Experimental Results of Motion Responses of High Speed Marine Crafts Magnús Þór Jónsson March 2016
Content 1. Introduction... 3 2. Evaluation criteria... 5 3. Instrumentation and measurements... 6 4. Results... 8 5. Summary... 15 References... 16 2
1. Introduction Human tolerance to vibration depends primarily on the interactions of motion magnitude, direction, duration, frequency and human characteristics. The interactions are complex and their effects on humans are not fully understood (Griffin, 1990). However, Whole Body Vibration (WBV), especially those associated with rough vehicle rides, can damage the human body (Griffin, 1998; Waters et al., 2007). In particular, crews of high speed marine vessels that are typically 6 15 m in length and capable of speeds in excess of 20 knots, are exposed to uncomfortable motions that can cause physical and mental fatigue. This report summarizes the results from vibration measurements or motion responses of two different hull designs for high speed marine crafts. The objective is to compare different hull designs and measure the impact or acceleration onboard when sailing against wind and sea waves. The two boats under consideration, which specification is listed in Table 1 and shown in Fig. 1, are Stefnir ÍS-7747 built by Rafnar, Iceland with a special designed Rafnar hull, and Þórður ÍS-7738 built by Holen Mek. Verksted, Norway, with a Vee hull design. Operator skills can have significant effect on high speed marine motion but that is not included in this research. Figure 1: High speed marine crafts, Þórður on the left side and Stefnir on the right side. 3
Table 1: Technical specifications Boat Stefnir ÍS-7747 Þórður ÍS-7738 Type Leiftur RIB 1100 RS #118 (Rauna) Material Glassfiber Aluminum Length overall 11.00 m 9.55 m Length 8.79 m 8.79 m Max Beam 3.20 m 3.60 m Draught 0.55 m 0.65 m Max Speed 40 kn 33 kn The report is organized as follows. In chapter 2, the whole body vibration criteria and the limits specified by standards are discussed. Chapter 3 describes the instrumentation used. In chapter 4, the results of the measurements are shown. Finally, the report is concluded with a summary. 4
2. Evaluation criteria and injury mechanisms This section discusses the evaluation criteria and injury mechanisms regarding the allowable vibration levels for Whole Body Vibration (WBV) in high speed marine vessels. Standards and regulations used for evaluation of WBV are ISO 2631-1:1997, BS 6841:1987, ANSI S3.19:2002 and European Directive 2002/44/EC. WBV standards suggest how whole body vibration should be measured, evaluated, and assessed. ISO 2631-1:1997 defines two different WBV health guidance caution zones. An RMS zone is assumed to be given by a range 2:1 of acceleration for duration between 1 min and 10 min with RMS limits of the health guidance caution zone as 2.8 m/s 2 and 5.6 m/s 2. From 10 min duration the acceleration decreases in inverse proportion to the square root of exposure time between 10 min and 24 hours. According to the standard the VDV health guidance caution zone is defined by the vibration dose values between 8.5 m/s 1.75 and 17 m/s 1.75. Typical acceleration limiting criteria and indicative scales of vibration magnitudes have been developed as shown in Table 2. Table 2: Maximum weighted RMS values for impulsive acceleration (g) Motion Impulsive vibration Residences velocity limits (BS 6472) Offices velocity limits (BS 6472) Workshops velocity limits(bs 6472) 5 Acceleration RMS limit 0.2 mm/s 0.6 mm/s 1.2 mm/s Rough rail or road vehicle motion 1.0 m/s 2 Off-road vehicle motion 2.0 m/s 2 Fast small craft acceleration limit (ISO 2631) 5.6 m/s 2 Hazardous motion 10.0 m/s 2 Various injury mechanisms are associated with WBV and repeated impacts associated with high speed marine craft motions. The lower back pain, diagnosable as damage to the vertebrae or intervertebral discs, is one of the most commonly reported effects of whole body vibration (Stayner, 2001) and Bovenzi and Betta (1994) have reported that there is a linear relationship between posture and the prevalence of lower back pain. According to Griffin, (1990), Stayner, (2001), and Myers et al. (2008), lower back pain is associated with vibration magnitudes between 1.0 m/s 2 and 10 m/s 2, rather than exposure durations and posture is considered a compounding factor in almost all epidemiological studies (Stayner, 2001). Although measures based on individual motion magnitudes, ignoring vibration frequency, cannot adequately describe motion severity, the purpose of this research is to compare two different hull designs based on impulsive acceleration and the acceleration RMS value in y direction above 5.6 m/s 2 will be used as a reference.
