Comparison of Comfort and Workability on board a Trimaran and a Monohull Yacht

Transcription

1 Comparison of Comfort and Workability on board a Trimaran and a Monohull Yacht H.R. Luth, Consultant Seakeeping Maritime Research Institute Netherlands (MARIN) Project 2000, November 2000, Amsterdam Abstract This paper discusses the possible merits and disadvantages of a trimaran over a monohull yacht. The powering performance and ship motion behaviour are evaluated for a modern 60 m monohull motor yacht and a trimaran with comparable displacement and installed power. Special emphasis is put on discomfort related to vertical and transverse accelerations. It is concluded that an increase in speed of 2 to 3 knots is possible for the trimaran. Comfort and workability on board a trimaran are superior over a monohull when sailing or at anchor in head seas. In quartering seas, the roll motions and transverse accelerations of this particular trimaran showed to be less tolerable than of the monohull. If these can be solved with special attention to the design of roll stabilising appendages, a trimaran can be a worthwhile option for owners who need a large deck area or who wish to stay onboard a longer than average period of the year. Introduction Over the years, more and more attention is being paid to the possible merits of trimaran huil forms. Up to now the focus has been on warships (for instance the TRITON demonstrator of the UK Navy) and fast transatlantic (container) transport ships (for instance the SeaLance project). The basic idea behind a trimaran is twofold: (1) the resistance of a long and narrow huil is lower than that of a short and beamy huil and (2) the wave induced vertical motions of a long huil are lower than of a short huil. The first will give a higher speed with the same propulsion plant; the latter will result in more comfort on board. Because the transverse stability of a long and narrow huil is significantly lower than of a short wide huil, two side hulls are required to keep the ship upright: the trimaran is born. Because of the large distance bet we en the side hulls and the main hult, the displacement of the side hulls can be relatively small to get the required stability. The side hulls should also be long and narrow to keep the resistance as low as possible. are addressed are the calm water trial and transit speed, workability in transit condition and comfort on board at low speeds and at anchor. Workability and comfort are investigated by evaluating commonly accepted criteria for roll angles, the levels of transverse and vertical accelerations and the effective gravity angles. Description of the monohull The monohull yacht that is used as reference is a modern 60 m de Voogt Ship Design motor yacht. A pair of roll stabilising fins and an anti-rolling tank are installed to reduce the roll motions at high and low speeds. An extensive investigation into the motions and measures to influence the motions and performance of a motor yacht is reported in [1]. Although the length of the present vessel is 20% larger than that used in this reference, the general statements still hold. Figure 1 shows a contour of the vessel. This paper discusses the possible merits of a trimaran huil for yachts from a hydrodynamic point of view by comparing a number of performance indicators of a (nonexisting) trimaran yacht and a typical conventional monohull yacht. Issues that Fig. 1: Contour monohull 1

2 Description of the trimaran To carry the same number of people and the same equipment, the trimaran yacht is designed to have the same displacement as the reference monohutl yacht. The width of the main huil is reduced to roughly 60% of that of the monohull. The total width of the ship (including side hulls) is almost twice that of the monohull. The length is increased by 50% to 90 m. The draft is comparable to that of the monohull. One set of active stabiliser fins is mounted to the inside of the outer hulls to reduce the roll motions of the vessel. Although not exactly the same as the ship used for the calculations, an example of how a trimaran yacht could look like is shown in figure 2, obtained from [2]. cabins and service rooms is also comparable to that of a monohull yacht. As the costs of a yacht depend heavily on the number of rooms and less on the overall length of the vessel alone, the costs for the trimaran are not necessary much higher than for a monohull. Maximum speed and transit speed Figure 3 provides the required and available power of both the monohull and the trimaran huil. These estimates are based on model test results of similar vessels. With the installed engine power the monohull will reach a maximum speed of 16 knots. To save fuel, the long duration transit speed will be somewhat lower. It is assumed that the transit power is in the order of 60% of the maximum power, leading to a transit speed of 14 knots Ship speed (kn) Fig. 3: Powering performance Fig. 2: Contour and front view trimaran Besides the main general improvements of a trimaran over a monohull described in the introduction, the differences for yacht applications are: larger decks accommodating more cabins with balconies, a larger open deck area aft that can be used for dining, recreational purposes or for a well accessible heli-deck and finally more space around the ship to launch and store small boats and accommodate diving facilities. Because the trimaran is designed to have the same number of people on board as the monohull, the number and size of As the resistance of the trimaran huil is lower, the maximum speed for the same installed power is 19 knots and the transit speed at reduced power is 16 knots. This shows one of the great advantages of the trimaran concept: the lower resistance leads to higher speeds at equal power levels; an increase of 2 to 3 knots is possible. This can also be inverted: smaller engines are possible to obtain the same speed as for a monohull; a reduction of 40% in power is possible. Smaller engines will lead to lower operational costs, a reduction in the weight of the ship and less environmental contamination.

