Study of a rational method of assigning freeboard

摘要

In Japan, the navigation area for domestic voyage ships had been divided two, namely the coastal area and the greater coastal area. The rule on load lines for the domestic voyage ship is relaxed compared to the international rule. Considering the difference of sea condition and so forth, the requirements for the rule in the greater coastal area are severer than that for ships in the coastal area. Recently, a new navigation area “the limited greater coastal area”, which is located among the coastal area and the greater coastal area, has been introduced for the efficiency of domestic transportation. In the present, rule for the greater coastal area is applied to the ships in the limited greater coastal area in spite of the difference of sea condition. The setting of the rational rule for them is needed. It is well known that deck wetness is one of factors for the basis of the rule on load lines. Deck wetness sometimes causes serious damage to the bow deck itself and structures on the deck. Many studies have been carried out on this phenomenon. It was found that the occurrence of deck wetness could be determined by the comparison of relative water height at stem with bow top height. However, only a few of the studies gave us quantitative information about water volume on deck, impulsive load and the relation between them and ship motion. A practical prediction method, which can be made use of by designers and rule-making people, is needed. An experiment was carried out using a Japanese domestic tanker and a cargo ship model in regular and irregular waves in order to measure the shipping water load and so forth. Based on experimental results, prediction methods for shipping water height distribution and its load are proposed. Using these prediction methods, estimation of the freeboard and bow height for ships navigating in the limited greater coastal area was carried out from the viewpoint of deck wetness.