. 3. Instrumentation and measurements The measurements are performed using Dyena Acceleration Recorder model no: 120301-04. The system shown in Fig. 2 includes solid state accelerometers ±16 g measuring simultaneously in three directions. Base unit GPS Receiver with power cable Micro SD Memory Card Figure 2: Dyena vibration measurement system. The Dyena Acceleration Recorder is fitted with a Garmin GPS 18 receiver and an antenna as shown in Fig. 2. The sampling frequency is f s = 1200 Hz and data logging time interval is ΔT = 1 sec. The data recorded are shown in table 3. Table 3: Data recorded by Dyena acceleration recorder and stored in a cvs file Number Columns 1 Time Local time 24 hour format 2 Lat Latitude coordinates from GPS signal 3 Long Longitudinal coordinates from GPS signal 4 SOG Speed Over Ground in Knots calculated from GPS data 5 COG Course Over Ground calculated from GPS data 6 x RMS Root Mean Square of x axis acceleration data in mg 7 x Peak Peak acceleration in x axis during that second in mg 8 y RMS Root Mean Square of y axis acceleration data in mg 9 y Peak Peak acceleration in y axis during that second in mg 10 z RMS Root Mean Square of z axis acceleration data in mg 11 z Peak Peak acceleration in z axis during that second in mg 12 xyz RMS Root Mean Square of combined xyz acceleration in mg 6
The measurements are performed using two Dyena acceleration recorders in each boat, one placed in the cockpit and the other at the bow as shown in Fig. 3 and Fig. 4. Figure 3: Location of recorders in the cockpit and at the bow of Þórður. As shown in Fig. 3 the distance from the stern to the longitudinal center of gravity (LCG) is 2.78 m, from the LCG to the recorder in the cockpit it is 1.7 m and from the LCG to the recorder in the bow the distance is 6.82 m. Figure 4: Location of recorders in the cockpit and at the bow of Stefnir. The distance from the stern to the LCG in Stefnir is 3.70 m and from the LCG to the recorder in the cockpit it is 1.60 m. The distance from the LCG to the recorder in the bow is 5.80 m as shown in Fig. 4. 7
4. Results The measurements were performed on the 8 th of January 2016. The sailing route is shown in Fig. 5, the green curve is the GPS tracking from Stefnir and the red curve is the tracking from Þórður. The first part marked as A is the route when sailing in the north direction close to wind and sea waves at around 70 o. Next part marked as B was directly against the sea waves and the last part marked as C was at a direction 45 o to the direction of wind and sea waves. Between the measurements taken for part A and part B the wind and waves did change directions. A B C. Figure 5: Sailing route tracked by GPS receivers, the green curve showing the route for Stefnir and the red curve showing the route for Þórður. The wind speed was 8-11 m/s with gusts around 13-15 m/s measured at Eyjagardur weather station. The sea conditions are shown in Fig. 6 where the estimate wave height is between 0.75 1.25 m. Figure 6: Sea conditions, estimated sea wave height is between 0.75 1.25 m. 8
Acceleration RMS (g) Boat speed (knots) Fig. 7 shows the overall boat speed for both boats during the measurements and Fig. 8 shows Root Mean Square of combined xyz acceleration in g at the cockpit location for both boats. Measurement: Boat speed 35 30 25 A B C Stefnir (Green) Þórður (Red) 20 15 10 5 0 0 500 1000 1500 2000 2500 3000 3500 Time (sek) (13500) Figure 7: Boat speed during the measurements. Measurement: RMS xyz acceleration 1.8 1.6 1.4 Stefnir (Green) Þórður (Red) A B C Impacts at hazardous conditions 1.2 1.0 0.8 0.6 ISO 2631 Limits 0.4 0.2 0.0 0 500 1000 1500 2000 2500 3000 3500 Time (sek) (13500) Figure 8: Root Mean Square of combined xyz acceleration in g at the cockpit location for both boats. 9