3 The ship motion calculations were carried out for two speeds: at anchor (0 knots) and at cruise speed (16 knots for the trimaran and 14 knots for the monohull. Criteria, waves and operability To judge the performance of a vessel it is common practice to adopt criteria for certain motions and to determine up to what wave height these criteria are not exceeded. The total percentage of time of occurrence of waves that lead to motions below the criteria is called the operability, the percentage of time that the criteria is exceeded is called the downtime. Figure 4 (from [1]) shows this in a simplified way. The downtime is often expressed in 1/1000,h and sometimes in percentages. GHT=^> r WAVE HE SIGNIFICAN 1 MEAN PERIOD = > Fig. 4: Performance analysis NON-OPERATIONAL --- WAVE CONDITIONS CRITERION 1 CRITERION 2 OPERATIONAL WAVE CONDITIONS The location on the worid where the ships sails will have an influence on the operability of the ship. The North Sea is known to have a high percentage of occurrence of high waves, in the Caribbean the percentage of occurrence of high wave is lower. Consequently, the operability in the Caribbean will be higher than on the North Sea. For the present work it is assumed that the vessels sail in the Caribbean. A scatter diagram of the Caribbean as found in Global Wave Statistics [3] is given in figure 5. Vertical accelerations (originattng mainly from the heave and pitch motion) are the leading factor in degradation of workability and comfort on board. In other words, if the vertical accelerations are too high, people will not be able to perform their assigned tasks or they wül get seasick. Two criteria are adopted: one higher level for the crew (as they are used to ship motions) and one lower level for the owner and his guests. According to [4] a vertical acceleration Root Mean Square (RMS) level of 1 m/s z (or 0.1 g) is the long-term tolerable limit for crews. A RMS level of 0.2 m/s 2 (or 0.02 g) is adopted for the owner and his guests, as this is close to the lower threshold below which vomiting is unlikely to take place. f» 2 ^iy 3N. JS y Oecutmne» ot n n conhafl ricaribtaanint'ioooth ^ < ; 14 t / 35 <ï -^^r 10 SB 1» z 37 1Z *s 23 e Zaro-upacrotatno; partod T3 ( ) Fig. 5: Wave scatter diagram Transverse accelerations (originating from the combined roll, sway and yaw motion) lead to reduced mobility on board. In other words, if the transverse accelerations are too high, people will not be able to stand upright or walk around the ship without support. Again, different maximum tolerable levels are adopted for the crew and the owner: 0.5 m/s 2 (or 0.05 g) RMS for the crew who is used to walk around on a rolling ship and 0.3 m/s 2 (or 0.03 g) for the owner who is less accustomed to the ship motions. Finally, a criterion is adopted to describe the limit of the vessel itself: for shipping water over the bow a limit of 5% is adopted. This means that when more than one out of 20 waves result in shipping of green water over the bow the captain will decide to reduce the speed of the vessel or change its course. As this interferes with the schedule, it can be regarded as a reduction of workability. Assessment of ship motions The motions of the monohull and the trimaran were calculated using MARIN's linear strip-theory program SHIPMO (monohull) and TRIMO (trimaran). Linear strip-theory calculates the wave induced force and the reaction forces of the vessel separately. This separate calculation provides good insight in the nature of the motion characteristics. As the method only uses the underwater part of the ship up to

