Page 183 - Technology Roadmap Transportation

TECHNOLOGY ROADMAP: TRANSPORTATION

metals (high impact resistance, durability, flexible 4.2.4 Ducted Propeller
manufacturing) with those of composites (high Efficiency of propulsion can be further increased
strength and stiffness to weight ratio, good by fitting auxiliary devices such as Wake equalizing
resistance to fatigue and corrosion). The metals duct or a Mewis duct. Ducted (shrouded)
can be of either aluminium or steel plates, propellers have some potential for very large
whereas the polymer core can be reinforced tankers. They become attractive with high thrust
with carbon or glass fibres. Successful application loading – a large mass moving at low speed – and
of these materials in the aeronautical industry since tanker speed is sensibly constant across the
and in specialized ships provides an opening for size range, then for very large units such as Ultra
introducing these materials into main shipping also. Large Crude Carriers (ULCC) ducts may show
However, widespread adoption in the near future some benefits.
is unlikely, the main constraints being high costs,
manufacturing and recycling challenges, and fire
resistance issues.

4.2.3 Optimization of propeller-hull interface, flow
devices and improvement of propulsion efficiency
Redesigning hull, rudder and propeller with
changes to ship’s aft body can improve the
interaction between these three elements and
improve propulsion and hydrodynamic efficiency.
Energy balance of a typical propeller is shown in
the following figure:

As can be seen, unfavourable wake flow from the FIG 4.5: BECKER MEWIS DUCT ON A BULK
hull into the propeller as well as energy losses CARRIER [5]
from propeller rotation reduce the potential thrust
that can be gained from a propeller to about 60%. 4.2.5 Pre and post-swirl devices
Some of the technologies that have the potential Most ships loose a substantial amount of energy
to improve ship’s propulsion efficiency such as through rotation of the propeller, imparting
appendages and supplements to propeller systems rotational rather than axial momentum on the
have been discussed in the following paragraphs. water. A large number of devices have been
Most of the technologies described, aim to proposed to recover some of this energy. These
recover part of the 40% losses. can be categorized into pre-swirl (upstream of
the propeller) and post-swirl (downstream of
FIG 4.4: PROPELLER ENERGY BALANCE[4]
the propeller) devices. They are simple, do not
FIG 4.6: PRE SWIRL DEVICE[6]

AXIAL
LOSSES
(20%)
EFFICIENCY
(60%)

FRICTIONAL
LOSSES
(15%)

ROTATIONAL
LOSSES
(5%)

WATERWAYS 181