Vessel having laterally offset propeller

A vessel equipped with a single screw, comprising a hull which is symmetrical with regard to a vertical center plane containing the center line of the hull, and a propeller shaft which is positioned laterally offset from the vertical center plane of the hull. The propeller shaft is positioned such that a ratio (d/D) is from 5 to 25%, where d represents a distance between the propeller shaft and the vertical center plane of the hull, and D represents the diameter of the propeller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hull form of a vessel, and more 
particularly to a position where a propeller shaft is installed. 
2. Description of the Related Art 
A body plan of a single-screw hull equipped with a conventional symmetric 
type stern is shown in FIG. 1, wherein reference numeral 1 denotes a 
transverse sectional shape, 2 denotes the hull center line, 3 denotes a 
propeller shaft, 4 denotes a propeller disc plane and WL denotes a load 
waterline. It is well known that the propeller shaft is usually provided 
on the vertical central plane of the hull, for a conventional type of a 
single-screw vessel. 
When the propeller shaft is installed in such a position, water inflows to 
the propeller disc plane are shown in FIGS. 2(A) and 2(B). FIGS. 2(A) and 
2(B) represent graphically water inflow speed to the propeller disc plane. 
FIG. 2(A) is a representation of wake distribution, and FIG. 2(B) is a 
vector diagram for transverse velocity of water. Curved line (a) shows a 
ratio of wake speed generated on the propeller disc plane in relation to 
vessel speed, and vector (b) shows the transverse direction of wake 
velocity generated on every point of the propeller disc plane. As clearly 
understood from these representations, inflows to the propeller disc plane 
are formed into symmetrical flows with regard to propeller shaft 3. In 
this manner, complicated distribution of wakes are generated while the 
vessel is sailing. As shown in FIG. 3, the wakes become symmetrical with 
regard to propeller shaft 3 positioned on the vertical center plane 2 of 
the hull. 
Vessels with high block coefficient and wide breadth have been increasing 
in number to raise loading capacity. Owing to this high blockage 
coefficient and wide breadth, vertical vortices around longitudinal axes 
are generated on the propeller disc plane, from the aforementioned wakes. 
These vertical vortices are generated in pairs by both sides of a vessel, 
unbalancing the wakes on the propeller disc plane. This results in 
reducing efficiency in propulsion and increasing hull-resistance. In those 
circumstances, there has been demanded a reduction of the ratio of fuel 
consumption for sailing as well as improvement in loading capacity. To 
satisfy this demand, improvement in propulsive efficiency is 
indispensable. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a vessel having high 
propulsive efficiency. 
In accordance with the present invention, there is provided a vessel 
comprising: 
a hull which is approximately symmetrical with respect to a vertical center 
plane containing the center line of the hull 
a propeller shaft which is positioned laterally offset from the vertical 
center plane of the hull; and 
a propeller installed on the propeller shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference characters designate 
like parts or corresponding parts throughout the several views, FIG. 4 
shows a plan view of an afterbody of a hull body plan of the present 
invention, as viewed from the back side. As shown in FIG. 4, the hull 
construction is symmetrical with regard to the vertical center plane 2 
containing the center line of the hull (hereinafter referred to as the 
vertical center plane) and the propeller shaft is positioned laterally 
offset from the vertical hull center place 2. Consequently, the only parts 
at which the propeller shaft is installed are asymmetrical. 
How the propeller shaft is positioned will now be described. 
With reference specifically to the drawing, FIG. 5 represents vector 
diagram illustrating movements of water inflows on the propeller disc 
plane of a vessel. As shown in FIG. 5, water inflow vector b is the 
transverse component of velocity which is symmetrical about vertical hull 
center plane 2. Propeller blades are rotated clockwise, about the axis of 
propeller shaft 3 which is positioned horizontally on the starboard side 
of the hull center plane. 
The relationship of the direction of the water inflows to the direction of 
the rotation of the propeller is shown in FIG. 6. In FIG. 6, arrows 5 
represent a direction of the water inflows which is indicated by vector b 
shown in FIG. 5. Arrow 6 represents a rotating direction of the propeller. 
As seen from FIG. 6, the propeller constantly receives the water inflows 
that circulate reverse to the direction to which the propeller shaft is 
rotated. This gives such an effect as if the rotating speed of the 
propeller shaft were increased. In other words, an increase of propulsion 
efficiency can be attained by this positioning of the propeller shaft. 
As described above, an increase of propulsion efficiency is attained by 
rotating the propeller shaft clockwise when the propeller shaft is 
positioned on the starboard side of the vertical hull center plane, and by 
rotating the propeller shaft counterclockwise when it is positioned on the 
port side. Otherwise, for example, when the propeller shaft is positioned 
on the starboard side and rotated counterclockwise, the rotating direction 
of the propeller shaft becomes the same as the circulating direction of 
water inflows. Consequently, the propulsion efficiency is lowered. When 
the propeller shaft is positioned on the port side and rotated clockwise, 
the propulsion efficiency is lowered as well. 
With reference now specifically to the drawing, 
FIGS. 7(A) and 7(B) show plan views of examples of the present invention. A 
rudder at the stern is positioned on the hull center plane. FIG. 7(A) is a 
schematic representation illustrating propeller shaft 3 being positioned 
horizontally in parallel to and offset from the vertical the hull center 
plane 2, without a horizontal rake. FIG. 7(B) is a schematic 
representation illustrating propeller shaft 3 arranged with a horizontal 
rake angle relative to the vertical hull center plane 2. The space of an 
engine room and the capacity of a main engine determined whether the 
arrangement of FIG. 7(A) or of 7(B) is adopted. According to test results, 
there was no difference between the types of FIGS. 7(A) and 7(B) with 
respect to steering ability and propulsion efficiency. In addition, there 
were no differences between one vessel equipped with a propeller shaft 
positioned laterally offset from the vertical hull center plane and 
another vessel equipped with a propeller shaft conventionally positioned, 
with respect to the steering ability. 
FIG. 8 graphically shows a relation of a distance between propeller shaft 3 
and the vertical hull center plane 2 to relative propulsive power ratio 
efficiency which was obtained through a water tank test of propelling a 
200,000 DWT ore carrier. In FIG. 8, the ordinate shows a ratio of 
HP(O)/HP(C) where HP(O) represents the propulsive horse power generated by 
an engine in the case of a propeller shaft positioned laterally offset 
from the vertical hull center plane and HP(C) represents the propulsive 
horse power generated in the case of a propeller shaft positioned on the 
vertical hull center plane, and the abscissa represents a ratio of d/D 
where d represents a distance between the propeller shaft and the vertical 
hull center plane and D represents a diameter of a propeller. As seen from 
FIG. 8, the relative propulsive power ratio shown by the HP(O)/HP(C) is 
remarkably improved when the d/D ranges from 5 to 25%. If the ratio is 
less than 5%, the propulsive efficiency does not increase. On the other 
hand, if the ratio is over 25%, the propulsive efficiency does not 
increase, either. The ratio range from 10 to 15% is most preferably. 
Other test results proved that rudder position was not required to be 
restricted owing to this positioning of the propeller shaft; the rudder 
position was not unfavorably affected. 
The present invention enabled the propulsive efficiency to be improved (by 
10% approximately) by making use of vertical vortices which had caused a 
reduced propulsive efficiency in a conventional vessel with wide breadth 
and high blockage. Moreover, the present invention also enables keeping 
the hull structure symmetrical on both sides of the vessel.