Evaluation of the Seakeeping Performance Indices for Enhancing Crew Safety and Workability in Small Fishing Vessels
Article information
Abstract
This study evaluated the applicability of the seakeeping performance indices for assessing the safety and crew workability of small fishing vessels, considering the high incidence of accidents in this type of vessel and the limited research on the impact of vessel motion on crew safety. Hydrodynamic analyses of 16 fishing vessels were conducted and the responses of roll, pitch, MII (Motion induced interruption), and MSI (Motion sickness incidence) were derived under various environmental conditions. A three-dimensional panel method based on potential flow theory was used to assess the motion responses of the vessels. Viscous roll damping coefficients were derived from computational fluid dynamics simulations for three representative vessels and applied to the remaining vessels. The results suggested that the magnitude of the seakeeping performance indices varied according to the vessel dimensions and main particulars. In particular, MII exceeded the established criteria for most vessels except in the lowest environmental condition, highlighting the need for criteria specific to small fishing vessels. The study also examined the influence of the forward speed on the indices and found that roll and pitch responses increase with the speed while MSI decreases marginally. Overall, these findings may help enhance the safety of vessels and crew members in small fishing vessels.
1. Introduction
Fishing vessel accidents comprise a significant proportion of overall vessel accidents (Papanikolaou et al., 2014; Eliopoulou et al., 2016). Such vessel accidents are more likely to result in casualties compared to commercial vessels because of their smaller size and the nature of work conducted on deck. The motion of a fishing vessel is a critical factor affecting the safety of the crew and the vessel itself during operation and fishing activities. In particular, small fishing vessels can experience large motions even under benign environmental conditions. Accordingly, various studies have been conducted on the seakeeping performance and safety criteria of fishing vessels.
Previous studies focused on the seakeeping performance characteristics of fishing vessels to assess their safety. Tello et al. (2009) performed a seakeeping analysis of four fishing vessels and identified the tendencies in response to changes in environmental conditions. They expanded their research to the responses of various fishing vessels and the characteristics according to the direction of the environmental conditions (Tello et al. 2011). Uzonoglu et al. (2013) confirmed the parametric rolling phenomenon in fishing vessels through numerical and experimental approaches. González and Bulian (2018) examined the accuracy of simplified models for predicting fishing vessel roll response in waves, compared to more complex 6-DOF analysis software. They suggested that simplified models can be a valuable tool in the early design stages, offering a balance between accuracy and computational efficiency. Liu et al. (2019) investigated the effect of the bilge keel design on the roll response of fishing boats. Computational fluid dynamics (CFD) were used to estimate roll motion response and resistance. They reported that the application of bilge keels reduced the roll response and resistance. Im and Lee (2022) also used CFD analysis to examine the effects of forward speed and wave height on the seakeeping performance. Nguyen et al. (2024) analyzed the influence of the length-to-beam (L/B) and beam-to-draft (B/T) ratios on the seakeeping performance of a trawler in the Bering Sea. The results suggest that these ratios significantly affect the motion of a trawler, especially roll and pitch, with the B/T ratio being particularly influential for good seakeeping performance. Youn et al. (2023) conducted experiments on three fishing vessels of different sizes to compare their motion responses. They found that wave steepness has a linear relationship with roll and pitch motions in the bow and stern seas.
Studies assessing the safety criteria of fishing vessels in relation to their motion performance focused on the safety of the fishing vessel itself. Tello et al. (2011) evaluated the seakeeping performance of various fishing vessels using strip theory and a short-term approach. They calculated the roll and pitch responses, as well as responses of greenwater, slamming, propeller emergence, and vertical and lateral accelerations at the bridge and working deck. They analyzed the effects of environmental conditions in terms of magnitude and direction. Mata-Álvarez-Santullano and Souto-Iglesias (2014) studied the impact of seakeeping criteria on small fishing vessel safety, showing that capsizing accidents often involved vessels that met the stability standards but failed the operability criteria under various sea conditions. Similarly, Iqbal et al. (2022) highlighted the significance of the loading conditions on the operability of small fishing vessels, showing that the changes in loading can significantly affect vessel safety, particularly in terms of roll motion and stability.
