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<strong>MAN</strong> B&W 2.02<br />
Propeller diameter and pitch, influence <strong>on</strong> the optimum propeller speed<br />
In general, the larger the propeller diameter D,<br />
the lower <str<strong>on</strong>g>is</str<strong>on</strong>g> the optimum propeller speed and the<br />
kW required for a certain design draught and ship<br />
speed, see curve D in the figure below.<br />
The maximum possible propeller diameter depends<br />
<strong>on</strong> the given design draught of the ship,<br />
and the clearance needed between the propeller<br />
and the aft body hull and the keel.<br />
The example shown in the figure <str<strong>on</strong>g>is</str<strong>on</strong>g> an 80,000 dwt<br />
crude oil tanker with a design draught of 12.2 m<br />
and a design speed of 14.5 knots.<br />
When the optimum propeller diameter D <str<strong>on</strong>g>is</str<strong>on</strong>g> increased<br />
from 6.6 m to 7.2. m, the power demand<br />
<str<strong>on</strong>g>is</str<strong>on</strong>g> reduced from about 9,290 kW to 8,820 kW, and<br />
the optimum propeller speed <str<strong>on</strong>g>is</str<strong>on</strong>g> reduced from 120<br />
r/min to 100 r/min, corresp<strong>on</strong>ding to the c<strong>on</strong>stant<br />
ship speed coefficient ∝ = 0.28 (see definiti<strong>on</strong> of<br />
∝ in Secti<strong>on</strong> 2.02, page 2).<br />
kW<br />
9.500<br />
9.400<br />
9.300<br />
9.200<br />
9.100<br />
9.000<br />
8.900<br />
8.800<br />
8.700<br />
8.600<br />
8.500<br />
Shaft power<br />
P/D<br />
1.00<br />
<strong>MAN</strong> B&W MC/MC-C, ME/ME-GI/ME -B engines<br />
0.95<br />
0.90<br />
0.85<br />
D = Optimum propeller diameters<br />
P/D = Pitch/diameter ratio<br />
0.80<br />
0.75<br />
D<br />
7.4m<br />
7.2m<br />
0.70<br />
<strong>MAN</strong> <strong>Diesel</strong><br />
Page 1 of 2<br />
Propeller<br />
speed<br />
70 80 90 100 110 120 130 r/min<br />
Fig. 2.02.01: Influence of diameter and pitch <strong>on</strong> propeller design<br />
Once an optimum propeller diameter of maximum<br />
7.2 m has been chosen, the corresp<strong>on</strong>ding optimum<br />
pitch in th<str<strong>on</strong>g>is</str<strong>on</strong>g> point <str<strong>on</strong>g>is</str<strong>on</strong>g> given for the design<br />
speed of 14.5 knots, i.e. P/D = 0.70.<br />
However, if the optimum propeller speed of 100<br />
r/min does not suit the preferred / selected main<br />
engine speed, a change of pitch away from optimum<br />
will <strong>on</strong>ly cause a relatively small extra power<br />
demand, keeping the same maximum propeller<br />
diameter:<br />
• going from 100 to 110 r/min (P/D = 0.62) requires<br />
8,900 kW i.e. an extra power demand of 80 kW.<br />
• going from 100 to 91 r/min (P/D = 0.81) requires<br />
8,900 kW i.e. an extra power demand of 80 kW.<br />
In both cases the extra power demand <str<strong>on</strong>g>is</str<strong>on</strong>g> <strong>on</strong>ly<br />
of 0.9%, and the corresp<strong>on</strong>ding ‘equal speed<br />
curves’ are ∝ =+0.1 and ∝ =�0.1, respectively, so<br />
there <str<strong>on</strong>g>is</str<strong>on</strong>g> a certain interval of propeller speeds in<br />
which the ‘power penalty’ <str<strong>on</strong>g>is</str<strong>on</strong>g> very limited.<br />
7.0m<br />
0.65<br />
6.8m<br />
0.60<br />
6.6m<br />
0.55<br />
D<br />
P/D<br />
0.50<br />
178 47 03�2.0<br />
198 38 78�2.5
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