4 the calm water level, the effect of bow flare and shallow aftship sections that might emerge in a wave are not accounted for. Still, experience has shown that the calculations give reasonable answers in moderate wave conditions. The results are certainly reliable enough to compare the motion behaviour of two different concepts, as in this paper. Heave, pitch and vertical accelerations Vertical accelerations are a result of the combined heave and pitch motion of a ship. In head and quartering seas, the pitch motion dominates the vertical accelerations near the ends of the ship. In the midship area and in beam seas, vertical accelerations are dominated by heave. The length of a ship determines the pitch motions to a great extent: the longer the ship is, the sooner it can not follow the wave slope anymore. This can be seen in Figure 6 when the lines for the pitch motion are foliowed from left to right. First both ships follow the wave slope. The longer trimaran drops down first, the shorter monohull later. The heave motion shows the same trend: the trimaran starts to drop below a value of one earlier than the monohull. In beam waves it is just the opposite: as the monohull is wider than the main huil of the trimaran, the monohull heave motions in beam seas are the lowest. Bow and stern quartering seas show similar behaviour as head seas. As a consequence of the lower heave and pitch motions, the vertical acceleration level of the trimaran is lower that that of the monohull for all headings except pure beam seas. Figure 7 compares the vertical motions at zero speed at the forward part of the deckhouse of the vessels. This figure shows the amount of acceleration per meter wave height as a function of wave frequency and heading. Especially in head and following seas the vertical accelerations are significantly lower. Only very short beam waves (to the right hand side of the middle of the figure, the vertical accelerations of the trimaran are higher than of the monohull. Trimaran Ship motions at 0 knots in head seas 1 E heave pitch Wave frequency (rad/s) Trimaran Monohull wave slope Fig. 6: Heave and pitch motions Wave frequency (rad/s) Fig. 7: Transfer functions of vertical accelerations

5 Table 1 shows the downtime figures associated with the vertical motions on a number of locations on the ship. A downtime of 0% indicates that the adopted criterion is never exceeded. This table provides a quantitative comparison between the trimaran and the monohull. It confirms that the downtime of the trimaran is lower for all headings except pure beam seas. Where the owner can be in the lounge in the aft part of the superstructure up to Beaufort 4 on the monohull without getting seasick, he can withstand up to Beaufort 7 on the trimaran. The crew can work at this location up to Beaufort 6 on the monohull, the trimaran allows working even beyond Beaufort 9. Table 1: Downtime (trimaran/monohull) in % due to exceeedance of vertical acceleration criteria Headinq (deg) at anchor deckhouse fore deckhouse aft wheelhouse 78/88 52/73 0/0 96/93 94/90 17/5 69/88 26/67 0/0 43/86 7/43 0/0 at cruise speed deckhouse fore deckhouse aft wheelhouse 7/27 2/8 0/0 95/90 94/90 15/2 96/97 85/92 22/32 93/96 76/89 15/38 Figure 8 compares the vertical acceleration for the aft part of the superstructure in another way. It shows the acceleration level as a function of Beaufort number. Fig. 8: Vertical acceleration as a function of Beaufort number Figure 9 shows the distribution of the vertical acceleration over the length of the ships. SlripLrajÜj Fig. 9: Distribution of vertical accelerations over length in Beaufort 5 head seas 5

6 When applying the criteria mentioned in the precious section, a usable length can be defined. The usable length is defined as the length of the ship where the vertical accelerations are lower than the criterion. At zero speed, the monohull shows a usable length of only 10 m while the trimaran shows a usable length equat to the whole length of the superstructure. This means that while at anchor in Beaufort 5 the owner feels comfortable around the midship on his monohull only (he may get seasick when he walks toward the forward or aft end of the deckhouse), he can freely use the whole deckhouse length on the trimaran. At forward speed the difference is smaller. In this case it is still very welcome for the crew. Trimaran Because the main huil is narrower than that of the monohull, the crew space is distributed over a longer length and thus closer to the more uncomfortable ends of the ship. With the better motion characteristics this has no negative effect on the ability of the crew to work on board. From Figure 8 can be seen that the advantage of the trimaran for the crew is found above mainly Beaufort 5 Roll motion and trans verse accelerations Transfer functions of the roll motions at zero speed and at cruise speed are given in Figure 10. At zero speed, the monohull shows the classic doublé peaked shape over the frequency that is due to the presence of the anti-rolling tank. Monohull W*Vt 1-WOJtNCY natt WAVI HttQUENCÏ Fig. 10: Transfer functions of roll

