Sec. 61.3] 



UNDERWATER-HULL DESIGN 



545 



propeller, or other causes. On the other hand it 

 may be worse because of increased drag due to 

 separation behind fuller waterlines at the stern. 



If the keel drag is limited to small amounts it 

 may easily become more of a nuisance in laying 

 out or building the ship than a help in its service 

 performance. 



67.30 Underwater Exhaust for Propelling 

 Machinery. A passenger-carrying vessel having 

 propelling machinery that produces gaseous 

 products of combustion and that is intended to 

 run at all times with the propeller(s) fully sub- 

 merged offers a favorable opportunity for installa- 

 tion of underwater gas exhaust. This is especially 

 true if the gas-producing portion of the propelling 

 machinery can be placed close to the stern, and 

 if the draft aft is reasonably constant. 



On the basis that an alternative vertical gas 

 outlet is provided in the form of a tall post or 

 slender stack, to care for lighting off, port opera- 

 tion, low-speed running, maneuvering, and emer- 

 gencies, it may be assumed that the duct area 

 required for underwater exhaust need be no 

 greater than that for vertical exhaust at full 

 power. This takes no account of the possible 

 condensation of steam in the exhaust gases or the 

 use of stack-gas coohng as a margin against too 

 high velocity of the combustion gases in the 

 ducts leading to the underwater stern outlet. 



To insure that the gases are discharged into 

 the separation zone deliberately formed abaft 

 the stern, within the variations in draft which 

 will occur there, a high-level as well as a low-level 

 gas outlet is required. Both may be connected 

 to the gas discharge lines which should enter the 

 high-level outlet box from the top, to prevent 

 entry of sea water back into the gas line. Escape 

 of gas from the high-level outlet when running at 

 the shallower draft at the stern is prevented by a 

 combined gravity- and buoyancy-operated flap 

 valve which closes when it is above the waterline 

 but opens when it is submerged. 



On the basis of limiting slopes of 15 deg for 

 separation at the light- and deep-draft waterlines 

 of a vessel of about the size of the ABC ship but 

 having a canoe stern, the separation zone is 



estimated to extend from the AP to points at 

 least 20 ft forward on either side. Assuming a 

 depth of 2 ft for each outlet screen, one-third of 

 the length of the separation zone on the two 

 sides is ample to provide the necessary outlet area. 

 The estimated back pressure is of the order of 

 zero, or it may be slightly negative, due to the 

 — Ap in the separation zone. Further model and 

 full-scale experiments are necessary to verify 

 this point. 



When underway in smooth water the stern-wave 

 crest may rise above the at-rest designed water- 

 Une by an amount estimated as from 2.5 ft to 

 4.0 ft, depending upon the stern shape and other 

 circumstances. It may be necessary to provide 

 several levels in the outlet boxes so that one may 

 be selected which best suits any given operating 

 condition. 



When the ship is pitching in waves so that the 

 selected underwater gas outlet for the mean draft 

 is alternately submerged and exposed, it may be 

 necessary to lock the flap valves closed in the 

 outlet boxes and to resort to vertical stack-gas 

 discharge. 



67.31 General Notes on Water Flow as 

 Applied to Hull Design. At the risk of boring 

 the reader with duplication it can not be too 

 strongly recommended that the flow pattern and 

 the wake diagram at the positions proposed for 

 any type of propulsion device be adequately 

 investigated and recorded on a model by chemical 

 or physical means, by strings or tufts, and by 

 spherical-ended pitot tube. These data should be 

 given great weight when finally fixing the form 

 of the ship and its appendages adjacent to the 

 propeller positions and when establishing the 

 propeller clearances. 



It may be taken as an axiom, in the detail 

 design of the underwater hulls of ships and their 

 appendages, that separation and cavitation and 

 all other forms of discontinuity in a liquid, 

 whenever and wherever occurring, are detri- 

 mental to good performance. As such they are 

 to be carefully and systematically minimized or 

 avoided altogether. 



