Page 132 - Practical Design Ships and Floating Structures

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long engaged in this industry, have made much efforts to the research on the hull form design
technology. As the result, several different theoretical methods have been prepared and published by
the authors. As far as the authors’ knowledge is concerned, no analytical optimum hull form design
method practically exists in the world except the authors’ works. Recently, the authors have developed
a theoretical hull form design method for displacement type super-high-speed ships utilizing the
“Minimum Resistance Theory” and “Sectionally-Varying Hull From Equation”[S]. This method
quickly produces displacement type super-high-speed ship hull forms which not only possess superior
resistance characteristics, but also require almost no correction or adjustment. The hull form design
concept and the method have been presented several times and shall not be discussed in this paper.

To improve effectiveness of the study, it was decided to first carry out the study on the demi-hull
forms and to design the catamaran ship hull form utilizing the result of the demi-hull form study. As
discussed earlier, the ship type in this study is the type whose resistance is greatly decreased during
normal sailing, since designated part of the ship hull is lifted up above the free-surface by the dynamic
lift. Therefore, it does not have much meaning to optimize the hull form for the full load condition.
The primary importance is to design the hull form to have the least resistance in the normal sailing
condition when desired part of the ship hull is lifted up. In order to do that, the optimum ratio between
buoyancy and lift should be known in advance. However, it is not known in the initial stage. From
the previous experiences, therefore, the buoyancy/lift ratio has been tentatively selected as 40 : 60.

In principle, two optimized hull forms were designed first - one for the fill load condition(upper hull
form) and the other for the normal sailing condition(1ower hull form), and the upper and the lower hull
forms were combined to a single hull form. In this way, total 4 demi-hull forms were prepared. Model
tests were carried out for 4 demi-hull forms, and the test results were compared. One best hull form
was selected focusing the performance characteristics to the condition of desired buoyancy/lift ratio.
The catamaran ship hull form was designed based on the selected demi-hull form, and is shown in
Figure 1.

All the model tests were carried out in deep water towing tank of Hyundai Maritime Research
Institute(HMR1). Since model tests for the demi-hull forms were regarded as qualitative ones, small
models of 2.5m length were manufactured using wood and polyurethane. For the catamaran hull form
tests, 6.0m long ship model (0=1/40) was manufactured using wood.

Figure 1 : Catamaran Ship Hull Form

4 STUDY ON THE OPTIMUM BUOYANCYLIFT RATIO

As the ship hull is lifted up above free-surface, the hull displacement is decreased, and hence, the hull
resistance is also decreased. On the other hand, resistance of the hydrofoil system is increased as
dynamic lift is increased. The sum of these two resistance components, that is, the sum of the hull
resistance and the foil system resistance is the overall resistance for buoyancyflift combination ships.
The overall resistance varies according to the buoyancyflift ratio. In general, therefore, the