of the Departments of Naval Architecture of the Universities of Genova, Trieste and Napoli and the University of Ancona. Comparison of the main resistance and seakeeping properties of two different trimaran design (having different hull forms but equivalent in terms of main geometric parameters and total displacement), tested in model scale with different side hulls positions will be addressed in the paper. The design exercise presented in the paper has comprehended also other aspects such as the implications that different hull typologies can have on the general layout of the vessel, problems related to the intact and damaged stability, internal spaces and propulsion arrangements.
INTRODUCTION After Triton (the trimaran demonstrator built by DERA in 2000 [1] the shipbuilding world became conscious that modern and efficient ships could also be built having a trimaran hull configuration. Nevertheless, the advantages and implication of trimarans in terms of the general ship design have been rarely published [2]. Aiming to fill this gap this paper proposes a comparison between the main characteristics of two different hull geometries for a fast ferry trimaran for commercial applications. The design of both trimaran hulls has been based on an initial market survey which brought to the definition of the basic reference design conditions, such as the merchant ship type, mission profile and payload. As a result of this study [3], the Mediterranean sea scenario and a medium size fast ferry able to carry passengers and cars having the following design characteristics were selected: - Main hull L/B>10 - Displacement of each side hull: about 5% of total - Maximum total breadth: approx. 28 m - Service speed: up to 40 knots - Payload: 800 passengers and 240 cars The value of L/B ratio of the main hull (taking into account that a total breadth around 28 m allows the vessel to use standard harbor facilities for monohull vessels), relatively lower with respect to other trimaran designs, was chosen in order to obtain a reasonable breadth of the lower car decks of the main hull, while preserving reasonably low wave resistance at high speed, and good seakeeping. Having a maximum speed exceeding (by more than 50%) the value of 3.7 0.1667 computed at the design displacement volume, the trimaran is to be considered a high-speed craft and falls under the regulations of IMO HSC Code. The amount of buoyancy reserved to the side hulls was selected as a compromise to obtain additional reserve of buoyancy for stability and to limit as much as possible the increase in total resistance. The first trimaran design was prepared as a consequence of the first design spiral performed for the feasibility project. Subsequently, an alternative hull design for the same targets has been prepared [4] allowing an interesting comparison between the two different hull geometries, equivalent from the main design specifications, i.e. payload, speed, range and main geometric parameters relevant to the resistance and seakeeping performance of the vessel. Both hull geometries are described in detail in the next sections. The two main and side hulls have been designed in such a way to be fully interchangeable, to permit a direct comparison of the effect of the hull typology on the hydrodynamic performance of the trimaran, without any other side effects due to the variation of main geometric parameters such as length and displacement or initial attitude, for instance.
HULL FORM GEOMETRIES Different design of both central and side hulls has been done from the first to the second model. Main geometrical characteristics of the two alternative main hulls and corresponding side hulls are given in Table 1. The first trimaran features a main typical fast round bilge hull (Figure 1) having V-type bow sections with low flair and stern U-type sections closed towards the transom in both vertical and transversal directions. The deadrise angle of the main section is around 15 deg.. The longitudinal prismatic coefficient was kept rather low (0.645) to limit wave resistance together with the high slenderness ratio L/B and L/ 1/3. Side hulls were designed like with hard chines forms (Figure 1) with simple slightly convex bow sections, high deadrise angle of the Tab. 1: Main geom. characteristics of both design main section (abt. 29 deg.) and slightly warped aft straight sections. The main hull of the second model was chosen of Deep-V type (Figure 2). In the case of high speed monohull ships, in fact, it was shown that Deep-V hull forms have considerable advantages over conventional round bilge hulls, in terms of resistance and seakeeping ([5] and [6]) at high speed. The idea, then, was that of check these advantages also in the case of a trimaran ship. The main Deep-V hull features simple transverse section made of straight line having high deadrise angle at the main section (abt. 34 deg.), same length and displacement of the previous hull as well as same C P and LCB for a fairer comparison of the residual resistance of the two alternative main hulls. The fullness of the main section results lower than that of the first round bilge hull so a higher breadth and draft were necessary to obtain an equal displacement. Stern bottom lines, still straight, are highly warped toward the transom where the bottom becomes almost horizontal and the sections close gradually in transversal direction. The same transom area was maintained for both main hulls, in order to permit the same waterjet arrangement and roughly the same contribution of the transom wave formation to the wave resistance. The side hulls of the second model (Figure 2) were re-designed as well, assuming simple symmetric U-type section with a very small transom. The models of the two trimarans were built to a scale of 1:60 and tested in the towing tank of the University of Genova for still water resistance tests and in the towing tank of the University of Trieste for seakeeping. The trimaran models were built in ABS material by means of a modern technique used in mechanical engineering for rapid prototyping (Fused
