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 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

Power Stern
Figure 21.3 Stern frames

gear, which does not matter a great deal with power steering gears but is 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 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 bottom 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 pintles which turn in lignum vitae (hardwood) bearings fitted in the gudgeons. Modern practice is to use synthetic materials like 'Tufnol' 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. Until recently it has not been found practicable to provide oil-lubricated metal bearings for the pintles, but Queen Elizabeth 2 has this innovation.

rudder stock A rudder stock may be of cast or forged steel, and its diameter is determined in accordance with the torque and any bending moment it is to withstand. At its lower end it is connected to the rudder by a horizontal or vertical bolted coupling, the bolts having a cross-sectional area which is adequate to withstand the torque applied to the stock. This coupling enables the rudder to be lifted from the pintles for inspection and service.

Rudder stock

Rudder stock

Bolted coupling bearing

Side plates welded to cast frame

Rudder stock

Rudder stock

Bolted coupling

Side plates welded to cast frame

Dimensions Ship Rudder

bearing

Side plates are slot welded to tube

Figure 21.4 Rudders

Side plates are slot welded to tube

Figure 21.4 Rudders rudder bearing The weight of the rudder may be carried partly by the lower pintle and partly by a rudder bearer within the hull. In some rudder types, for example, the spade type which is only supported within the hull, the full weight is borne by the bearer. A rudder bearer may incorporate the watertight gland fitted at the upper end of the rudder trunk as shown in Figure 21.5. Most of the rudder's weight may come onto the bearer if excessive wear down of the lower pintle occurs, and the bearers illustrated have cast iron cones which limit their wear down.

RUDDER TRUNK Rudder stocks are carried in the rudder trunk, which as a rule is not made watertight at its lower end, but a watertight gland is fitted at the top of the trunk where the stock enters the intact hull (Figure 21.5). This trunk is kept reasonably short so that the stock has a minimum unsupported length, and may be constructed of plates welded in a box form with the transom floor forming its forward end. A small opening with watertight cover may be provided in one side of the trunk which allows inspection of the stock from inside the hull in an emergency.

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Responses

  • RITA
    What is the minor features of the rudder?
    3 years ago
  • Paula
    How to fabricated modern rudder?
    3 years ago
  • duncan
    Why drain hole is provided on the rudder?
    3 years ago
  • Tim Gottschalk
    What in ship construction pintle?
    3 years ago
  • tahvo kankaanper
    How rudder frame constructed?
    2 years ago

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