Textos de Apoio (pdf)

Engine power, %R1
100
R1
R1+
Engine power, %R1
100
R1
90
R3
90
80
R3
Rx1
Rx2
Rating line
slope = α
80
R4
R2
R2+
70
R4
R2
80
90
100
Engine speed, %R1
60
70
80
90
100
Engine speed, %R1
Fig. 2: For the RT-flex82C, RTA82C, RT-flex82T and
RTA82T engines the layout fields are extended to the ratings
R1+ and R2+ at the same powers as R1 and R2 respectively
but with increased shaft speed.
[08#049]
that the lower CMCR speeds allow flexibility in
selection of the optimum propeller with consequent
benefits in propulsion efficiency and thus lower fuel
consumption in terms of tonnes per day.
One feature to be borne in mind when selecting
the rating point for the derated engine is the rating
Fig. 4: Since the 1980s engine ratings have been selected over
a steadily smaller area of the layout field.
[08#051]
Engine power, %R1
100
90
80
70
60
70
R3
R4
Area of recent
CMCR selection
80
Area of CMCR
selection in
the 1980s
90
R1
100
R2
Engine speed, %R1
Fig. 3: For a given ship, a rating line (slope α) can be applied
to the layout field so that all rating points on that line would
give the same ship speed with a suitably optimized propeller.
Rating points at lower speeds on the rating line require
a larger propeller diameter and give a greater propulsive
efficiency.
line (Fig. 3). This is the line through a CMCR rating
point such that any point on the line represents
a new power/speed combination that will give
the same ship speed in knots. The points on the
rating line all require the same propeller type but
with different adaptations to suit the power/speed
combination. In general, lower speeds of rotation
require larger propeller diameters and thereby
increase the total propulsive efficiency. Usually the
selected propeller speed depends on the maximum
permissible propeller diameter. The maximum
diameter is often determined by operational
requirements, such as design draught and ballast
draught limitations, as well as class recommendations
concerning propeller–hull clearance (pressure
impulse induced by the propeller on the hull).
The slope of the rating line (α) depends broadly
upon the ship type. It can range from 0.15 for
tankers, bulk carriers and general cargo ships up to
about 10,000 tdw to 0.22 for container ships larger
than 3000 TEU and 0.25 for tankers and bulk
carriers larger than 30,000 tdw.
Changing engine selection strategies
When the broad layout field was introduced in
RTA engines in 1984 it was widely welcomed by
shipowners and shipbuilders. Afterwards RTA
engines were frequently selected at ratings in the
lower part of the layout field to gain the benefits of
— 2 — © Wärtsilä Corporation, June 2008