Acceleration RMS (g) Although the mean value for the speed of Stefnir over the entire sailing route is higher than of Þórður the number of impacts at hazardous conditions, i.e. the RMS amplitude for combined xyz acceleration higher than 1 g, is 42 for Þórður but for Stefnir the number of hazardous impacts or combined xyz acceleration amplitude higher than 1 g is only 9. Measurement: RMS xyz acceleration 5 4 Þórður (Red) 3 Stefnir (Green) 2 1 0 0 500 1000 1500 2000 2500 3000 3500 Time (sek) (135000) Figure 9: Root Mean Square of combined xyz acceleration in g at the bow location for both boats. Fig. 9 shows the RMS value of combined xyz acceleration at the bow. The mean value of RMS combined xyz acceleration for Stefnir measured for the entire sailing route is 0.42 g at the bow but for Þórður the value is 0.78 g. Fig. 11 and Fig. 12 show the peak values of y acceleration in the cockpit and at the bow location for Stefnir and Þórður when the boats are sailing route A against the wind and sea waves at a speed shown in Fig. 10. The difference between Stefnir and Þórður is significant as the graph shows. In table 4 the number of RMS acceleration amplitudes higher than 1 g and 0.57 g are counted and the mean value for each part is shown. Fig. 13 shows the boat speed when sailing route B against the wind and sea waves and Fig. 14 shows the RMS acceleration in y direction in the cockpit for both Stefnir and Þórður. Fig. 15 shows the RMS acceleration at the bow for Stefnir and Þórður. The difference here is also significant. Fig. 16 and Fig. 17 show the peak acceleration in the cockpit and at the bow for Stefnir and Þórður in y direction when sailing route B. The scale of the figures are different because of higher peak acceleration in Þórður. Fig. 19 shows the RMS acceleration in y direction in the cockpit for both Stefnir and Þórður when sailing route C against the wind and sea waves. Fig. 20 shows the RMS acceleration at the bow for Stefnir and Þórður and Fig. 18 shows the boat speed. 10
Acceleration RMS (g) Acceleration RMS (g) Boat speed (knots) Measurement: Boat speed (knots) 30 25 Þórður (Red) Stefnir (Green) 20 15 10 5 0 0 50 100 150 200 250 Time (sek) (135200) Figure 10: Boat speed when sailing route A against the wind and sea waves. Measurement: RMS y acceleration 1.2 1.0 Þórður cockpit (Red) 0.8 0.6 Stefnir cockpit (Green) 0.4 0.2 0.0 0 50 100 150 200 250 Time (sek) (135200) Figure 11: RMS y acceleration (g) in the cockpit for both Stefnir and Þórður when sailing against the wind and sea waves the route marked A in Fig. 5. Measurement: RMS y acceleration 3.5 3.0 Þórður bow (Magenta) 2.5 2.0 1.5 1.0 Stefnir bow (Blue) 0.5 0.0 0 50 100 150 200 250 Time (sek) (135200) Figure 12: RMS y acceleration (g) at the bow for both Stefnir and Þórður when sailing against the wind and sea waves the route marked A in Fig. 5. 11
Acceleration RMS (g) Acceleration RMS (g) Boat speed (knots) Measurement: Boat speed (knots) 24 Stefnir (Green) Þórður (Red) 22 20 18 16 14 12 0 50 100 150 200 250 Time (sek) (141200) Figure 13: Boat speed when sailing route B against the wind and sea waves. Measurement: RMS acceleration, y direction 0.6 0.5 Þórður cockpit (Red) 0.4 Stefnir cockpit (Green) 0.3 0.2 0.1 0.0 0 50 100 150 200 250 Time (sek) (141200) Figure 14: RMS y acceleration (g) in the cockpit for both Stefnir and Þórður when sailing against the wind and sea waves the route marked B in Fig. 5. Measurement: RMS acceleration, y direction 1.4 Þórður bow (magenta) 1.2 1.0 0.8 0.6 Stefnir bow (Blue) 0.4 0.2 0.0 0 50 100 150 200 250 Time (sek) (141200) Figure 15: RMS y acceleration (g) at the bow for both Stefnir and Þórður when sailing against the wind and sea waves the route marked B in Fig. 5. 12