Figure 1: Body plan of the main round bilge hull and of the asymmetric side hulls (half side of the body plan) of the first trimaran deposition Modeling), with an overall accuracy of about +/- 0.15%. Having defined Clearance and Stagger as follows, it was decided to test the model with a systematic variation of stagger and clearance of the side hulls side hulls according to the test matrix presented in Table 2. Stagger (%) = Long. distance between the side hulls transom and the main hull transom, in percent of a reference main hull length equal to 120m. Clearance (%) = Lateral distance between the external side (in case of first model) or symmetry Table 2: Clearance and Stagger test values Figure 3: Variation of the absolute clearance (CL) and the absolute stagger (ST) for symmetric and asymmetric side hulls Figure 2: Body plan of the main Deep-V hull and of the symmetric side hulls of the second trimaran plane (in case of second model) of the side hulls and the main hull symmetry plane, in percent of a reference main hull length equal to 120m. A sketch illustrating the relative hulls positions for different values of Clearance and Stagger is provided in Figure 3.
Figure 4: General plan of the first trimaran fast ferry design Figure 5: Embarking car deck and engine room views of the second trimaran GENERAL ARRANGEMENTS Two possible solutions regarding the general arrangements of the trimarans are shown in Figure 4 and 5 at a Stagger and Clearance values close to the optimal solutions from the resistance point of view (PL1-PT1 for both trimarans as explained in resistance paragraph).
As reported in the following the comparison between the trimarans will start from the just mentioned figures as explanatory enough. The main car deck, in fact, although strongly conditioned by the configuration chosen, is not influenced by the effect of the side hulls shape, extremely narrow for both trimarans. Focusing the analysis on the consequences that a choice of the main hull geometry has on the payload, it is possible to note that the embarking car deck, thanks to an highest local hull breadth, allows an increase of about 10% for the loaded cars (from 65 to 72). Since also the hoistable car deck and the lower car deck behave in the same way, the main hull shape of the second trimaran is more effective in terms of load capacity of the ship. As visible from the engine room views of Figure 4 and 5, the distribution of the hull volume at the stern (especially with regards to 2 ½ and 3 water line) allow to maintain the same engine arrangement for both trimarans and hence the theoretical validity of the present comparison. STILL WATER RESISTANCE Both models of trimarans have been tested at DINAV Towing Tank at Froude numbers in the range to 0.25 0.64 (full scale speed range 16.7 42.8 knots). For some of the hull configurations of the first trimaran, the tests at the highest speeds were not performed due to an unfavorable interference between the wave formation of the central and the side hulls, so bad to produce a very Figure 6: Full scale effective power (PL1 configurations) Figure 7: Full scale effective power (PL2 configurations)
Figure 8: Full scale effective power (PL3 configurations) Position L1 L2 L3 T0 x x x T1 x T2 x T3 x high internal wave crest, ultimately causing danger of model flooding. In this paper the comparison of performance between the two hulls is discussed in terms of effective power at full scale, the resistance being predicted with ITTC-57 procedure. For the first model, as presented in [7] and [8], the towing tank tests indicated that the minimum value of P ES in the intermediate speed range (25-30 knots) is occurring at high clearance and low stagger values (PL1-PT3) or for low stagger and intermediate clearance values (PL1-PT2). On the contrary, the trends at higher speeds show that the minimum P ES is obtained for low stagger and clearance values (PL1-PT0 and PL1-PT1, transom in line and side hulls closer to the main hull), taking into account that also the configuration PL3- PT3 provides comparable results, against a less realistic solution as regards internal arrangements. This finding is in quite good agreement with results from Ackers et al. [9] which were used in the preliminary power estimation of the research project. This agreement is obviously qualitative, since Ackers results are given in terms of percent interference while our results are total resistance at full scale. In Figures 6, 7, and 8 trimaran full scale effective power curves P ES are given for the different clearances at constant stagger values for both trimarans (first number indicating the trimaran design version, so 1PT0 is the first trimaran with clearance PT0). Tests results, in fact, were performed fisrt for six configurations of the second model, which have been tested first since they resulted to provide the lowest values of resistance for the first model (see table 3). In all the configuration tested, the second hull design showed a better resistance than the first. This difference can be ascribed essentially at the difference Table 3: Second model configurations between hull form typologies, having the main parameters affecting the residual resistance almost the same value among the two models (L WL, C P,B/T,). Moreover being the wetted surface of the second model notably higher than the first one, the relative advantage on the residual resistance in even more pronounced than what appears from the total resistance. More details about the tests results and analysis can be found in [7,10]. In any case already in previous studies in the case of monohulls, deep-v hull forms showed advantages over conventional round bilge hulls especially at highest speeds. In this case, at V S =42 knots the 2PT1-PL1 solution (P ES = 26MW) is in absolute the best denoting an advantage of about 10% over the first trimaran (1 PL1-PT1; P ES = 29MW).
