Propulsion Systems - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 126

blades. In reality, there is a practical limit to seven blades that is used on the
biggest vessels such as container ships. Smaller ships have usually much fewer
numbers of blades [ 26 ].
Ventilation and cavitation appear if air is vacuumed by the water surface or
waste gases are vacuumed from the exhaust gas flow into the propeller, especially
in fast sporting boats and in small, fast marine ships with exhaust gas end pipe at a
side in the water, e.g., in stealth ships. This process reduces the performance of the
propeller, and the thrust decreases. The reason for this is the decreased boiling of
water if the pressure is low enough. In this case, steam formation starts at lower
temperatures than 100C (212F). The high propeller speed can decrease the water
pressure at the back surface of the propeller's blade and steam bubbles can appear
which could cause mechanical damage. Whirls at the blades formed both in air and
in water lead to perforations in the propeller's material [ 27 ].
The suitable measures against cavitation are:
• Slow steaming; and
• Sub-water coating.
New coatings such as finely powdered Polyamide-11 perform better than metals,
including stainless steel, and have exceptional resistance to cavitation erosion [ 28 ].
Ships like tugboats and icebreakers which must have a very high thrust require
a propeller with a nozzle, a large number of blades and a highly skewed design.
The Voith-Schneider propeller (VSP) can adjust the size and the direction of
thrust in a broad range without changing the speed [ 29 ].
8.4.2 New Propeller Technology
Important examples of new propeller systems are:
• Large propellers;
• Carbon fiber propellers;
• Pod propulsion systems;
• Linear jets; and
• Ring propeller driving and maneuvering systems.
8.4.2.1 Large Propellers
Large ship propellers are up to 12 m in diameter (39.3 ft) and have a weight
between 80 and 120 t (176,211 and 264,317 lb). They are produced with efficient
casting methods. The application field attaches large diesel engines in the range
from 50,000 kW (67,114 HP) to 75,000 kW (100,671 HP). The technological
parameters range from a relatively low number of revolutions between 60 and
125 rpm to a directly coupled transmission without a gearbox and with high torque.