In contrast, research on commercial and naval vessels has focused on the applicability of various safety indices based on their seakeeping performance. O’Hanlon and McCauley (1974) investigated the relationship between vertical sinusoidal motion and motion sickness incidence (MSI). Their study showed that the MSI is highest at a frequency of 0.167 Hz and increases monotonically with acceleration for all frequencies tested. Graham et al. (1992) proposed motion-induced interruptions (MII) as a criterion for ship operability to assess the impact of ship motion on the ability of crew members to perform tasks. MII refers to instances where excessive ship motions, particularly lateral accelerations, force individuals to momentarily stop their work and focus on maintaining balance to avoid slipping or falling. Bales and Cieslowski (1981) proposed seakeeping performance criteria for destroyers based on their primary missions. They identified motion, velocity, and acceleration responses, as well as deck wetness and slamming, as limiting-motion criteria depending on the operational mission. Stevens and Parsons (2002) reviewed the effects of ship motion on crew performance. They highlighted the negative impacts on various tasks, fatigue, and motivation and emphasized the importance of considering human factors in ship design and operation to enhance safety and efficiency. These criteria have been used and combined in previous studies to examine the relationship between seakeeping performance and crew workability and safety (Sariöz and Narli, 2005; Zu et al., 2022).
Although existing operability and seakeeping criteria have been used to evaluate the safety of fishing vessels, they have limitations in demonstrating a direct relationship with crew workability and safety. Although deck accelerations and slamming can affect the crew, they do not fully represent crew safety during work. Therefore, this study applied additional criteria related to crew workability and safety, which were previously established for commercial and naval vessels, to fishing vessels, and to assess their applicability. Hence, hydrodynamic analysis was conducted using various small fishing vessels, and the motion responses were derived. This study also applied MII and MSI criteria with the roll and pitch responses to small fishing vessels. The tendencies of the criteria were identified, and their characteristics were evaluated according to environmental conditions and the main dimensions of the fishing vessels.
2. Fishing Vessels and Numerical Approach
2.1 Fishing Vessels
Sixteen fishing vessels were considered to evaluate their seakeeping performance. The target vessels were T-shaped fishing vessels with box keels, as shown in Fig. 1. The vessels were designated FV01 to FV16, and their principal particulars are listed in Table 1. Among them, FV02, FV03, and FV11 are Korean standard fishing vessels. The displacement and gross tonnages of the fishing vessels ranged from 8.7 t to 46.4 t and from 1.99 t to 9.77 t, respectively.
2.2 Governing Equation
Three-dimensional hydrodynamic analysis based on Radiation–Diffraction theory was conducted to assess the seakeeping performance of the fishing vessels. The six degrees of freedom equations of motion for a fishing vessel in the frequency domain can be written as follows:
The response amplitude operators (RAOs) for unit wave amplitude, obtained through the analysis, are combined with the wave spectrum to calculate the response spectrum.
Fig. 2 shows the coordinate system used in the analysis, along with the definition of environmental condition directions. The environmental directions are defined as 0° for the stern, 90° for the beam, and 180° for the bow.
2.3 Environmental Conditions
Four environmental conditions were considered to evaluate the seakeeping response of the fishing vessels, as listed in Table 2. The range of environmental conditions was selected within the significant wave height of 1.5 m, which is the threshold for typical fishing vessel operations. The JONSWAP (Joint North Sea Wave Project) spectrum was used for the wave spectrum, with a peak shape parameter (γ) of 3.3. The environmental directions were considered at 15° intervals from the stern (0°), as shown in Fig. 2.
2.4 Seakeeping Performance Indexes
Four seakeeping performance indices were selected to assess the safety of fishing vessel operations. First, the roll and pitch responses were considered as seakeeping indices. MII and MSI were also included in the assessment.
The MII is an indicator of the loss of balance experienced by a person on board a vessel (Graham et al., 1992). It quantifies the number of occurrences caused by a combination of sliding and tipping caused by vessel motion. The definition of MII per minute is expressed as follows:
MSI quantifies the percentage of individuals experiencing motion sickness symptoms, such as nausea and vomiting, within a two-hour window of motion exposure (O’Hanlon and McCauley, 1974). Vertical acceleration at the individual’s location is a primary contributor to motion sickness, often amplified by the combined effects of roll and pitch motions.
