A new hull form for a Venice urban transport waterbus: Design, experimental and computational optimisation.'

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1 1998 Elsevier Science B. V. All rights reserved. Practical Design a/ships and Mobile Units M w.e. Oosterveld ands.g. Tan, editors. 395 A new hull form for a Venice urban transport waterbus: Design, experimental and computational optimisation.' a Maritime Research Institute Netherlands (MARIN), P.O.Box 28, 6700AA Wageningen, Netherlands b Universita degli Studi di Napoli "Federico II", Dipartimento di Ingegneria Navale (D.LN.), Via Claudio 21, Napoli, Italy A combination of naval architects experience, modern CFD tools and model testing has been used in the hydrodynamic design of a new waterbus for use in the city of Venice, required to have minimum wave generation at low speed in shallow water, and minimum resistance at higher speed. The free-surface panel code RAPID has been extended for wave making in shallow channels with sloping sidewalls, and has been found to adequately predict the effect of hull form changes and the phenomena occurring in shallow water. 1. INTRODUCTION With the expanding use of fast passenger ferries and an increasing awareness of environmental issues, the wave wash of ships is a topic of growing interest worldwide. Ship waves may disturb or damage moored or sailing ships, waterway banks, or marine life. A particular example is found in Venice, Italy. The passenger transport system in the city and lagoon of Venice is mostly based on public services, managed by Azienda del Consorzio Trasporti Veneziano (A.C.T.V.). The A.C.T.Y. fleet consists of 54 waterbusses or "vaporetti" and 59 motor crafts, plus a few larger units. Almost all of the vessels are steel built, equipped with fixed axis propellers and manoeuvring rudders fitted at the stern, and powered by diesel engines. Since the transport service takes place along the main canals of the city, the vessels operate in restricted waters, sometimes very shallow. The size of the public water traffic flows (5-10 minutes between subsequent passes) is not very large, but together with the freight traffic and private transport it causes a very high risk of erosion to monumental buildings, as a result of wave impact on the channel walls, pressure fields and flow along the walls, and propeller slipstreams. The EC sponsored Brite-Euram project "LIUTO" (Low Impact Urban Transport water Omnibus) has two major innovative aspects in this regard: the design and development of a novel urban waterbus to be used on the Venice waterways in the next decade. the extension, application and validation of the latest tools for predicting and minimising wave wash. The new vessel will be the prototype of the Venice urban transport fleet for the 2000's, and shall substantially improve the passenger comfort and reduce the overall impact of transport through Venice. It will be built in GRP, will have a hybrid diesel-electric propulsion and steerable twinpropeller units. While other aspects of the project are discussed in other papers in this symposium [1,2], the present paper addresses the hull form development of the LIUTO MIB and the wave generation and wave impact issues which posed additional requirements to the design. A basic hull form design has been made by the D.LN. and has been refined with model tests (Section 2). The next stage was a further optimisation study by MARIN, based on wave pattern calculations using the CFD-code RAPID for several variations of the DIN design (Section 3). The same code has then been extended to include the conditions critical for erosion (Section 4). Further tank tests have then been carried out at MARIN, first in deep water, then in shallow water and in a typical channel cross section, both to study the performance of the vessel and to validate the predictions by the I This research project was sponsored by the EC under contract BRPR-CT O. This support is gratefully acknowledged.

