Stability and hull characteristics of small sized fishing vessels below 10 GT according to hull type

1. Introduction

Fishing vessels play a critical role in the Korean coastal and fishery industries. Among these, small fishing vessels below 10 gross tons (GT) are the most common, accounting for approximately 96% of the national fleet. Despite their dominance, these vessels are disproportionately involved in maritime accidents, and are responsible for more than 8% of all accidents [1][2]. This high accident rate highlights significant safety concerns for small-vessel operations. Figure 1 illustrates the overall distribution of fishing vessels by gross tonnage, while Figure 2 highlights the accident rates by size.

Figure 1:

Distribution of fishing vessels registered in Korea by gross tonnage [1]

Figure 2:

Marine accident rates of fishing vessels by size range [2]

The high accident rate of small-sized vessels is attributed to several factors, including poor stability, absence of statutory surveys, and limited awareness of safety among operators. In particular, unlike larger vessels, fishing vessels under 24 m in length are exempt from statutory requirements such as intact stability assessment and load line assignment according to current Korean regulations, for example “Structure and Equipment Standards for Small Fishing Vessels with a Gross Tonnage of Less than 10 Tons” and “Stability and Load Line Standards for Fishing Vessels” [3][4]. Consequently, their stability characteristics often remain unverified in practice, creating difficulties in evaluating safety margins. This regulatory gap has been identified as a key factor contributing to the vulnerability of small vessels.

Standards and studies regarding stability have been developed for fishing vessels. While some previous studies have addressed stability, most have focused on case-specific analyses or larger vessel categories, leaving a gap in systematic research on the hull types of small fishing vessels. The International Maritime Organization (IMO) provides detailed stability standards for multiple parameters, such as length (L), breadth (B), depth (D), draft (T), and freeboard (f) [5]. In contrast, Korean domestic standards are comparatively simplified, relying mainly on B, D, and freeboard ratio, which may lead to an incomplete evaluation of stability and safety [6]. Kwon and Lee compared the actual GM with the GM requirement of fishing vessels more than 12 m in length and proposed stability criteria for small vessels by reviewing international regulations and combining empirical data with non-dimensional parameters T/D and B/D to evaluate stability and safety margins [7]. Oh and Lim compared the stability standards in Korea, Japan, China, and Canada together with IMO standards [8]. A couple of studies were conducted to estimate stability for small sized fishing vessels [9][10]. Recent studies have highlighted that inadequate stability assessment contributes significantly to accidents involving fishing vessels [11][12]. These studies still focused on accident statistics and stability rather than systematic relationships with hull types and hydrostatic characteristics.

In practice, small fishing vessels in Korea exhibit diverse hull forms, most commonly classified as V-type-, U-type-, or T-type. These hull types have markedly different hydrostatic properties and stability behaviors, which influence both operational safety and seaworthiness. However, few studies have comprehensively analyzed how these hull types affect the metacentric height (GM) and how domestic stability standards compare with international frameworks in evaluating vessel safety. Therefore, this study aimed to:

• 1. Classify the hull types of small sized fishing vessels below 10 GT.
• 2. Review and compare stability standards between IMO and Korean domestic regulations.
• 3. Evaluate hydrostatic characteristics and GM values through computational analysis using the K-SHIP program.

From these results, a practical method is proposed for estimating GM in the initial design, providing engineering insights to support the improvement of domestic safety standards for small-sized fishing vessels.

2. Preparation of Data and Analysis

2.1 Hull Type Classification

Most small fishing vessels in Korea are constructed using fiber-reinforced plastic (FRP) molds, which results in a relatively low production cost but high variability in hull dimensions. Variability in length (L), breadth (B), and depth (D) among vessels of similar tonnage was significant (Figure 3).

Figure 3:

Principal hull dimensions (L, B, D) of small sized fishing vessels registered in Korea

Based on hull data representative of 105 fishing vessels, three major hull types were identified: V-type, U-type, and T-type. Their primary characteristics are listed in Table 1.

Table 1:

Classification of small sized fishing vessels by hull type

Figure 4 shows the distribution of hull types across the gross tonnage. The vessels were distinctly categorized into V-type-, U-type-, or T-type. As the gross tonnage increased, a transition from V-type to T-type was observed, and the T-type was common in fishing vessels over 8 GT.

Figure 4:

Distribution of hull types across gross tonnage ranges

This distribution pattern reflects practical considerations. The V-type hulls perform better at higher speeds but provide less stability, whereas the T-type hulls sacrifice speed for greater stability and load-carrying capacity.

2.2 Review of Stability Standards

Stability criteria are crucial for preventing capsizing accidents. The IMO Intact Stability Code (IMO IS Code) specifies criteria based on GM and dynamic stability, applicable primarily to vessels over 24 m. Although recommendations for the minimum GM exist for fishing vessels under 30 m, the scope remains limited. Stability criteria according to the IMO IS Code are given in Equation (1).

$\[ \begin{array}{c}{\mathrm{G}}_0\mathrm{M}=0.53+2B\left[0.075-0.37\left(\frac{f}{B}\right)+0.82{\left(\frac{f}{B}\right)}^2-\right.\hfill \\ \left.0.014\frac{B}{D}-0.032\frac{l_s}{L}\right]\hfill \end{array} \]$

(1)

where G₀M is the minimum stability criteria, L is the length, B is the breadth, D is the depth, and f is the freeboard height.