2.5 Viscous Roll Damping Coefficient
Hydrodynamic analysis is based on potential theory, which requires accounting for viscous effects that are essential when considering roll motion. Thus, an additional viscous roll damping coefficient is required as an input to the potential-based approach by the free roll decay test.
The free roll decay test begins with a forced initial heeling angle, and the fishing vessel is released to decay under fluid forces until it reaches equilibrium. The roll damping coefficient is derived using the relative decrement method, analyzing the peak values from the roll angle time history (van’t Veer and Fathi, 2011). The extinction curve was fitted using the mean amplitude (Ai ) and decrease in amplitude (Di ) divided by the mean amplitude difference, as shown in Fig. 3.
The linear (Bl) and quadratic (Bq ) roll damping coefficients were derived using the least-squares method to fit coefficients p and q to the decrement data points.
The equivalent linear damping can be expressed as
The equivalent linear damping coefficient can be non-dimensionalized by the critical damping coefficient (
3. Results and Discussion
3.1 Estimation of Viscous Roll Damping Coefficient
The roll damping coefficient was obtained by conducting decay simulations using CFD. Performing decay simulations for all 16 fishing vessels is impractical. Thus, CFD simulations were conducted on three representative vessels (FV01, FV02, and FV03), considering the size of the fishing vessels. Fig. 4 shows the computational domain and boundary conditions used in the decay simulation.
The number of grids was selected through a grid dependency test using FV01 as a representative. A coarse, medium, and fine mesh system was used with 1.5 million, 3 million, and 4.5 million grids, respectively. Fig. 5 presents the grid distributions of each system. The time histories of free roll decay motion for three different grid systems were compared (Fig. 6(a)). The result of the coarse grid system showed an increasing difference along with time marching, but the difference between the medium and fine grid systems was minor. As a result, the medium grid system was applied to the following analysis.

Comparison of the time histories of roll motion: (a) Grid dependency test, (b) Time step dependency test
A time step dependency test was conducted to find a suitable time step for free roll decay simulation. Time steps of 0.01 s, 0.005 s, and 0.001 s were applied in the time step dependency test as DT01, DT02, and DT03, respectively. Similar to the result of the grid dependency test, time histories of free roll decay motion were compared between three different results with different time steps in Fig. 6(b). The results showed a minor difference between DT02 and DT03, and the DT01 result showed a slightly larger roll angle than DT02 and DT03. Thus, a time step of DT02 was selected as the representative.
Fig. 7(a) shows the results of the free roll decay simulation for FV01, FV02, and FV03. The time history of roll motion obtained from the decay simulation suggests that the period increases as the size of the fishing vessel increases, as shown in Fig. 7(a). The relative damping coefficients were calculated using the decay period and peak angle, as listed in Table 3.
The relationship between the roll damping coefficient and main particulars was investigated to expand these results to other vessels. GM, breadth, and draft were combined to find the best relationship. GM/(B ⋅ d) had a close linear relationship with the roll damping coefficient, as shown in Fig. 7(b). Thus, the roll-damping coefficients for the remaining fishing vessels were estimated using the GM/(B ⋅ d) values.
The roll damping coefficients were validated by comparing the result with the roll motion RAO obtained from the experiment conducted on the FV01 fishing vessel under the beam sea condition. The experimental result was compared with the hydrodynamic analysis result using the estimated roll damping coefficient, as shown in Fig. 8. The roll amplitudes at the peak frequency showed good agreement between the experimental and hydrodynamic analysis results, confirming the suitability of the estimated roll damping coefficient obtained from the decay simulation using CFD. As a result, the estimated relative damping coefficients were then linearly applied, accounting for the size variation of the other fishing vessels.
