Sec. 73.4 



FIXED-APPENDAGE DESIGN 



677 



Waterline slope at the stem nearly to zero and 

 smooths out local discontinuities at the same time. 

 In fact, shell plating may be applied to the outside 

 of a stem without a rabbet if faired by such a 

 device. The slope of a cutwater, in a horizontal 

 plane, need rarely average less than 5 deg, or 

 about 1 in 12. Usually, a slope of 6 or 7 deg, or 

 about 1 in 10 or 1 in 8, is small enough. The 

 radius at the extreme leading end may be 0.04 ft 

 or less. The waterline section at the extreme 

 nose is elliptic rather than circular. 



A design lending itself to modern fabricating 

 methods, and adaptable to a bulb bow, is sketched 

 for the ABC ship in Fig. 73. B. The space inside 

 the false stem or cutwater is filled with a light- 

 weight, water-excluding, and rust-resisting mate- 

 rial such as a foamed-in-place resin. This prevents 

 the nearly flat sides from panting under the 

 pressure variations they are likely to encounter 

 at high speed and takes care of maintenance for 

 an indefinite period. 



The sharp, "soft" cutwater is obviously not 

 adaptable to a vessel which must, during a 

 turn-around, have its nose pushed up against a 

 pier or quay. For a ship with bower anchors in 

 side hawsepipes, the cutwater is made sturdy 

 enough to withstand the pull of a chain crossing 

 the bow. 



The wetted surface of a stem cutwater is a 

 continuation of that of the main hull. It is there- 



Q545->]]- 



I 



Ur 4;o 



FP Plan Saction at 26 Designed 



Waterline 



Fig. 73. B Design of Cutwater fob the ABC Ship 



fore added to the latter as an appendage and its 

 surface taken into account when calculating 

 friction drag. Since it lies directly behind the 

 leading edge, where the local specific friction 

 resistance coefficient Clf has its highest value, 

 the cutwater surface should be exceptionally 

 smooth. 



73.4 Selection of Struts or Bossings. The 

 designer may find useful a summary of the ad- 

 vantages and disadvantages of both struts and 

 bossings, so that all phases of the selection problem 



TABLE 73. a Comparison of Design, Construction, and Operation Features of SuArr Struts and 



Bossings 



ADVANTAGES 



Struts 

 Lighter overall weight 

 Less volume and weight displacement 

 More precise alignment with flow 

 Smaller shadowing effect of appendages projecting from 



hull 

 Less overall first cost 

 Less liability of vibration due to periodic forces exerted 



on hull by propeller 



Bossings 

 Access to more shafting and shaft bearings without docking 

 Protection of shafting and bearings (except propeller 



bearing) from foreign matter, wear, corrosion, incidental 



damage, and major damage from striking piles, buoys, 



and chains 

 Some degree of pitch damping and steadying effect in 



a following sea 

 Appreciable reduction in shaft power due to deflection or 



contra-guide features, if employed 

 Greater average wake fraction at propeller positions 



DISADVANTAGES 

 Struts 

 Inadequate protection of exposed shafting from corrosion 



or from damage to corrosion-resisting coating or 



covering 

 Less protection of shaft and propeller bearings from 



foreign matter, wear, and striking large objects such 



as buoys and their mooring chains 

 Greater liability of cavitation ahead of propeller, with 



erosion and corrosion of strut arms 



Greater overall weight 



Probably greater overall first cost 



Greater hability of irregular flow abaft bossing termi- 

 nations 



Greater periodic vibratory forces exerted on hull by 

 propeller 



Reduction of maneuverabihty and turning characteristics 