2 396 extended RAPID code. Altogether the design problem, the tools used and the experimental validation have several features justifying their presentation. 2. THE BASIC HULL FORM DESIGN The basic hull form design has been made by the D.I.N. based on the following data supplied by A.C.T.V.: 1. Main dimensions: LoA::;; m, B~IAX::;; 5.00 m 2. Pay loa d: 250±5 passengers (= 17.5 t) 3. Speed requirements and water depth (h): Urban speed V I = 5.94 kn; h = 4.5 m Max full load speed V2= 10.0 kn; h = 10.0 m Max half load speed V 3 = 10.8 kn; h = 10.0 m 4. Maximum power: 147 kw (power margin 59C at 10.8 kn); 5. Transverse metacentricheight ~ 0.30 m; 6. Free board ~ 0.20 m, half of the passengers staying on one side with a crowd factor of 4 / m 2 ; 7. Minimum hydrodynamic pollution, i.e. low wave generation and weak propeller slipstream. 8. Manoeuvrability as high as possible 9. Minimum shallow water effects on wave pattern characteristics and resistance. Regarding points 7 and 9, the CMO (Wave Motion Committee of Venice Municipality) imposes a maximum value of the residual resistance. This somewhat unusual requirement is meant to limit the wave wash intensity. Item 3 asks for a compromise between best performance in two, very different conditions: low speed in the confined waterways inside the city, and high speed in the deeper water of the Venetian lagoon. At low speed the focus is on wave wash and on the overspeed of the flow next to the hull. For high speed the required propulsive power should be minimised, since most of the fuel is consumed during navigation on the lagoon. In a first step an analysis of the A. C.T,V. ships currently in service and D.I.N. database was carried out. Afterward the new hull was designed starting from a probable value of the ship displacement and with the following guidelines: To keep as high as possible the ratio LwdBwL in order to reduce the height of the waves generated and the wave resistance; To keep as high as possible the ratio LwdLoA To keep as high as possible the differencbetween L CB and L CF in order to oppose the tendency to trim at high Froude numbers; To minimise the slope of buttocks astern to improve the ship's ability at high relative speeds; To minimise the inertia moment of ahead half waterplane area in order to reduce the height of the second transverse wave; To reach a compromise between the best values of Cp for maximum speed and for lower speed in a canal; To optimise the ratio between the values of afterbody and fore body prismatic coefficients; To minimise the lateral area coefficient in order to improve the manoeuvrability and still to obtain a ship length larger than that of the ships currently in service; To adopt a sectional area curve that minimises shallow water effects. On the basis of these considerations, three hulls.) (LIUTO 1.3, LIUTO la, LIUTO 1.5) were designed. Two of these were derived from the third one characterised by the lowest value of Cs. Particularly the transverse sections were moved by Lackenby method in order to obtain the given ratio between the values of astern and ahead prismatic coefficients. LIUTO 1.3 LIUTO 1.4 LIUTO 1.5 C p C PF CP.-\ Three models with scale ratio 12 were made and resistance tests in deep water were carried out in the ship speed range kn, in the towing tank of D.I.N. This has a length of 145 m, the width is 9 m and the depth 4.5 m. For the shallow water experiments a false bottom has been constructed on a length of 60 m. In these tests, the LIUTO 1.3 hull showed the best performance. Based on this result and a closer examination of the stability requirements, two new hulls (LIUTO and LIUTO 2.1) were designed. The LIUTO hull was derived from LIUTO 1.3 by affine transformation. Both the breadths and the length were slightly expanded so the value of transverse metacentric radius was increased and the length was adapted to the maximum value allowed by A.C.T.V. LIUTO 2.1 was designed by increasing both the waterplane area and the maximum draught

3 397 at ballast (i.e. higher metacentric height); and minimising the consequent changes of the sectional area curve. The two models, with scale ratio 5.6, were tested in deep water and in shallow water at two load conditions. Additionally several tests were carried out to evaluate the sensitivity to trim and load variations. During the tests, wave elevations were measured using three capacitive probes located in the tank at 20%, 40%, 60% of model L WL from the hull centreplane. The analysis of the experimental results extrapolated to the ship was made by comparing: The performance ofthe new hulls; The performance of the chosen hull (LIUTO 2.1) with the available data of the boats in service. i.e.: E- I hull in shallow water and Serie 80 hull in deep water (Fig.2) The residual resistance curve of the chosen hull with the maximum curve imposed by the CMO; The ship performances at different displacements and trim angles; After these comparisons LIUTO 2. I was selected on the basis of its best stability characteristics and powering performance very close to LIUTO 1.3. The results obtained, as showed by the figures, ' ' 30 25, L1UTO 1.3 LIUTO 1.4 _. L1UTO1.5 -L1UT Fn 15.; ~ Fig. I Experimental resistance curves for initial design variations. I, 1/ were good, the values of the effective power of the new hulls are lower by 20+35Ck than the values of the hulls currently in service. The main characteristics of LIUTO 2. I hull are as follows: LOA = m; B = m; L WL = m B WL = m; C B = 0.379: Cp = 0.529; Cw p = The general form of the vessel is shown in Fig O'.. CT'('x UUTO 2.1 DEEP WATER,,,,,"' ;. ',-' --- SERlE 80 DEEP WATER / 30 i i 25! i ; 20 ' I --El SHALLOWWATER,./ :::::~-~:_~~-~~--_..