Page 140 - Practical Design Ships and Floating Structures

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TABLE 1
PRINCIPAL PARTICULARS OF GEOMETRICALLY SIMILAR MODEL SHIPS
Model Name L M S
Scale Ratio, h 1 1.33 2
LWL (m) 1.8 1.35 0.9
Breadth. B (m) 0.4 0.3 0.2
Dratf, T (m) 0.0756 0.0567 0.0378
Step Height, h (mm) 5 3.75 2.5
Deadrise, a (") 10

2.2 Air Cavity and Scale Law
From the previous experience, it is known that area of the air cavity formed behind a step usually
increases with increase in flow rates of air but do not above a certain flow rate of air, i.e., the critical
flow rate of air. The amount of drag reduction also ceases to increase at the critical flow rate of air and
so increase in flow rate of air beyond the limit is useless. The critical flow rates of air for each
geometrically similar model ships have been measured over the range of the Froude number,
Fn I 0.35-0.6. The flow rate of air, Q,,, , non-dimensionalized with V, , the towing velocity of the
model ship, B , the breadth of a model ship, and h, the step height as shown in Eqn. 1 [Sato et ul.
(1997)l and the results are summarized in Figure 2.

The area of air cavity, A, at the critical flow rate of air is measured and also shown in Figure 2 in the
form of the ratio A, /S where S is the wetted surface area when air is not supplied. Both the figures(2a
and 2b) show that scale effects are not dominant and the two variables CQV and Ac/S have almost
identical values for all three models at a given Froude number.

0.05 : 40
.
:
ModelM
ModelM
o.04. :LJ A M-IL 30 - A M-IL
, 0.03 Fitted Curve :20 j 0 Models
Fitted Curve
~
0"
0.02 - 2:
10 -
0.01 -