Acceleration peak (g) Acceleration peak (g) Measurement: Stefnir peak acceleration, y direction 4.0 3.5 Stefnir bow (Blue) 3.0 2.5 2.0 1.5 Stefnir cockpit (Green) 1.0 0.5 0.0 0 50 100 150 200 250 Time (sek) (141200) Figure 16: Peak y acceleration (g) in the cockpit and at the bow location for Stefnir when sailing against the wind and sea waves, route marked B in Fig. 5. Measurement: Þórður peak acceleration, y direction 10 9 Þórður bow (Blue) 8 7 6 5 4 3 2 Þórður cockpit (Red) 1 0 0 50 100 150 200 250 Time (sek) (141200) Figure 17: Peak y acceleration (g) in the cockpit and at the bow location for Þórður when sailing against the wind and sea waves, route marked B in Fig. 5. Table 4 shows the performance for different hull designs based on counting the RMS acceleration values in y direction above the hazardous condition limits (1 g) and ISO 2631 WBV health guidance caution zone (0.57 g). The mean RMS value for each period is also listed. As shown in the table the number of RMS acceleration values in y direction above 0.57 g in the cockpit is 33 for Þórður against only 6 counted for Stefnir for all sailing periods. The mean RMS values for Þórður are between 35% - 121% higher than the values for Stefnir. Table 4: The number of RMS acceleration amplitudes in y direction above the limits and the RMS mean value for each sailing route. 13
Acceleration RMS (g) Acceleration RMS (g) Boat speed (knots) Measurement: Boat speed (knots) 24 22 Þórður (Red) Stefnir (Green) 20 18 16 14 12 10 8 6 0 20 40 60 80 100 120 Time (sek) (142100) Figure 18: Boat speed when sailing route C against the wind and sea waves. Measurement: RMS acceleration, y direction 1.2 Þórður cockpit (Red) 1.0 Stefnir cockpit (Green) 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 120 Time (sek) (142100) Figure 19: RMS y acceleration (g) in the cockpit for both Stefnir and Þórður when sailing against the wind and sea waves the route marked C in Fig. 5. Measurement: RMS acceleration, y direction 3.0 2.5 Þórður bow (Magneta) 2.0 Stefnir bow (Blue) 1.5 1.0 0.5 0.0 0 20 40 60 80 100 120 Time (sek) (142100) Figure 20: RMS y acceleration (g) at the bow for both Stefnir and Þórður when sailing against the wind and sea waves the route marked C in Fig. 5. 14
5. Conclusion This report summarizes the results from vibration measurements or the motion responses of two high speed marine crafts, Stefnir ÍS-7747 built by Rafnar, Iceland with a special designed Rafnar hull, and Þórður ÍS-7738 built by Holen Mek. Verksted, Norway, with a Vee-hull. The objective is to compare different hull designs and measure the impact or acceleration onboard when sailing against wind and sea waves. The main results show that the number of impacts at hazardous conditions, i.e. the RMS acceleration higher than 1 g as shown in table 4, measured in the cockpit of Þórður is two compared to none measured in the cockpit of Stefnir. At the bow the number of impacts is 112 for Þórður and only 6 for Stefnir. The number of values of RMS acceleration in y direction above 0.57 g in the cockpit is 33 for Þórður but only 6 for Stefnir. When sailing against the wind and sea waves the mean value of RMS combined y acceleration is 35% - 121% higher at Þórður compared to Stefnir for different locations. Reykjavík, 16.03.2016 ---------------------------------------------- Magnús Þór Jónsson kt. 120357-5499 prófessor í vélaverkfræði við verkfræðideild Háskóla Íslands 15
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