Man B&W S50MC-C8-TII Project Manual Download

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MAN B&W

2.01

Page 2 of 2

MAN Diesel

198 38 338.4

MAN B&W MC/MCC, ME/MEGI/ME-B engines

placed on the light running propeller curve 6. See
below figure. On the other hand, some shipyards,
and/or propeller manufacturers sometimes use a
propeller design point (PD) that incorporates all or
part of the socalled sea margin described below.

the socalled sea margin, which is traditionally
about 15% of the propeller design (PD) power.

Engine layout (heavy propeller)

When determining the necessary engine layout
speed that considers the influence of a heavy run-
ning propeller for operating at high extra ship resis-
tance, it is (compared to line 6) recommended to
choose a heavier propeller line 2. The propeller
curve for clean hull and calm weather line 6 may
then be said to represent a ‘light running’ (LR)
propeller.

Compared to the heavy engine layout line 2, we
recommend using a light running of

3.07.0%

for

design of the propeller.

Engine margin

Besides the sea margin, a socalled ‘engine mar-
gin’ of some 10% or 15% is frequently added. The
corresponding point is called the ‘specified MCR
for propulsion’ (MP), and refers to the fact that the
power for point SP is 10% or 15% lower than for
point MP.

Point MP is identical to the engine’s specified
MCR point (M) unless a main engine driven shaft
generator is installed. In such a case, the extra
power demand of the shaft generator must also
be considered.

Constant ship speed lines

The constant ship speed lines

∝

, are shown at

the very top of the figure. They indicate the power
required at various propeller speeds in order to
keep the same ship speed. It is assumed that, for
each ship speed, the optimum propeller diameter
is used, taking into consideration the total propul-
sion efficiency. See definition of

∝

in Section 2.02.

Note:

Light/heavy running, fouling and sea margin are
overlapping terms. Light/heavy running of the
propeller refers to hull and propeller deterioration
and heavy weather, whereas sea margin i.e. extra
power to the propeller, refers to the influence of
the wind and the sea. However, the degree of light
running must be decided upon experience from
the actual trade and hull design of the vessel.

Fig. 2.01.03: Ship propulsion running points and engine
layout

Power, % af L

100%

= 0,15

= 0,20

= 0,25 = 0,30

100%

Engine margin
(SP=90% of MP)

Sea margin
(15% of PD)

Engine speed, % of L

2 6

Line 2 Propulsion curve, fouled hull and heavy weather

(heavy running), recommended for engine layout

Line 6 Propulsion curve, clean hull and calm weather (light

running), for propeller layout

Specified MCR for propulsion

Continuous service rating for propulsion

Propeller design point

Heavy running

Light running

Fouled hull

When the ship has sailed for some time, the hull
and propeller become fouled and the hull’s re-
sistance will increase. Consequently, the ship’s
speed will be reduced unless the engine delivers
more power to the propeller, i.e. the propeller will
be further loaded and will be heavy running (HR).

As modern vessels with a relatively high service
speed are prepared with very smooth propeller
and hull surfaces, the gradual fouling after sea
trial will increase the hull’s resistance and make
the propeller heavier running.

Sea margin and heavy weather

If, at the same time the weather is bad, with head
winds, the ship’s resistance may increase com-
pared to operating in calm weather conditions.
When determining the necessary engine power, it
is normal practice to add an extra power margin,

178 05 415.3