3.2 Motion RAOs
Hydrodynamic analysis was conducted on 16 fishing vessels. Considering all fishing vessels and presenting their results in the limited pages is difficult. Thus, three fishing vessels were selected as representatives. FV02, FV03, and FV11, which are Korean standard fishing vessels, were chosen as primary representatives. The gross tonnages of FV02, FV03, and FV11 were 9.77, 4.99, and 7.93 t, respectively; FV02 was the largest, and FV03 was the smallest. Fig. 9 shows the panel models used for the hydrodynamic analysis of each representative vessel. An average of 3,000 underwater panels was applied for each fishing vessel.
Fig. 10 shows the RAOs for representative fishing vessels calculated by the hydrodynamic analysis. In terms of roll motion, FV11 exhibited the smallest roll response. This was attributed to its relatively large breadth compared to its displacement, resulting in reduced roll motion. The peak period of the roll RAO for all three vessels was similar, around 3.0 s. For pitch motion, FV03 shows the most significant response, which can be attributed to its relatively short LBP. In addition, the period of pitch motion was smaller for FV03 and FV11 than for FV02.
3.3 Comparison of Seakeeping Performance Index
The selected seakeeping performance indices, including roll, pitch, MII, and MSI, were calculated using the hydrodynamic analysis results. The variations in these indices according to the direction and magnitude of environmental conditions were examined using rose plots. Figs. 11, 12 and 13 presents the tendencies for FV02, FV03, and FV11, respectively. Each radial axis on the polar plot corresponds to a specific wave incident angle, with 0° and 180° representing the stern and bow, respectively. The magnitude of the indices for each environmental condition is indicated by the symbols.
For all representative fishing vessels, the roll response distributions exhibit the most significant responses at 90° and 270°, respectively (Figs. 11(a), 12(a), 13(a)). The magnitude of the roll response was similar for FV02 and FV03, but FV11 exhibited a relatively smaller roll response. This can be attributed to the relatively larger breadth of FV11 than FV02 and FV03. On the other hand, large pitch responses appeared close to 0° and 180° (Figs. 11(b), 12(b), 13(b)). FV03, with the smallest LBP, exhibited the largest pitch response, while FV02 and FV11 showed similar pitch responses. The magnitudes of MII and MSI, similar to the roll response, were largest near 90° and 270° (Figs. 11(c), 11(d), 12(c), 12(d), 13(c), 13(d)). This suggests that the roll component of acceleration holds a significant weight in the estimation of MII and MSI. Similar to the tendency observed in roll responses, MII and MSI exhibit comparable magnitudes for FV02 and FV03, while FV11 showed a reduced magnitude. The MSI exhibited a marginally increased response at 180° under low environmental conditions, potentially because of the influence of acceleration measured at the bow.
3.4 Relationship Between Seakeeping Performance Index and Main Particulars
The relationship between the four indices and the main particulars of the fishing vessels was examined by measuring the roll, pitch, MII, and MSI for all fishing vessels with respect to the displacement, LBP/B, and B/dm, as shown in Fig. 14. The dots represent the responses obtained from the analysis. The lines indicate the linear regression lines for the dots.
The roll response tended to decrease as the displacement and B/dm increased (Fig. 14(a)). Hence, increasing the breadth of fishing vessels contributes to a decrease in roll motion responses. Conversely, the roll response tended to increase as LBP/B increased because of the relative decrease in breadth compared to the length of the fishing vessel. The tendency for a roll response in ENV3 and ENV4 exhibited a cross-over pattern with increasing B/dm. This is consistent with the observations in Figs. 11(a) and 12(a), where the position of the resonant period leads to a larger roll response in ENV3 for the representative fishing vessels. On the other hand, for larger fishing vessels, the roll peak period of the fishing vessel and the wave period separate, leading to a greater influence of the wave height on the roll response. The pitch response and displacement exhibited a strong relationship, with the pitch response decreasing as the displacement increased (Fig. 14(b)). In contrast, the influence of LBP/B and B/dm on pitch response was less significant.
A similar trend to the relationship between the roll response and main particulars was observed in the case of MII (Fig. 14(c)) because MII is defined as a function of transverse acceleration and is strongly related to the roll response. The results from the representative fishing vessels also confirmed that the tendency of MII was similar to that of the roll response (Figs. 11–13). MSI tended to decrease as B/dm increased, as shown in Fig. 14(d). On the other hand, the dependence on displacement and LBP/B was weak.