SEAKEEPING Besides the still water resistance tests, a campaign of tests in head regular waves to check seakeeping qualities of the two hulls was planned on the basis of the best configurations identified as regards performance in calm water. The results, in this paper reported in terms of added resistance and heave transfer functions compared with a same length monohull (frigate) optimised for seakeeping, are reported in Figure 9 and in Figure 10. More detailed results concerning seakeeping are going to be presented in [11]. To avoid excessive flooding of the models due to green water in head waves, a maximum ship speed of 30 knots and a small wave amplitude (at fixed steepness equal to 1/100 for both trimarans) was adopted. Due to the difference in longitudinal position of the centre of gravity of the different trimaran configurations, heave was reported in all cases to the middle perpendicular. The lower vertical motions of the second design shown by the heave transfer function in Figure 9 can be correlated with the deep-v shape of the central hull, usually having more vertical motion damping than equivalent round bilge monohull [6] at higher speeds (Fn=0.5 0.8). Present study was more oriented to the analysis of the effect of position of outriggers. Tests are actually under schedule with different combinations of side-main hulls (such as all deep-v hulls). The cross analysis of these tests will allow the assessment of the influence of the side hull geometry and form on the global behaviour of the trimaran ship. For sake of comparison between the two trimarans, the case of added resistance corresponding to wavelength equal to ship length (which is close to the maximum), is reported in Fig. 10. Taking into account that the same values of displacement and L WL of both trimarans allow a direct comparison between the two Figure 9: Heave transfer function of first trimaran design at Vs=30 kn (Frigate: without marks ) models, both diagrams show a certain improvement in performance of the second design,
Figure 10: Added Resistance comparison of both trimaran design at different speeds especially in the PT3-PL3 configuration at high speed. The configuration PT0-PL1, identified as the best for to side resistance, does not result to be the best for the added resistance, where the case PL3-PT3 results in advantage. In these terms a compromise in the design solution is in general necessary, depending on the priority scale of the performance assumed (comfort and operability against fuel consumption and speed). STABILITY As regards stability, both trimaran designs have been checked with respect to present criteria of the IMO/HSC rules. A trimaran ship, as expected, has in general an intermediate static stability between those of monohulls and catamarans. Despite a very high intact stability, the conditions become critical when an asymmetric flooding is considered, as arguable from figure 12. Figure 12, in fact, synthesises the verification of the most stringent stability requirements in the intact condition (a) and for two damaged condition corresponding to the side (b) and bottom(c) damage cases foreseen by actual IMO-HSC rules, i.e. the sided + gusting + heeling due to passenger crowding or high speed turning. Especially in the asymmetric damaged condition the righting arm curve of the trimaran has an evident slope discontinuity around 14-18 Figure 11: Initial and modified watertight volumes of the second trimaran design (in the static equilibrium angle for the asymmetric damaged condition c) degree (depending on the analysed case and trimaran design) caused by the emergence of the intact side hull opposite to the heeled side. The subsequent drastic loss of righting moment (that would be dramatic in the case of a catamaran) is mitigated by the contribution to stability of the trimaran main hull and by the above water volumes of the damaged side. Monohull fast ships, on
Figure 12: Verification of worst dynamic criteria of intact (wind + gusting + high speed turning) and damage stability (crowding +wind) for multihull ships. First trimaran design (left) and second trimaran design (right).
