Page 138 - Practical Design Ships and Floating Structures

Practical Design of Ships and Other Floating Structures
You-Sheng Wu, Wei-Cheng Cui and Guo-Jun Zhou (Eds)
8 2001 Elsevier Science Ltd. All rights reserved

A PRACTICAL APPLICATION OF AIR LUBRICATION
ON A SMALL HIGH SPEED BOAT

Jinho Jang ', 11 Jun Ahn Jaesung Kim I, Jung-Chun Suh Hyochul Kim I,
Seung-Hee Lee and Museok Song

' Dept. of Naval Architecture & Ocean Eng., RIMSE, Seoul Nat. Univ., Seoul, Korea
Dept. of Naval Architecture & Ocean Eng., Inha Univ., Inchon, Korea
Dept. of Naval Architecture & Ocean Eng., Hongik Univ., Jochiwon, Korea

ABSTRACT

The reduction of resistance with lubrication of an air cavity and the similarity relations involved are
investigated with a series of towing tank tests on geometrically similar models. Three geometrically
similar models of different size are tested in the towing tank of Seoul National University. The results
indicate that an introduction of air cavity by artificially supplying air beneath the bottom of a model
ship with a backward-facing step is effective for reduction of the model resistance. The areas of air
cavity and the required flow rates of air, both of which are directly related to the effective wetted
surface area and the overall energy saving are found to be dominated by the Froude number scaling.
Both the traditional two-dimensional method and Telfer's three-dimensional method seem to be
applicable to the extension of the model resistances in the tested range if corrections are made to
account the changes in the frictional resistance caused by the changes in the effective wetted surface
area. Based on the findings obtained above, a small test boat of practical size is constructed and the
effect of air cavity on the reduction of ship resistance is studied. The results will confirm the results
found in the model tests and assure the practical applicability of the air cavity for the resistance
reduction of a real ship.

KEYWORDS

Air lubrication, Frictional resistance, Drag reduction, Bottom Step, Geometrically similar models,
Model experiment, Scale law

1 INTRODUCTION
The researches on drag reduction of displacement type ships have been focused mainly on reduction of
wave resistance or form drag, and many valuable concepts are derived already and practically utilized
to achieve the high energy efficiencies from the modem hull forms. On the contrary, however, little
attempts have been made to reduce the frictional resistance in spite of the fact that the frictional
resistance occupies a major portion of the total resistance of conventional ships. In these days, with
increase in the understanding of the phenomena involved in the friction and the harsh request for