// Fn 15 c..' ~ Fig. 2. Comparison of resistance curves of Liuto 2.1 and existing vessels. 3. CO;\IPUTATIONAL OPTIMISATION 3.1 The RAPID method The main features of the design thus having being fixed, the second stage was started: a refinement of the hull form using CFD tools. This study was performed by MARIN through a succession of RAPID calculations and hull form modifications. The well-known code RAPID [3,4] is a panel method that computes the inviscid flow and wave pattern generated by a ship at steady forward speed. Source panels are distributed over the wetted part of the hull and over a plane at a specified distance above the wave surface. An iterative procedure. with repeated adjustments of the wave surface. the boundary conditions, the panellings and the attitude of the hull, leads to the final solution satisfying the complete nonlinear boundary conditions. The resulting streamline pattern and pressure distribution on the hull, the wave pattern, and the wave resistance. dynamic trim and sinkage provide much insight in the flow and its relation with the hull form, and indicate how further improvements can be achieved. RAPID thus has become the main workhorse for hydrodynamic hull form optimisation at MARIN, hundreds of calculations being carried out per year. 4.2 Optimisation study Calculations were made both for the low-speed shallow water operation and for the high-speed deepwater conditions. These are partially conflicting, and a decision had to made on the primary focus of the study. The first data obtained on LIUTO 2. I showed very moderate wave generation at low speed: According to the Naples experiments. at 6 knots waves of about 0.06 m (peak-to-trough) are generated

4 398 at 14 m distance from the path of the vessel. These values are very moderate in comparison with the fleet currently in operation. Figure 3 compares longitudinal cuts through the computed low speed wave patterns for LIUTO 2.1 and two existing designs, at 4.65 m off the vessel centreplane. The wave height divided by ship length is here plotted against a non-dimensional length coordinate X/L, which is zero at the mid ship section (1/2 Lpp) and +0.5 at the stern. The bow wave system, at the left side of the figures, is responsible for the largest wave heights. According to these initial calculations, the E I prototype (C0312) generates waves of about 0.15 m, the ACTV Series 80/A MIb (C0313) a little more, compared to 0.11 m for LIUTO 2.1. r ~._------,- ~ l,..ro-e011'."..0..""_...".c..1 o-r--...,... Fig. 3. Low speed wave cuts of C0311, C0312 and C0313, water depth 4.5 m. In the actual situation in Venice, significant wave heights of m are measured in the centre of the Canal Grande close to Rialto Bridge. Values up to 0.1 m and higher are frequently observed due to passing boats. The waves generated by LIUTO 2.1 at 5.94 knots are well within the highest one third of the spectrum and will hardly increase the energy contained in the wave spectrum. Therefore it was decided to focus the further optimisation primarily on the high-speed condition. Even so, througout the following process the lowspeed wavemaking has been monitored. As an example. Figure 4 shows the calculated wave profile along the hull, for C0311 (DIN-LIUTO 2.!) at 5.94 knots. The most prominent wave system predicted is the stern wave; but from experience and the initial comparisons with the DIN tests it was obvious that in reality this will be strongly reduced by viscous effects. At this speed the flow detaches ahead of the transom edge, a phenomenon quite sensitive to viscous effects and hard to model in a calculation anyway. Therefore the low-speed stern wave 1 \ 1 J predictions were disregarded in the optimisation. On the other hand, the distinct second wave crest of the bow wave system is to be taken seriously and asks for reducing the waterline entrance angle or fitting a bulbous bow. o.sr i "'lui W'VEP~OF[LE ALONG THE HULL Fig. 4. Low speed wave profile along C0311 Turning to the 10 knots calculations now, these in the first place indicated the transom to be too high above the still water surface. Fig.5 (top) clearly shows that the wave surface is drawn upwards by the rising afterbody before being released at the transom. In the second variant C0311 A, the transom was therefore lowered and the buttock slope aft was reduced. Fig.5 (bottom) shows that this produces a much smoother stern wave system, and it has therefore been basically adopted in all following variations. With the new afterbody, the calculations indicated that the additional lift caused by the more concave buttocks resulted in a small increase of the forward dynamic trim and thus of the bow wave height at high speed. In the next versions the buttock curvature in the afterbody was therefore carefully adjusted to find the right balance between suppressing the stern wave system and limiting the bow-down trim. At the same time, for C0311A the waterline entrance was made sharper by introducing slightly hollow waterlines and lowering the bow contour, thus lengthening the submerged hull. Together with the modified stern, the calculated wave resistance coefficient at 10 kn decreased by 15CJc. Since the resistance predicted by RAPID corresponded well with the DIN measurements, this figure may be considered realistic. On the basis of statistical data and experience, a