The safety of fishing vessels and the magnitude of the estimated seakeeping performance indices were assessed by comparing the estimated indices against the criteria from previous studies under different environmental conditions. For roll and pitch, 6° and 3° were selected as criteria, respectively (Tello et al., 2009, 2011; Mata-Álvarez-Santullano and Souto-Iglesias, 2014). For MII and MSI, 1 and 20 were applied based on the previous studies, respectively (Sariöz and Narli, 2005; Zu et al., 2022). Fig. 15 compares the responses of each index and the criteria for each fishing vessel under different environmental conditions. The red dotted line represents the criteria, and the points represent the responses of each fishing vessel.
The results of roll and pitch responses show that the seakeeping performance remains below the criteria under all environmental conditions except for several fishing vessels in ENV3 and ENV4 conditions, as shown in Figs. 15(a) and (b). In contrast, the MSI exceeds the criteria for a larger number of fishing vessels under ENV3 and ENV4 conditions compared to roll or pitch. On the other hand, under the milder conditions of ENV1 and ENV2, all fishing vessels consistently met the MSI criteria (Fig. 15(d)). In the case of MII, the criteria were exceeded under all ENV3 and ENV4 conditions and even under some ENV2 conditions. This is probably because the current MII criteria, designed for general vessels, do not adequately account for the inherently larger motions experienced by fishing vessels, even under relatively benign environmental conditions. Therefore, it will be necessary to consider the MII criteria that specifically account for the motion characteristics of small fishing vessels.
In addition, the influence of the fishing vessel speed on the seakeeping performance indices was investigated. Forward speeds of 2.5 knots and 5.0 knots were considered, corresponding to a Froude number (
4. Conclusions
The seakeeping performance indices were derived and compared against the criteria to assess their applicability for evaluating the safety and crew workability of small fishing vessels. Hydrodynamic analyses were conducted on 16 fishing vessels, and the responses of the seakeeping performance indices were obtained. The seakeeping performance indices consisted of roll and pitch, which represent the motion performance of the fishing vessel, as well as MII, which indicates tipping and sliding during work, and MSI, which indicates seasickness. Free decay simulations using CFD were performed on three fishing vessels of different sizes to derive the viscous roll-damping coefficient. These are applied to the remaining vessels as the input for hydrodynamic analysis. The motion RAO and the tendencies of seakeeping performance indices according to the magnitude and direction of environmental conditions were examined for three representative fishing vessels. The magnitude of the seakeeping performance indices varied according to the main particulars of the vessels. In the case of the roll response, the differences in response occurred depending on the difference between the wave peak period and the roll motion peak period. MII and MSI showed larger responses when the environmental conditions occurred from the side of the fishing vessel due to the influence of roll motion. The relationship between the main particulars and seakeeping performance indices was examined, and differences were observed according to seakeeping performance characteristics depending on the changes in the main particulars.
The seakeeping performance indices for all fishing vessels were compared against the criteria. For roll motion, pitch motion, and MSI, the criteria were relatively less conservative, and most fishing vessels met the criteria. On the other hand, for MII, most fishing vessels did not meet the criteria except for the lowest environmental conditions. The MII criteria were considerably conservative, and evaluating them by considering the characteristics of small fishing vessels is essential. The responses of roll and pitch motion increased as the forward speed increased, while the influence of forward speed on MII was relatively small. On the other hand, the MSI showed a slight decrease in trend with increasing forward speed.
Throughout this study, the responses of the seakeeping performance indices considering the crew workability and safety of small fishing vessels were derived, and their appropriateness was identified. These results are expected to contribute to the safety of vessels and workers or crew members on small fishing vessels. In the future, it will be necessary to evaluate the seakeeping performance comprehensively, considering various criteria, such as acceleration, slamming, and water on deck.
Notes
The authors declare no potential conflict of interest.
This research was funded by the establishment of software predicting the roll motion and safety of fishing vessels based on a D.N.A. grant funded by the Korea Maritime Transportation Safety Authority (KOMSA).