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 coursekeeping 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 coursekeeping 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 coursekeeping 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 coursekeeping 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 coursekeeping speed in the simplified assessment,
V_{ck}, is selected to facilitate coursekeeping of the ships in
waves and wind from all directions. This speed is defined on the basis of the
reference coursekeeping speed
V_{ck, ref}, related to ships with the
rudder area
A_{R} equal to 0.9% of the submerged lateral area
corrected for breadth effect, and an adjustment factor taking into account the
actual rudder area:

V_{ck} =
V_{ck, ref}  10.0 ×
(A_{R%}  0.9)

(1)

where V_{ck} in knots, is the minimum coursekeeping speed,
V_{ck, ref} in knots, is the reference coursekeeping 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} =
L_{pp}T_{m}(1.0+25.0(B_{wl}/L_{pp})^{2}),
where L_{pp} is the length between perpendiculars in m, B_{wl} is
the water line breadth in m and T_{m} is the draft a midship in m. In case
of highlift rudders or other alternative steering devices, the equivalent rudder
area to the conventional rudder area is to be used.
3.7 The reference coursekeeping 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, AFW, 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:

T =
(R_{cw} + R_{air} +
R_{aw} + R_{app})
/(1 t)

(2)

3.9 The calmwater resistance for bulk carriers, tankers and combination carriers can
be calculated neglecting the wavemaking resistance as , where k is the form factor, is the frictional resistance coefficient, is the Reynolds number, ρ is water density in
kg/m^{3}, S is the wetted area of the bare hull in m^{2},
V_{s} is the ship advance speed in m/s, and ν is the
kinematic viscosity of water in m^{2}/s.
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:


(3)

where C_{B} is the block coefficient based on L_{pp}.
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/m^{3}, A_{F} is the frontal windage area of the hull
and superstructure in m^{2}, 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:


(4)

where is the quadratic transfer function of the added
resistance, depending on the advance speed V_{s} in m/s, wave
frequency ω
in rad/s, the wave amplitude, ζ_{a}
in m and the wave spectrum, S_{ζζ}
in m^{2}s. 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 7.502 0702.1 and 7.502
0702.2, 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

0.55

0.23




3.14 The required advance coefficient of the propeller is found from the equation:


(5)

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} (1w) . J can be found from the curve of
K_{T} (J)/J^{2}.
3.15 The required rotation rate of the propeller, n, in revolutions per second, is
found from the relation:


(6)

3.16 The required delivered power to the propeller at this rotation rate
n,
P
_{D} in watt, is then defined from the relation:


(7)

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
torquespeed 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 torquespeed 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.