5 399 small bulbous bow was expected to be advantageous at high speed without noticeable negative effects at low speed. An initial bulbous bow design, with sharp waterlines and little rounding, appeared to yield some improvement at high speed but to be unfavourable for lower speeds. The next version was given a slightly larger and more conventional bulbous bow, which was lifted towards the stili water surface. At high speed it performed similarly to the previous version, at low speed it was marginally better than without bulb. Fig. 5. Computed wave pattern around the stern, for LIUTO 2. I (top) and C0311 F (bottom). Fig.6 (above). Hull form of C03 I IF Statistics suggested that more could be gained at high speed. Therefore, in C03 I ID the bulbous bow was slightly enlarged by further rounding of the bulb waterline entrance. Evidently all these adjustments involved a careful tuning of the bulbous bow and forebody design, for which visualisations of the detailed flow field were indispensable. This final form meant a considerable improvement at high speed, whereas the low speed wave making was comparable with C03 I IA. From a propulsive performance and wave making point of view, the bulbous bow version C031 ID therefore is preferred for the design condition at both high and low speed. It was then to the owner ACTV to decide whether this would justify the larger construction costs, vulnerability and sensitivity to operational draft and trim variations. The decision was then taken not to pursue the bulbous bow. Prime reason were the expected loading variations, and the unfeasibility to study the performance for the entire spectrum of operational conditions that emerged from the design work. The challenge was now to further improve the latest non-bulbous form (C0311A) in one final attempt. The fore body was given a somewhat deeper forefoot and the beam at the water line was marginally reduced. The fore body sections were given more S shape, moving the section centroid a little downward. The combined effect of the S curvature and beam reduction resulted in a finer waterline entrance. Also a last adjustment of the afterbody form was made. The resulting final form and the hull panelling used are illustrated in Fig.6. This hull form C0311 F appeared to indeed outperform C03 I 1A, in particular at high speed. In Fig. 7 the 10 knots wave profile of C03l IF is compared with that of the LIUTO 2.1 design and of the bulbous bow design C031lD. The latter obviously has the lowest bow wave and a shallower wave trough next to the hull. The significantly lower stern wave of C0311 F reflects the last successful

6 400 ~ 8 0 U adjustment to the afterbody and transom. The versions A to F discussed above represent a sequence of attempts to improve on the DIN design. The combination of advanced CFD tools and expert analysis of the results again permitted a directed refinement of an already quite good design. While this design C0311D was eventually abandoned, also the non-bulbous final version (C0311F), a derivative of versions A and D, showed a significant improvement at high speed and comparable results at low speed and was chosen to be model tested. ~r---'-'::-:-tliffi1i:l]5iil'6ciuseiowll--"-----' N 0<:i " While a precise comparison with the tests fo LIUTO 2.1 required corrections for the differei displacements, model scales and appendages, it was found that the new design had a 10% lower total model resistance at 5.94 kn, and 7.3% at 10 kn. If the latter figure is entirely attributed to a reduced wave resistance, that reduction would amount to a further 29% compared to LIUTO 2.1. The reduction predicted by RAPID was 26 %! Fig. 8 illustrates the typical level of agreement of measured and computed longitudinal wave cuts for the higher speeds. The deviations are limited to some local differences in amplitude of the shortest wave components; and a minimal overestimation of the amplitude of the stern wave system due to the neglect of viscous effects. Fig. 9 shows that also the difference between the wave patterns generated by LIUTO 2.1 and C0311F is very well predicted. save for some local deviations in the first wave trough latioc.-io".7i..." x/l Fig.7 Calculated hull wave profiles for Liuto 2.1, C0311D and C0311F. 10 kn, deep water. One aspect that played a role in the analysis of the calculated results was that an accurate prediction for the low speed was not easy, as initial validations with the DIN measurements demonstrated. The slender hull, low speed and relatively small hull form modifications required considering minor changes in minor waves and optimising in the margins of the code. At higher speed the assessment was easier and more accurate, and more representative of a common design problem for a merchant vessel. Although the calculations for C0311F indicate improvements at both low and high-speed conditions this is deemed more reliable at 10 knots than at 6 knots. 3.3 Experimental validation For the hull form thus found, resistance and propulsion tests and wave height measurements were done with a 1:3.4 model in the Deep Water Towing Tank at MARIN; and with a 1:5 model in the Shallow Water Basin, in a simulated waterdepth of 4.5 m. These provided an interesting validation of the ability to predict the effect of hull form changes on the wave making.!..... t...wl.l.iaild~iou.".u.s. -oo)iif,&a.i'ld~ ~:L..,.,.-----t.;------t;;-----'t; ;.. Fig. 8. Calculated and measured longitudinal wave cut. C0311F, 10 kn, deep water, z =4.65 m. :1""" -1lIll)11f...-.M.OI.AIN...,._... I ~" G 1Q 1.G Fig. 9. Comparison of longitudinal wave cuts for Liuto 2.1 and C0311F; as calculated (top) and as measured (bottom). 10 kn, deep water, Z =4.65 m.