In contrast, Korean domestic regulations focus exclusively on vessels shorter than 24 m. These regulations consider breadth (B), depth (D), and freeboard ratio. The minimum G₀M of fishing vessels according to “Stability and Load Line Standards for Fishing Vessels” are given in Equation (2).

$\[ {\mathrm{G}}_0\mathrm{M}=0.04B+ \alpha \frac{B}{D}- \beta \]$

(2)

Here, G₀M is the minimum stability requirement, B is the breadth, D is the depth, α is a coefficient dependent on material (FRP: 0.54, wooden: 0.28), and β is the freeboard ratio.

Table 2 summarizes the differences between IMO standards and Korean domestic regulations.

Table 2:

Comparison of IMO and Korean stability standards.

Korean domestic regulations are simplified formulations based on an empirical method and do not adequately represent the actual stability conditions of small fishing vessels, underscoring the need for refinement.

2.3 Data Collection and Analysis Vessels

A total of 28 small sized fishing vessels below 10 GT were selected as representative vessels. These vessels were selected based on the most recent deliveries (latest registrations) of 10 GT in Korea to ensure data availability and reliability. The following vessel data were collected:

• ● Hull drawings and lines
• ● Compartments and loading conditions
• ● Input data for K-SHIP software

Principal dimensions (L, B, D, T) were considered for each vessel.

Figure 5:

Definition of hull parameters for hydrostatic calculations

Table 3 summarizes the specifications of small fishing vessels for hydrostatic calculations.

Table 3:

Specifications of small sized fishing vessels for hydrostatic calculations

For the hydrostatic calculations, 28 vessels were analyzed, comprising 25 T-type and 3 V-type fishing vessels. U-type hulls were not included because they are extremely rare among recently delivered small fishing vessels under 10 GT in Korea and their hydrostatic data were insufficient for consistent comparison. The number of V-type vessels was relatively small because only a limited sample was available among conventional fishing vessels. Therefore, while the analysis provided meaningful insights into the hydrostatic characteristics of both hull types, the representativeness of the V-type sample was limited and should be interpreted with caution.

2.4 Hydrostatic Calculation

Hydrostatic particulars and GM values were computed using K-SHIP software. The K-SHIP software was developed by the Korea Maritime Transportation Safety Authority to verify the stability of Korean vessels in accordance with Korean regulations. K-SHIP generally considers the stability calculation of coastal vessels in accordance with Korean regulations. The procedure involved the following steps.

• 1. Input of principal dimensions and hull form data.
• 2. Computation of displacement and buoyancy distribution from the hull form and calculation of hydrostatic parameters such as KG and GM.
• 3. Verification of GM.

The GM values were then compared across hull types (V-type- and T-type) to identify their stability. This analysis resulted in GM values ranging from 0.86 to 5.65, covering a wide range of stability parameters. Figures 6–11 illustrate the relationships between GM and the principal hull parameters. First, as shown in Figures 6–9, GM exhibited weak correlations with single parameters, such as L, B, D, and T. This suggests that stability cannot be reliably evaluated using single parameters. In contrast, strong linear correlations were observed between the B/D and B/T ratios (Figures 10 and Figure 11), which can serve as practical indicators for preliminary stability evaluation and estimation in the initial design stage.

Figure 6:

The relationship between G0M and length (L)

Figure 7:

The relationship between G0M and depth (D)

Figure 8:

The relationship between G0M and breadth (B)

Figure 9:

The relationship between G0M and draft (T)

Figure 10:

The relationship between G0M and B/D

Figure 11:

The relationship between G0M and B/T

3. Results and Discussion

4. Conclusion

This study classified the hull types of small Korean fishing vessels, compared IMO and Korean stability standards, and conducted hydrostatic analyses of representative vessels. The findings of this study were as follows:

• 1. The hull types of conventional Korean small-sized fishing vessels below 10 GT were categorized as V-type, U-type, and T-type, with a transition from V-type to T-type dominance as tonnage increased.
• 2. The IMO stability standards consider multiple hull parameters and are more comprehensive, whereas Korean domestic regulations are simplified and account for only B, D, and freeboard ratio, making them insufficient for consistent comparison.
• 3. Hydrostatic analyses of 28 fishing vessels were conducted and the results showed GM values widely ranging from 0.86 to 5.65 according to independent manufactures.
• 4. V-type hulls exhibit low GM and require design modifications or stricter loading limits.
• 5. GM correlated strongly with B/D and B/T ratios, whereas single dimensions such as L, B, D and T were less influential. In particular, B/T can serve as an effective design indicator according to the hull types of Korean fishing vessels.

From this study, implications from an engineering perspective were highlighted.

• ● Design application: the strong relationship between GM and dimensional ratios offers ship designers a simple yet reliable tool for evaluating stability at the earliest design stages, reducing trial-and-error in hull form selection.
• ● Inspection practice: for inspectors and regulators, ratio-based GM estimation provides a practical method for rapid assessment of small fishing vessels, especially in cases where detailed hydrostatic data are unavailable.
• ● Regulatory improvement: Korean domestic standards may be revised to align with the correlations of design parameters, such as B/D and B/T ratios, and safety margins.

Finally, this study established a foundation for further research. Future work should expand the dataset to include a broader range of vessels, integrate sea trial validations, and apply advanced computational methods such as CFD and time-domain simulation to capture dynamic stability behavior under realistic sea states.

Acknowledgments

This research was funded by the Korea Maritime Transportation Safety Authority (KOMSA) as part of the 2025 internal research and development project, “Update and Development of the K-SHIP Program”

References

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