Page 258 - Ship Construction.DJ Eyres 6Ed

Ch21-H8070.fm Page 247 Wednesday, October 18, 2006 6:59 AM
Aft End Structure 247
to be absorbed under towing or trawling conditions, and also when the
vessel is running freely at full revolutions and a higher speed. It is possible
to reverse the pitch in order to stop the vessel rapidly and go astern, with
the propeller shaft and propeller still rotating in the one direction.
Large controllable pitch propellers have been fitted to large diesel-driven
bulk carriers in recent years.

--- ใช้เพื่อการศึกษาเท่านั้น---
SHROUDED PROPELLERS To increase the thrust provided by a pro-
peller of given diameter at low speeds and high slips it may be enclosed in a
fixed nozzle. Single-screw tugs and trawlers are often fitted with the fixed
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
patent ‘Kort nozzle’ where under a heavy tow the propeller is working at a
high slip. This nozzle has a reducing diameter aft and is relatively short in
relation to the diameter, to avoid increasing the directional stability thus
making steering difficult.
When running freely the slip is much lower, and it might appear that
there was little advantage in fitting a shrouded propeller. However serious
consideration has been given to fitting shrouded propellers on very large
single screw ships where there is a problem of absorbing the large powers
on the limited diameter.
Some ships are fitted with a steering nozzle which not only fulfils the
ึ
purposes of a fixed nozzle but having a turning stock and fin and pivoting
about the ships longitudinal axis can be used to steer the ship.
The fabricated construction is similar for both the fixed and steering nozzles
and is shown in Figure 21.8 for a nozzle of 2.5 to 3 metres in diameter. The
inner plating of the nozzle has a heavier ‘shroud’ plate in way of the propel-
ler tips and this is carried some distance fore and aft of this position. At this
diameter the nozzle section has two full ring webs and a half-depth ring web
supporting the inner and outer plating.

Electric Podded Propulsors

In recent years a number of large cruise ships have been fitted with electric
podded propulsors. Pod propulsion has also been considered for large con-
tainer ships and fast ro-ro passenger ships.
The advantages of electric podded propulsors are –

1. lengths of propeller shafting within the hull are eliminated thus pro-
viding more revenue earning space.
2. the hull form can be designed for minimum resistance.
3. the thrusters provide good propeller clearances and can be aligned
with the local waterflow to provide a clean flow of water to the pull-
ing propeller.
4. lower noise and vibrations are claimed but it is reported that with
some installations this has not always been the case.