Page 133 - Practical Design Ships and Floating Structures

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buoyancynift ratio exists at which the overall resistance for the given ship speed becomes minimum.
This ratio is the optimum ratio. Figure 2 shows the concept of the optimum buoyancyhift ratio. It is
important to design the hydrofoil system to produce the lift as much as the amount designated by the
optimum ratio.

- -
- __ ._
Higher Lift Higher Buoy.
BuoyancylLifI Ratio
Figure 2: Concept of the Optimum BuoyancyLift Ratio

The general method to find out the optimum ratio is as follows :
- pre-select several buoyancyflift ratios
- design several hydrofoil systems for each of ratios
- manufacture ship and foil system models
- conduct model tests for all the combinations of buoyancynift ratio and foil systems
- analyze the test results and determine the optimum ratio

The above procedure could possibly be carried out only by an expert who possesses the high level
technologies on the buoyancyflift combination ship and the foil system. Furthermore, it will take
enormously long time and high cost to carry out the model tests for such numerous combinations. In
practice, therefore, it is almost impossible to per€orm the above procedure.
In the way of long continuous studies on super-high-speed foil catamaran ships, however, the authors
have developed a theoretical and empirical method that quickly determines the optimum buoyancyAift
ratio with suffkient practical accuracy. It is considered that the authors' method is the only method in
the world which could determine the optimum buoyancy/lift ratio without model tests. This method is
being actively utilized in actual designs.

In order to determine the optimum buoyancy/lift ratio, a number of ratios should be pre-selected
systematically and resistance components for the following items should be calculated for each of
pre-selected ratios :

- bare-hull resistance
- foil resistance
- side strut resistance
- center strut resistance (if any)
-blockage resistance at comers

The overall resistance is obtained by summing up the resistance components, and the buoyancyflift
ratio versus overall resistance curve could be prepared. The optimum ratio is the ratio where the
overall resistance becomes minimum in the curve. The method of calculation or estimation for each of