## 4 Discussion
Figure 17 depicts a polar diagram of the obtained results after analysing the 21 sea state conditions. It is shown that potential synchronism situations are produced for 1 s \(<\) Tw \(<\) 10 s, i.e., for the lowest Tw or similar, without being considered very rough seas. In this case, it is possible to find synchronism in more sea state conditions (Tw = 9 and 10 s) than in the previously mentioned study. Furthermore, the graph shows us that the potential synchronism is concentrated from forward to a little after the beam. Figure 16: Polar diagram of synchronism representing \(\alpha\) angle for different ship speeds. Therefore, with the results obtained from polar diagrams, it is deduced that the extreme peaks registered in Figure 15, corresponding to erratic behaviour, are related to moments of synchronism occurring. Furthermore, the results of both approaches are concordant. The Spanish Commission for Investigation of Maritime Accidents and Incidents (CIAIM) issued a technical report about the accident of the fishing vessel SIN QUERER DOS [35], where four crewmembers died in Galician waters in December 2018. It was concluded that the sudden capsizing and sinking of the vessel was caused by deficient stability, mainly related to a negligent shipment of fishing nets with excessive weight and size. From this report, it was obtained that the ship was sailing with a course of about 160\({}^{\circ}\); with wind from the west at a force 5 (gusts of force 7) on the Beaufort scale; and a swell from the west with wave heights between 4 m and 5 m, corresponding to 8 s < Tw with a median < 9 s. The ship was sailing freely after a fishing campaign, so it could have been sailing with a speed between 7 and 10 knots. According to the data included in the report, the \(\alpha\) angle between the ship's heading and the waves' direction was about 110\({}^{\circ}\). Therefore, considering the parameters of the ship's speed and \(\alpha\) angle, according to our conclusions expressed in Figure 17, at the moment of the accident the ship could have been involved in a synchronism phenomenon. However, in spite of this, there was no mention included in the report of the ship's behaviour and seakeeping in dynamic situations when sailing between waves. Therefore, the mathematical models, graphs, and results presented in this paper could be considered by ship operators, by investigating equipment regarding sudden losses of stability (mainly fishing vessels), or by investigating the potential damages due to cargo shifting on board merchant vessels due to an excessive angle of rolling. With the use of these guides, shipmasters have a useful tool when they decide to change the ship's speed and heading, depending on the available time, to reach a new stable condition. Furthermore, it was deduced that when a shipmaster alters the heading in order to receive the waves forward the beam, the Te tendency to reach the coupling is linear with the ship's speed. However, if after performing the heading alteration the waves are received abaft the beam, the shipmaster must carry out a deep study of the situation. The shipmaster may need to reach the Te quickly or, conversely, he may need to lengthen the time at which the rendezvous is reached in order to alter, for example, any other situations on board such as reinforcing the lashing system or changing the loading conditions. In consequence, as a guide for shipmasters, it is proposed to improve navigation by selecting adequate values of ship velocity and heading angle in accordance with our previous conclusions. This relation, due to the limitations in reducing the ship's speed  
Figure 17: Polar diagram of the synchronism representing ship speeds for different \(\alpha\) angles. to reasonable and energetic minimum requirements, has a limitation in changes. Despite this, changing the heading angle will help to reach, until the coupling process is finished, an adequate rolling angle and, what is more, change the time to adjust to these new sea conditions.