Fig 2-1
Operating field for CP Propeller (DAAF007339)
2.2.2
Fixed pitch propellers
The thrust and power absorption of a given fixed pitch propeller is determined by the relation
between ship speed and propeller revolution speed. The power absorption during acceleration,
manoeuvring or towing is considerably higher than during free sailing for the same revolution
speed. Increased ship resistance, for reason or another, reduces the ship speed, which
increases the power absorption of the propeller over the whole operating range.
Loading conditions, weather conditions, ice conditions, fouling of hull, shallow water, and
manoeuvring requirements must be carefully considered, when matching a fixed pitch propeller
to the engine. The nominal propeller curve shown in the diagram must not be exceeded in
service, except temporarily during acceleration and manoeuvring. A fixed pitch propeller for
a free sailing ship is therefore dimensioned so that it absorbs max. 85% of the engine output
at nominal engine speed during trial with loaded ship. Typically this corresponds to about
82% for the propeller itself.
If the vessel is intended for towing, the propeller is dimensioned to absorb 95% of the engine
power at nominal engine speed in bollard pull or towing condition. It is allowed to increase
the engine speed to 101.7% in order to reach 100% MCR during bollard pull.
A shaft brake should be used to enable faster reversing and shorter stopping distance (crash
stop). The ship speed at which the propeller can be engaged in reverse direction is still limited
by the windmilling torque of the propeller and the torque capability of the engine at low
revolution speed.
2-2
Wärtsilä 20 Product Guide - a13 - 6 September 2016
Wärtsilä 20 Product Guide
2. Operating Ranges
Содержание WARTSILA 20
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