Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



shaft, the stern tube shaft, and the propeller 

 or tail shaft. These portions of the shafting may 

 be seen in figure 5-15. 



Segments of the line shaft and the thrust 

 shaft are joined together with integral flange- 

 type couplings. The stern tube shaft is joined to 

 the after end of the line shaft with an inboard 

 stern tube coupling which has a removable after- 

 sleeve flange. The tail shaft is joined to the 

 stern tube shaft by a muff-type outboard coup- 

 ling. 



On single-screw ships, the portion of the out- 

 board shaft which turns in the stern tube bearing 

 is normally covered with a shrunk-on composi- 

 tion sleeve. This is done to protect the shaft 

 from corrosion and to provide a suitable journal 

 for the water-lubricated bearings. On multiple- 

 screw ships, these sleeves normally cover only 

 the bearing areas; on such ships, the exposed 

 shafting between the sleeves is covered with 

 synthetic sheet rubber to protect the shafting 

 from sea water corrosion. 



On carriers and cruisers, the wet shafting— 

 that is, the shafting outboard in the sea— is 

 composed of three sections: a tail shaft, an 

 intermediate or dropout section, and a stern 

 tube section. Integral flanged ends of these sec- 

 tions are usually used for joining the sections 

 together. 



Circular steel or composition shields known 

 as fairwaters are secured to the bearing bush- 

 ings of the stern tube and strut bearings and to 

 both the forward and the after ends of the under- 



water outboard couplings. These are intended 

 primarily to reduce underwater resistance. The 

 coupling fairwaters are secured to both the shaft 

 and coupling flanges and are filled with tallow 

 to protect the coupling from corrosion. 



REDUCTION GEARS 



Reduction gears are used in many propulsion 

 plants to allow both the prime mover and the 

 propeller to operate at the most efficient speed. 

 Reduction gears are also used in many kinds of 

 auxiliary machinery, where they serve the same 

 purpose. Some of the gear forms commonly used 

 in shipboard machinery are shown in figure 

 5-16. 



Reduction gears are classified by the number 

 of steps used to bring about speed reduction and 

 by the general arrangement of the gearing. A 

 single reduction gear consists of a small pinion 

 gear which is driven by the turbine shaft and a 

 large main gear (or bull gear) which is driven 

 by the pinion. In this type of arrangement, the 

 ratio of speed reduction is proportional to the 

 diameters of the pinion and the bull gear. In a 

 2 to 1 single reduction gear, for example, the 

 diameter of the driven gear is twice that of the 

 driving pinion. In a 10 to 1 single reduction gear, 

 the diameter of the driven gear is ten times 

 that of the pinion. 



All main reduction gearing in current com- 

 batant ships makes use of double helical gears 

 (sometimes referred to as herringbone gears). 



STRUT BUmNE 



F«IIIW«TER 

 , Sl[E»ES 



AFTER STERN TUBE 

 BEARINC 



BU|i GEAR SHAFT 

 (THRUST SHAH) 



PORT MAIN REDUCTION 



GEAR 



MAIN THRUST 

 BEARING 



AFTER ENGINE ROOM 



47.42B 



Figure 5-15.— Propulsion shafting, twin-screw ship. 



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