7 The trimaran shows a single peak over the frequency, with a higher maximum value than for the monohull, and, more parttcular, a doublé peak over the heading. This means that at zero speed the trimaran will roll more in quartering seas than in beam seas. Obviously, this is caused by the side hulls of the trimaran. At cruise speed both ships show only one peak in stern quartering seas; the trimaran having a peak value twice that of the monohull. The reason for the higher roll angles of the trimaran was implicitly mentioned before: due to the large distance from the centre line, the side hulls provide a high restoring roll moment at low roll angles. This leads to an excellent increase of stability in calm water, but at the same time to a high roll excitation moment when a wave passes the ship. Figure 11 shows that the trimaran wave excitation moment around the natural roll frequency in beam seas is roughly twice that of the monohull. Combined with a similar level of damping this leads to roughly twice the roll angles of a monohull when the wave encounter frequency matches the natural frequency of roll. The very high peak value found for _the trimaran is confirmed by seakeeping model tests carried out for another trimaran project at MARIN. Without stabiliser fins even higher roll angles we re found: at the worst heading a 20 degree roll amplitude was measured in a 1 m wave. Roll excitation in beam seas at zero speed Range of natural rolt frequencies of yachts accelerations in the order of twice that of the monohull. At forward speed, the sway and yaw motion have a larger influence. The highest peak value for the monohull is even found in bow quartering seas. The trimaran shows comparable acceleration levels in bow quartering seas, but significantly higher levels in stern quartering seas. Figure 12 compares the transverse accelerations in the aft part of the superstructure for the worst heading: stern quartering waves S Transverse accelerations In aft part of superstructure in stem quartering seas zero speed Trimaran Beaufort nr. Monohull Fig. 12: Transverse acceleration function of Beaufort number While the acceleration levels on monohull are acceptable for the owner up to Beaufort 7, the trimaran is not acceptable even in Beaufort 3. The presented stern quartering sea case is the worst case that can be found. However, as can be seen in Table 2, other headings show similar trends. as Wave frequency (rad/s) -Trimaran Fig. 11: Wave excitation forces Ktonohul At zero speed, the transverse acceleration is entirely dominated by the roll motion. Consequently, the trimaran shows the highest values in bow and stern quartering seas, with maximum 1.2 Table 2: Downtime (trimaran/monohull) in % due to exceedance of roll and transverse acceleration criteria Heading (deg) at anchor Roll Deckhouse fore Deckhouse aft Wheelhouse 79/49 90/79 92/88 87/61 51/41 93/91 92/90 88/81 74/41 88/81 90/86 83/63 at cruise speed Roll Deckhouse fore Deckhouse aft Wheelhouse 51/6 79/9 75/7 49/0 0/0 86/75 82/69 62/38 0/0 40/57 63/48 18/17 7

8 The presented trimaran is certainly not designed to show minimum roll motions. By changing the longitudinal location and, possibly more important, reducing the volume and waterplane area of the side hulls the excitation can be reduced. Increasing the activity of the roll stabilising fins, installing larger fins, or even two pairs of fins at each side will reduce the roll motion at forward speed even more. A suitable solution for the zero speed case stil has to be developed. The results indicate clearly that sufficiënt measures should be taken to reduce roll motions when designing a trimaran. Shipping of water Figure 13 shows that the percentage of time that green water is shipped over the bow when the monohull sails a whole year at cruise speed in head seas in the Caribbean is 4.2%. downtime asfunction of bow height (all year, single heading, cruise speed ) \ Head seas Bow quartering seas % \ \ \ ^_ Msnohuü f leve) -** trimaran bow height /monohull bow height Fig. 13: Downtime as a function of bow height The all year value for bow quartering seas is 0.7%. These numbers are lower in reality because waves come from other directions also, the ship is not always sailing at cruise speed and most important, it is not sailing 365 days per year. However, it is a good measure to compare the performance of the trimaran to that of the monohull. The figure shows that the bow height of the trimaran has to be in the order of 15% to 30% higher than that of a monohull to have the same probability of shipping water over the bow. Obviously, the combined vertical water and ship motions are higher near the bow of the trimaran than they are for the monohull. A consequence of the last conclusion is that the relative vertical water velocity at the bow of the trimaran is higher also. As this vertical water velocity is considered to be the driving force behind bow flare slamming, bow flare angles (the angle between the horizontal and the bow) of the trimaran should be higher than is common practise for existing monohulls. Conclusions A trimaran yacht with the same displacement as a monohull yacht offers larger decks accommodating more cabins with balconies, a larger open deck area aft to be used for dintng, recreational purposes or for a wel) accessible heli-deck and finally more space around the ship to launch and store small boats and accommodate diving facilities. The calm water speed of a trimaran will be in the order of 2 to 3 knots higher than a monohull with the same displacement. When the same speed is required, the in stal led propulsion power can be reduced by some 40%, leading to lower operational costs, a reduction in weight and less environmental contamination. If the heading of the ship can be adjusted such that the bow of the ship points towards the waves, comfort at anchor and workability at cruise speed onboard a trimaran, expressed in the vertical accelerations and transverse accelerations, is significantly higher than onboard a monohull. However, when the waves arrive at the ship under an angle with the bow, and especially in stern quartering seas, rolling and transverse accelerations will be higher onboard a trimaran. Therefore special attention should be paid to the design of the side hulls and the roll stabilising appendages of a trimaran. More than the conventional pair of stabiliser fins is required to match the performance of the trimaran in stern quartering seas with that of the monohull. Zero speed solutions still need to be developed The design of the bow of a trimaran, both in terms of bow height and flare angle,

