3 Simplified assessment
3.1 The simplified assessment procedure is based
on the principle that, if the ship has sufficient installed power
to move with a certain advance speed in head waves and wind, the ship
will also be able to keep course in waves and wind from any other
direction. The minimum ship speed of advance in head waves and wind
is thus selected depending on ship design, in such a way that the
fulfilment of the ship speed of advance requirements means fulfilment
of course-keeping requirements. For example, ships with larger rudder
areas will be able to keep course even if the engine is less powerful;
similarly, ships with a larger lateral windage area will require more
power to keep course than ships with a smaller windage area.
3.2 The simplification in this procedure is that
only the equation of steady motion in longitudinal direction is considered;
the requirements of course-keeping in wind and waves are taken into
account indirectly, by adjusting the required ship speed of advance
in head wind and waves.
3.3 The assessment procedure consists of two steps:
-
.1 definition of the required advance speed in
head wind and waves, ensuring course-keeping in all wave and wind
directions; and
-
.2 assessment whether the installed power is sufficient
to achieve the required advance speed in head wind and waves.
Definition of required ship speed of advance
3.4 The required ship advance speed through the
water in head wind and waves, V
s, is set to
the larger of:
-
.1 minimum navigational speed, V
nav; or
-
.2 minimum course-keeping speed, V
ck.
3.5 The minimum navigational speed, V
nav, facilitates leaving coastal area within a sufficient time
before the storm escalates, to reduce navigational risk and risk of
excessive motions in waves due to unfavourable heading with respect
to wind and waves. The minimum navigational speed is set to 4.0 knots.
3.6 The minimum course-keeping speed in the simplified
assessment, V
ck, is selected to facilitate
course-keeping of the ships in waves and wind from all directions.
This speed is defined on the basis of the reference course-keeping
speed V
ck, ref, related to ships with the
rudder area A
R equal to 0.9 per cent of the
submerged lateral area corrected for breadth effect, and an adjustment
factor taking into account the actual rudder area:
 where V
ck in knots, is the
minimum course-keeping speed, V
ck, ref in
knots, is the reference course-keeping speed, and A
R% is
the actual rudder area, A
R, as percentage
of the submerged lateral area of the ship corrected for breadth effect, A
LS, cor, calculated as A
R% = A
R/A
LS, cor •100%. The submerged
lateral area corrected for breadth effect is calculated as A
LS, cor =LppTm(1.0+25.0(Bwl/Lpp)2), where Lpp is the length between
perpendiculars in m, Bwl is the water line breadth in m
and Tm is the draft a midship in m. In case of high-lift
rudders or other alternative steering devices, the equivalent rudder
area to the conventional rudder area is to be used.
3.7 The reference course-keeping speed V
ck, ref for bulk carriers, tankers and combination carriers
is defined, depending on the ratio A
FW/A
LW of the frontal windage area, A
FW,
to the lateral windage area, A
LW, as follows:
Procedure of assessment of installed power
3.8 The assessment is to be performed in maximum
draught conditions at the required ship speed of advance, V
s, defined above. The principle of the assessment is that the
required propeller thrust, T in N, defined from the sum
of bare hull resistance in calm water R
cw,
resistance due to appendages R
app, aerodynamic
resistance R
air, and added resistance in waves R
aw, can be provided by the ship's propulsion system,
taking into account the thrust deduction factor t:
3.10 The form factor k should be
obtained from model tests. Where model tests are not available the
empirical formula below may be used:
where CB is the block coefficient based
on Lpp.
3.11 Aerodynamic resistance can be calculated
as  , where C
air is the aerodynamic
resistance coefficient, ρ
a is the density
of air in kg/m3, A
F is the frontal
windage area of the hull and superstructure in m2, and V
w rel is the relative wind speed in m/s, defined
by the adverse conditions in paragraph 1.1 of the interim guidelines, V
w, added to the ship advance speed, V
s. The coefficient C
air can be obtained
from model tests or empirical data. If none of the above is available,
the value 1.0 is to be assumed.
3.12 The added resistance in waves, R
aw, defined by the adverse conditions and wave spectrum in paragraph
1 of the interim guidelines, is calculated as:
where R
aw
(V
s
,ω)/ζ
a
2 is
the quadratic transfer function of the added resistance, depending
on the advance speed V
s in m/s, wave frequency ωin
rad/s, the wave amplitudeζ
a
in
m and the wave spectrum, S
ζζ in
m2s. The quadratic transfer function of the added resistance
can be obtained from the added resistance test in regular waves at
the required ship advance speed V
s as per
ITTC procedures 7.5-02 07-02.1 and 7.5-02 07-02.2, or from equivalent
method verified by the Administration.
3.13 The thrust deduction factor t can
be obtained either from model tests or empirical formula. Default
conservative estimate is t=0.7w, where w is
the wake fraction. Wake fraction w can be obtained from
model tests or empirical formula; default conservative estimates are
given in table 2.
Table 2 Recommended values for
wake fraction w
| Block coefficient
|
One propeller
|
Two propellers
|
| 0.5
|
0.14
|
0.15
|
| 0.6
|
0.23
|
0.17
|
| 0.7
|
0.29
|
0.19
|
| 0.8 and above
|
0.35
|
0.23
|
3.14 The required advance coefficient of the propeller
is found from the equation:
where D
P is the propeller
diameter, K
T (J) is the open
water propeller thrust coefficient, J = u
a/nD
P, and u
a= V
s (1–w) . J can be found from
the curve of K
T
(J)/J2.
3.15 The required rotation rate of the propeller, n, in revolutions per second, is found from the relation:
3.16 The required delivered power to the propeller
at this rotation rate n, PD in watts, is then
defined from the relation:
where K
Q(J)
is the open water propeller torque coefficient curve. Relative rotative
efficiency is assumed to be close to 1.0.
3.17 For diesel engines, the available power is
limited because of the torque-speed limitation of the engine, Q ≤Q
max (n) , where Q
max(n) is
the maximum torque that the engine can deliver at the given propeller
rotation rate n. Therefore, the required minimum installed
MCR is calculated taking into account:
-
.1 torque-speed limitation curve of the engine
which is specified by the engine manufacturer; and
-
.2 transmission efficiency η
s which
is to be assumed 0.98 for aft engine and 0.97 for midship engine,
unless exact measurements are available.
|