Page 250 - Ship Construction.DJ Eyres 6Ed

Ch21-H8070.fm Page 239 Wednesday, October 18, 2006 6:59 AM
Aft End Structure 239
Rudders

Many of the rudders which are found on present-day ships are semibalanced,
i.e. they have a small proportion of their lateral area forward of the turning
axis (less than 20 per cent). Balanced rudders with a larger area forward of
the axis (25 to 30 per cent), and un-balanced rudders with the full area aft
of the axis are also fitted. The object of balance is to achieve a reduction in
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torque since the centre of lateral pressure is brought nearer the turning
axis. However the fully balanced rudder will at low angles tend to drive the
gear, which does not matter a great deal with power steering gears but is
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
less satisfactory with any form of direct hand gear.
Designs of rudders are various, and patent types are available, all of which
claim increased efficiencies of one form or another. Two common forms of
rudder are shown in Figure 21.4 each being associated with one of the stern
frames shown in Figure 21.3.

RUDDER CONSTRUCTION Modern rudders are of streamlined form
except those on small vessels, and are fabricated from steel plate, the plate
sides being stiffened by internal webs. Where the rudder is fully fabricated,
one side plate is prepared and the vertical and horizontal stiffening webs
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are welded to this plate. The other plate, often called the ‘closing plate’, is
then welded to the internal framing from the exterior only. This may be
achieved by welding flat bars to the webs prior to fitting the closing plate,
and then slot welding the plate as shown in Figure 21.4. Other rudders may
have a cast frame and webs with welded side and closing plates which are
also shown in Figure 21.4.
Minor features of the rudders are the provision of a drain hole at the bot-
tom with a plug, and a lifting hole which can take the form of a short piece
of tube welded through the rudder with doubling at the side and closing
plates. To prevent internal corrosion the interior surfaces are suitably
coated, and in some cases the rudder may be filled with an inert plastic
foam. The rudder is tested when complete under a head of water 2.45 m
above the top of the rudder.

RUDDER PINTLES Pintles on which the rudder turns in the gudgeons
have a taper on the radius, and a bearing length which exceeds the
diameter. Older ships may have a brass or bronze liner shrunk on the pin-
tles which turn in lignum vitae (hardwood) bearings fitted in the gudgeons.
Modern practice is to use synthetic materials or composites for the
bearings, and in some cases stainless steels for the liners. In either case
lubrication of the bearing is provided by the water in which it is immersed.
For many years it has not been found practicable to provide oil-lubricated
metal bearings for the pintles, but a number of larger ships now have this
innovation.