9 should be different from that of a conventional monohull. Larger freeboard and higher flare angles are required to obtain the same comfort and workability reduction due to slamming and shipping of green water. The combined features of a trimaran yacht offer excellent possibilities for two specific groups of yacht owners: (1) those who require a wide range of activities and related equipment onboard will profit from the larger deck area and (2) those who want to stay onboard more time and sail longer distances will profit from the higher speed or lower fuel consumption and of the large deck area providing a well accessible helicopter landing platform. Ref eren ces [1] "Passenger Comfort on Board Motor Yachts". Dallinga, R.P. and van Wieringen, H.M..HISWA1997 [2] The Feadship Story", TRP Magazines Ltd, London, ISBN [3] PC Global Wave Statistics, BMT, London, 1987 [4] NORDFORSK, Assessment of ship performance in a seaway ISBN ,1987 9

10 RESULTS OF THE ON-BOARD MEASUREMENTS Data The records cover in total two three-month periods on board the two yachts. Figure 3 shows a sample time history of the data statistics. From each "reported" comfort rating (Figure 4) statistics are available of the roll response and the vertical and transverse accelerations at key locations. The volume of the data in the various categories is indicated in Figure 5. Figure 6 indicates the correlation between the recorded roll motion and the recorded transverse acceleration. A very good coherence is obtained; indicating that roll is a prime factor in the transverse accelerations. The lower values in the cloud correspond closely with the inevitable gravity component along the deck. The dominance of the roll suggests that sway and yaw contributions are relatively small; this implies that by reducing the roll response the acceleration levels can be reduced as well. Figures 7 and 8 indicate the trend of the vertical and transverse accelerations with the comfort ratings. It shows that in the case of the vertical accelerations there is no clear relation between the comfort rating and the mean or median acceleration level. In the case of the transverse acceleration and the roll motion a very clear and similar trend is obtained. These values are summarised in the following table. Failed Very bad Bad Good Very good Rms Tr.Acc. m/s" 1 Mean Median Rins Roll deg Mean Median Taking the median value as the most robust estimate the transition between bad and good is around 0.08 m/s 2. A value, which is quite close to the 0.1 m/s 2 suggested by the motion simulator trials. The value for very bad (0.19 m/s 2 rms) is close to the 0.14 nyvsuggested by the coffee-break and lunch trials on board the motion simulator. One obvious characteristic of the present data is that the rms values can not take negative values. The fact that the Rayleigh distibution reflects this characteristic and the reasonable fit with the measured data motivated its use to predict 10% and 90% exceedance values. Rating level Very Bad & Failed Bad Good Very Good Rms Tr. Ace. m/s' i 10% % % Rms Roll deg 10% % % Comparing the median values from the fitted Rayleigh distribution with the median valuefromthe data a reasonable agreement is obtained. DISCOMFORT CRTTER1A AND DESIGN 1MPL1CAT10NS Discomfort criteria The reported comfort ratings correspond with a volume of accelerations statistics. These statistics show a considerable scatter, which indicates that the issue of discomfort is only partly understood. Despite the scatter, the correlation between the median values and the comfort ratings suggest strongly that the transverse accelerations are a prime indicator for passenger discomfort. These median values, which correspond with a 50% score, suggest that an rms transverse acceleration around 0.1 m/s 2 is the level in which in 50% of the reported cases the motions are characterised as "bad". It is encouraging to note that the on board measurements and the Motion Simulator Trials in schematised conditions yield very similar levels.

