Ships – Building – Antifriction surfaces
Reexamination Certificate
2001-11-09
2004-06-15
Swinehart, Ed (Department: 3617)
Ships
Building
Antifriction surfaces
Reexamination Certificate
active
06748891
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for reducing the frictional drag of a ship and to a ship with reduced frictional drag. In particular, the present invention relates to a technique by which energy required for delivering gas into water is reduced, and thereby power required for sailing a ship is effectively decreased.
This application is based on Japanese Patent Application No. 20000-067649, the contents of which are incorporated herein by reference.
BACKGROUND ART
A technique relating to a ship with reduced frictional drag is disclosed in Japanese Unexamined Patent Application, First Publication, No. 50-83992, Japanese Unexamined Patent Application, First Publication, No.53-136289, Japanese Unexamined Patent Application, First Publication, No. 60-139586, Japanese Unexamined Patent Application, First Publication, No. 61-71290, Japanese Utility Model Application No. 61-128185, and Japanese Utility Model Application No. 61-128185, etc. In a ship with reduced frictional drag, a number of microbubbles are produced so as to be present on a shell plating of a hull by delivering gas, such as air, into water from the outer surface of the hull (i.e., the shell plating) while the ship is in a sailing mode so that frictional drag generated between the hull and the water may be reduced by the presence of the microbubbles.
As a technique relating to the above-mentioned ship with reduced frictional drag, the applicant of the present invention proposed a technique by which microbubbles are produced to be present at a shell plating of a hull by delivering gas (for instance, air) into water from the vicinity of the bow. The aim of the technique is to cover the shell plating with microbubbles, which are generated by delivering gas from the vicinity of the bow, by diffusing the microbubbles along the flow of water on the shell plating. Conventionally, a gas supply device, such as a blower, has been used as a driving source for delivering the gas into water.
However, if a gas supply device, such as a blower, is used for delivering gas, a portion of the sailing power which is saved by the generation of microbubbles is lost because the power is consumed for driving the gas supply device. In the case where microbubbles are delivered at the vicinity of the ship's bottom, in particular, a large driving force tends to be required for the delivery of gas since it is necessary to deliver the gas using a larger force than the static pressure present in the vicinity of the ship's bottom. Also, a large cost for the device, operation, etc., will be required if a gas supply device is installed.
DISCLOSURE OF THE INVENTION
The present invention takes into consideration the above-mentioned circumstances with the following objects:
(1) To effectively decrease the power required for sailing a ship by reducing power necessary for delivering gas into water, and
(2) To reduce the construction cost of a ship.
In order to achieve the above objects, the present invention provides a method for reducing frictional drag between a hull and the water by generating bubbles in the vicinity of the surface of a shell plating of the hull in which a technique to discharge bubbles into water is adopted so that a negative pressure portion, whose pressure becomes lower with respect to a gas space as the ship travels, is formed in water, and gas is introduced into the negative pressure portion in water from the gas space.
Also, the present invention provides a ship with reduced frictional drag which reduces frictional drag between a hull and the water by generating bubbles in the vicinity of the surface of a shell plating of the hull, in which a technique to provide a negative pressure formation part which is disposed at the shell plating of the hull in order to form a negative pressure portion, whose pressure is lower than the pressure of a gas space, in water, and a gas path which introduces gas into the negative pressure portion in water from the gas space, is adopted.
Here, an explanation is made for reducing frictional drag of a hull by using the above-mentioned techniques.
FIG. 1
is a schematic diagram showing a ship
10
with reduced frictional drag according to an embodiment of the present invention. In
FIG. 1
, the numeral
11
indicates a shell plating of a hull, the numeral
12
indicates a negative pressure formation part, the numeral
13
indicates a gas path, and the numeral
14
indicates a water level (i.e., the waterline). A flow of water
20
is created relative to the ship
10
as the ship
10
sails in the direction indicated by the arrow Xa in the figure at a predetermined speed Vh.
The ship
10
with reduced frictional drag forms a negative pressure portion
21
in water whose pressure becomes lower (negative pressure, vacuum) than the pressure of a gas space (atmospheric air) during the travel. That is, the flowing state of water is changed to a desirable state using the negative pressure formation part
12
which is disposed at the shell plating
11
, in order to form the negative pressure portion
21
in water (a negative pressure system).
As a means for forming the negative pressure portion
21
in water, for instance, as shown in
FIG. 2
, the flow rate of water passing through a passage along the shell plating
11
of the hull may be increased by narrowing the passage by means of the negative pressure formation part
12
(Bemoulli's theorem). In this case, the pressure P at the passage may be expressed by the following equation:
P=P
0
+&rgr;·g·h−&rgr;·
(
V
1
2
−Vh
2
)/2 (1)
where V
1
indicates the flow rate of water, P
0
indicates the pressure of the gas space (atmospheric pressure), &rgr; indicates the density of water, g indicates the gravitational acceleration, and h indicates the depth of water. As is apparent from equation (1), it is possible to form the negative pressure portion
21
in water by sufficiently increasing the flow rate of water V
1
at a particular portion with respect to the speed of the ship Vh.
Also, it is known that a low-pressure portion (a flow separation area) tends to be generated behind an object placed in a flow because the matter changes the flowing state and causes a separation of a boundary layer of the fluid. That is, the negative pressure portion
21
may also be formed by generating a flow separation area in water by the above-mentioned negative pressure formation part
12
.
In general, if a bluff body which increases the resistance against a flow is placed in a uniform flow, a flow separation area associated with irregular vortexes is generated in the downstream area immediately behind the bluff body. For example, if a cylinder is put in a uniform flow, a fluid flows along the cylinder at decreasing pressure until it reaches a minimum pressure point, and immediately after that, it separates from the surface of the cylinder and a flow separation area is formed. In this case, it is confirmed by experiment that the pressure at the minimum pressure point may be expressed as, for instance,
(
P−P
0
)/(&rgr;V
2
/2)≈−2.2 (2)
(where P is static pressure, P
0
is reference pressure, &rgr; is the density of fluid, and V is the flow rate). Accordingly, if the flow rate V is 7 m/s (about 14 knots), and the reference pressure P
0
is 1 kgf/cm
2
(atmospheric pressure), the static pressure P (absolute pressure) is calculated to be about 0.45 kgf/cm
2
, which is a negative pressure with respect to the atmospheric pressure. This indicates that it is possible, if the depth of water is about 5.5 m or shallower, to generate a flow separation area of negative pressure on the surface of a cylinder by flowing the cylinder in water at a speed V of 7 m/s.
According to the ship
10
with reduced frictional drag of an embodiment of the present invention, the negative pressure portion
21
is formed in water in the above-mentioned manner, and gas is supplied from a gas space at the high pressure side to the negative pressure portion
21
at the low pressure side in water via the gas path
Fish & Richardson P.C.
Ishikawajima-Harima Heavy Industries Co. Ltd.
Swinehart Ed
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