Abstract
The authors have developed an advanced propulsion system, called the Overlapping Propellers System (OLP), which utilizes bilge vortices to obtain high propulsive performance. A pair of bilge vortices symmetric about the hull centerline occurs around the propeller disk, which has an inward rotational component.
OLP fully utilizes the rotational component of bilge vortices because the center of each propeller is close to that of the bilge vortex and the turning direction is outward, opposite to the rotational direction of bilge vortices. In OLP, since the center of each propeller is close to the hull centerline and about half of each propeller closely overlaps, the added resistance due to the shaft bracket and so on is negligibly small. The hydrodynamic characteristics of OLP, in which the propellers are in the vicinity of each other, are especially important in designing a ship with OLP. The authors have designed OLPs for various types of ships such as large LNG carriers with various propeller inflow improving devices. Energy savings by OLP were proven to be over 10 %, compared with single screw ships.
This paper describes the hydrodynamic characteristics of the propulsive performance, the cavitation behavior, the propeller excited hull pressure pulses and the shaft bearing force variations of OLP.