11 The results of the on-board measurements as well as the Motion Simulator Trials agree also on the result that the vertical accelerations are only of minor influence (within the range of the present data set). Design implications From literature the following five 'human factors engineering principles' to optimise human performance in ship operations and design are suggested: locate critica! stations near the ship's effective centre of rotation minimize head movements align an operator with the principal axis of the ship's huil avoid combining provocative sources provide an external frame of reference In the light of the present results the remark on combined sources in terms of combined motions bay be less relevant. The same is true for the longitudinal location on board the ship; since vertical accelerations are less important the locations w.r.t. to the pitch centre is also less important. The present results do suggest that the emphasis on reducing roll is important to improve yacht design. As outlined in earlier work on behalf of Feadship measures to increase the roll damping are quite effective. On-board recognition of the actual wave conditon (wind sea, swell) and direction by means of ship radar and subsequent evaluation of the optimum heading on basis of the known motion characteristics seem feasible with contemporary radar and computer technology. By means of a stem thruster or increasing the wind-vaning tendency of the vessel it might be easier to maintain the optimum heading. CONCLUSIONS In the course of 1997 and 1998 the motions of two motor yachts were continuously recorded. The results of measurements were related to questionaire based comfort ratings. The results were given an interpretation in the light of dedicated Motion Simulator Trials by TNO and existing literature on human factors. Based on the results it seems justified to conclude that: 1. The retumed comfort rating shows considerable scatter which suggests that the rating is both rather subjective and situational. 2. Both the on-board measurements as well as the simulator trials suggest that the transverse accelerations are a relevant measure for discomfort 3. Both data sets also suggest that within the present data sets the vertical accelerations are less important than the transverse accelerations. 4. The data suggest that an rmstransverse acceleration of 0.1 m/s 2 will be rated as 'bad' by 50% of the passengere. This value is considerable lower than suggested by the existing literature. Based on these findings it seems right to conclude that the ongoing efforts to reduce the roll motions Of the present class of ships is very relevant to design more comfortable motor yachts. Suggestions for improvements in yacht operations were made.

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13 TENDER OPERATIONS CARMACVII I f Cfr- SZX+^Z Hl 'i Eigei-;^ i Pi; se B?3S^ Tl to' c (B ro

14 Carmac VII: July 1997 Day Accy Std Accy Min Accy Max X Accy Marker

15 Figure 4 M.Y. Respondent Date Posiüon or location Number of guests ^ADSHIP Captain's Questionnaire 'Comfort at Anchof i^; Local time: \2ca Greenwich Mean Time: 2S0O Number of sick guests: NIU. The relevant event was: El Q Q Q tender operations & water sports, eating & drinking; galley, sleeping & relaxation, other, namely: My judgement on that. event is: ^ Q fafled Q n verybad O bad Q good Q verygood Comments ^w/s.m-.htt.<&irjc* TTw&tf f%ss Local time: / 3 3 ö. Greenwich Mean Time: The relevant event was: Q Q Q tender operations & water sports, eating & drinking; galley, sleeping & relaxation, other, namely: My judgement on that event is: Q failed Q verybad Comments 232C. Number of sick guests: J 0 bad Q good Q verygood Local time: /*/ oo Greenwich Mean Time: ac? o<r> The relevant event was: U Q Q tender operations & water sports, eating & drinking; gajiey, sleeping & relaxation, other, namely: Number of sick guests: f My judgement on that event is: El failed Q ven verybad Q bad Q good Q verygood Comments 'T. A> t I r-. LJH* t- f <j My overall judgement of the day Q very bad Comments / bad Q good Q very good

16 No Data Entries II Enterprise BCarmac D Total failed very bad bad good yery good Comfort Rating

17 jjjiaa JMSfl jtua rfoaa tst^ia rtijé*üi CEUfiOI Relation between Roll and Transverse Acceleratlons E o < ^ u rms Roll [deg]

18 O ^MfiA ogtttb tmsba f Comfort Rating & Vertical Accelerations av mean av mëdian failed very bad good very bad good Comfort Rating co' c -*j

19 Figure 8 c c.2 03 "O CD CD e E co co w c o co jy ü < CD W CD > CO c oö c w t: o E o o LO O LO O LD O co CM CM v- O O o O O O o O O ms Tr. Accel. i_ FT W >* 'S i- O 0) o > CD "O o O) ö) c -*-J co "O Q co XI t o <+- E "O o co ^ <D > "O CD CO o