7 401 - ~~...c03llr.:r..6ji.. cak iaim Fig. 10. Calculated and measured longitudinal wave cut for C031 IF kn,waterdepth 4.5m, z=4.65 m. and the first stern wave crest. These results reconfirm that a nonlinear inviscid flow model may be most useful also for afterbody design, provided there is a smooth flow off the transom. Fig. 10, for 5.94 knots in 4.5 m of water, gives a less convincing agreement. The stern wave system is again overestimated, as discussed above. Unexpected, however, was the underestimation of the second bow wave crest. Inspection of the experimental data indicates that this has been a tiny wave on the verge of breaking, quite sensitive to experimental conditions and hard to fully resolve in the calculations. The precise cause for the inability of the method to correctly indicate the presence or absence of this wave peak and its relation with hull form details is still to be found. However, it should be noted that the peak-to-trough wave heights found amount to only some 0.1 m at full scale. Otherwise, comparison of the measured wave heights at 5.94 kn in deep and shallow water indicates just a quite small effect of the water depth, as expected for a depth Froude number of CHANNEL AND SLOPING BOTTOM EFFECTS 4.1 Extensions of the CFD code The final stage was an evaluation of the wavemaking and performance in confined waterways. The wider scope of the EC project asked for the development and validation of a prediction tool of more general application, requiring some extensions of the RAPID code. While shallow water effects on the wave making and flow conventionally are included by mirroring the source distribution in a bottom, this only works well for uniform water depth. However, in many instances of severe wave wash effects. wave propagation from deep into shallow water and the resulting amplification play an important role. In the case of the Canal Grande, the depth typically decreases from 4.5 m in the centre to 1.0 m near the banks. At 5.94 kn, the longest (transverse) waves just become critical near the banks (depth Froude number = 0.98). The possibly resulting wave amplification therefore is to be taken into account by including the sloping bottom and bank in the calculation. To do this, the channel wall and bottom are covered with additional source panels, and a boundary condition of zero normal velocity is imposed. The flow field then follows from the solution of all boundary conditions simultaneously: the zero normal velocity conditions on ship hull, channel wall and bottom, and the free surface condition on the wave surface. Numerical experiments were carried out to determine the required panel densities and length of the channel to be represented. A second complication was that the most critical situation for erosion is one with the vessel out of the centre of the channel. It was decided to choose a distance of 15 m from the bank as representative. The other bank then is 30 m away, and the resulting asymmetry also needs to be taken into account, since supposing a symmetrical situation would exaggerate the blockage. Therefore, the calculations had to be done for asymmetric flows, such that just the nearby channel wall and slope would be included. The asymmetry of the flow, the low speed (Fn=0.196), the sharply diverging waves, the required resolution of the wave steepening near the banks and of the reflection at the channel walls all added up to a calculation with panels. requiring some min CPU per iteration, on a single processor of a CRAY C916 computer; more than 20 times usual CPU times for this method. Obviously, even larger problems will ask for a less 'brute-force' approach. 4.2 Experimental validation Experiments have been carried out in the MARIN shallow water basin in which the channel effects were simulated. Besides permitting to estimate the required propulsive performance in such conditions, these tests provided unique information and validation material regarding the pronounced channel effects on the wave propagation. A sloping bank was constructed along one side over a part of the length of the tank. Wave height probes were mounted at 4 lateral positions. both in the rectangular and in the trapezoidal part of