the other hand, do not suffer at all by this problem, having in general symmetric flooding which can bring, instead, to freeboard problems with respect to the lower car deck. To better appreciate the effect of buoyancy reserve of the side hulls during dynamic heeling moment, first the second trimaran has been intentionally verified with slender side hulls also in the above water portion (see figure 11). And in fact this extreme layout (unrealistic also for structural reasons) cannot satisfy the dynamic criteria in the case of worst asymmetric damaged condition (i.e. side damage of case c) of figure 12), while intact and bottom damaged criteria (case a and b) are satisfied. In summary, the trimaran designer as in the case of catamarans has to be very well aware of the risks of asymmetric flooding and in particular he is required to provide a convenient shape to the above water side hull to cope with the dynamic stability requirements of the IMO-HSC code, once having fixed the lateral position of side hulls from resistance or seakeeping considerations. CONCLUSIONS Two different designs for the same target ship, a trimaran fast ferry operating in the Mediterranean sea, differing mainly in the hull form typologies, have been described and analyzed in the paper with regards to basic design characteristics (general arrangement, propulsion, stability, etc.) and hydrodynamic performance as regards resistance and seakeeping. The second hull form is based on a deep-v main hull, similar to those used in modern monohull fast ferries, in alternative to the more conventional round bilge first main hull. The design exercise confirmed in both cases a appreciable freedom in the plan of internal arrangement and a great margin on intact and damaged stability if proper volumes in the above water portion of the side hull are reserved. As regards damaged stability the trimaran, in fact, behaves in a intermediate way respect to a monohull and a catamaran, mitigating the negative effects of an a- symmetric flooding (of side hull), that can result to be dramatic for a catamaran. Predicted still water resistance at full scale of both designs, based on model tests with different relative position of side hulls, show a general marked advantage of the second design against the first one, in the medium-high speed range for all the best positions of lateral hulls found for the first design, the best overall position of side hulls, at high speed, being the less advanced and relatively closest one. Seakeeping performance of the second model results also advantageous, especially at high speed for heave motions and added resistance. The configurations featuring the best resistance and seakeeping performance are not the same in both designs, so the designer needs to do, in general, a compromise taking into account damage stability and general layout at the same time. Further activities will comprise the completion of systematic tests for all all the other possible configuration (obtained by interchanging main and side hulls). A deeper analysis will be devoted to assess the relative
performance of designed trimarans against actual monohull fast ferries, since from preliminary consideration they do not show marked advantages. NOMENCLATURE SYMBOL S.I. UNIT DESCRIPTION A TR [m 2 ] transom area A X [m 2 ] main section area B WL [m] max beam on WL C B = /(B WL L WL T) block coefficient C P =/(A T L WL ) prismatic coefficient C WL waterplane area coefficient C X main section area coefficient D [m] depth of ship on MP Fn= V / (g L WL ) 1/2 Froude number g [m/s 2 ] gravity acceleration h trans [m] transom draft L OA [m] over all length LCB longitudinal centre of buoyancy L or L PP [m] length between perpendiculars L WL [m] length of waterline P ES [kw] full scale effective power S [m 2 ] wetted surface T [m] draft V S [m/s,knots] ship speed V m [m/s,] model speed [m 3 ] volume w [m] wave length h w [m] wave height HTF [m] heave transfer function e [rad/s] encounter frequency REFERENCES 1. Various, R.V. TRITON: Trimaran Demonstrator Project (2000) Proceeding RINA Conference, Southampton (U.K.), April 2000. 2. Pattison D. R., Zhang J.W., (1994), Trimaran Ships, Transactions of Royal Institution of Naval Architects, Vol. pp.143-161. 3. Benvenuto, G., Brizzolara, S., Figari, M., Podenzana Bonvino, C., (2001a) Fast Trimaran Ships: Some Examples for Commercial Application, Proc. HIPER'01, 2nd Int. Euroconference on High Performance Marine Vehicles, Hamburg, July 2001, pp. 64-78. 4. Brizzolara, S, (2002) Design of New Trimaran Hull Forms with Deep-V Main Hull and Round Bilge Side Hulls, in Alternative to the First DINAV Trimaran Design: Body Plan, Hydrostatics and Stability, DINAV Internal Report SB-01-02 (in Italian). 5. Brizzolara, S., Grossi, L., (1997) Design Aspects and Applications of Deep- V Hull Forms to High Speed Crafts, Proc. of IMDEX 97 International
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