8 402 the tank. The model was towed at a distance from '"' the bank corresponding to 15m full scale. g-_. - es - calculation. 1.=4.65 m.... experiment 8!---~-~---'---~-~----1 ci - calculation. z=9.3m.... experiment..'., ~.. j;, :' -, : '. ; ~,.,.,.. \J '.r\/....: A'.... ',:...' XII. Fig. II. Calcurateo and measured Ionguucmat wave cuts for C0311 F in shallow channel with sloping bank kn. z = 4.65m, 9.30 m and 15 meat the channel wall). Fig. 12 (below). Calculated wave pattern for C0311 F in shallow channel with sloping bank. Speed 8 kn. Fig. I I compares calculated and measureo longitudinal wave cuts for this situation at 5.94 kn. At 4.65 m off the ship centreline, the agreement is reasonable but displays the underestimation of the second crest and the overestimation of the stern wave system discussed before. At z=9.3 m the agreement is actually quite good, with almost equal maximum wave height and primary disturbance next to the hull. Closer to the channel wall the prediction has a somewhat lower wave height but basically the right shape. The wave amplification in the shallower area can only just be observed in the experiments. Much stronger effects were observed at 8 knots. While propagating into the shallower area near the bank the wave pattern strongly deforms, wave crests become essentially transverse, and the wave amplitude increases drastically, as the computed wave pattern in Fig. 12 illustrates. The measured waves along the channel wall are radically different from that in a rectangular channel section, with <: wave amplitude more than twice as large. This illustrates the dominant effect of wave propagation into shallow water on erosion. Fig.13 shows that this phenomenon is quite well predicted; the reflection, however, is somewhat incomplete, reducing the wave amplitude further aft; and some slight phase differences occur. It goes without saying that in this condition the erosion effects are drastically stronger

9 _dllfljm S. _ cj,iu...""....ji. : -:":':=dllrljm iu. ; ~o t;;/o ~------! Fig. 13. Calculated and measured longitudinal wave cuts for C0311F in shallow channel with sloping bank. Speed 8 kn. z =4.65m, 9.30 m and 15 m(at the channel wall). than at 5.94 kn, and the speed limit imposed appears to be a sensible choice for this channel profile. At 10 knots in the rectangular channel, pronounced shallow water effects were found, and the prediction was quite good. With sloping bank a giant breaking wave was found at this speed, making validation a useless affair. s. CONCLUSIONS The combination of a classical approach for the initial design, Cf'Dssupported optimisation, and model testing has resulted in a new Venice waterbus design with quite substantial power savings compared to existing vessels, and a significant improvement in wave making both at low and at high speed. The project has, however, a much wider scope, as a demonstration of the application of the best available techniques to a partly unusual design problem. Moreover, the further extension of the RAPID code and the validation material collected have contributed to making it a versatile and accurate tool for predicting wave wash effects and their dependence on hull form and channel cross section, of potential use in many other situations of current interest. REFERENCES 1. R. Schultze, S. Kaul, A. Brighenti, "LIUTO Development and optimisation of the propulsion system; study, design and tests," 7th PRADS Symposium, The Hague. Netherlands, F. Balsamo, A. Paciolla, F. Quaranta, "A system for the experimental determination of MIB's operating in Venice," 7th PRADS Symposium, The Hague. Netherlands, H.C. Raven, H.H.Valkhof, "Application of nonlinear ship wave calculations in design," PRADS95 symposium, Seoul, Korea, H.C.Raven, "A solution method for the nonlinear ship wave resistance problem", Doctor's Thesis, Delft Univ. Techn., Comparison of erosional effects To assess the achieved reduction of erosional effects with the new design compared to existing vessels, it is of interest to compare the wave amplitudes and flow disturbances caused. Full scale data still lacking, at present we can only base ourselves on the calculations; with the qualitative, and in many cases also quantitative agreement found between calculations and experiments, such comparisons seem justified. It was found that according to the RAPID calculations for 5.94 kn in the channel with sloping sidewalls, the wave heights generated at the channel wall are some 15-20% lower for the new design, and the flow velocity along the walls and bottom as well.