778 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.14 



Each of these pipes has a scraper of some sort 

 at its lower end and a large mouth adjacent to 

 it through which the bed material is carried by 

 suction, suspended in a powerful stream of water. 

 At its upper end, possibly also at other points 

 along its length, the drag pipe is hinged or swiveled 

 about a horizontal, transverse axis. The scraper 

 and the pipe may thus be lowered to the desired 

 depth when digging or drawn up above the 

 baseplane (and the waterplane) when the dredge 

 is running free. 



Self-propelled dredges may have two drag 

 pipes, one outboard on either side, or a single 

 pipe, lowered through a long center well. A 

 diesel-electric twin-screw hopper dredge of the 

 latter type, having a length of 290 ft and a beam 

 of 58 ft, is illustrated in the technical hterature 

 [SBSR, 8 May 1952, p. 48; also 9 Apr 1953, p. 42]. 



Most large model-basin estabUshments have 

 tested models of self-propelled dredges (for ex- 

 ample, TMB models 3132, 3175, 3597, 3633, and 

 3779), and some of them have made resistance 

 tests of drag-pipe assemblies at various attitudes 

 and speeds. Unfortunately, however, much of 

 this information is so speciahzed that it has not 

 found its way into the technical and reference 

 hterature. The drag encountered by the scraper 

 units depends upon the nature of the bottom, 

 the depth of cut being made, and other factors. 



The ahead resistance and the lateral forces 

 occasioned by the mouth of the drag pipe when 

 scraping on the bottom, and the hydrodynamic 

 resistance of the inchned drag pipe(s) moving 

 ahead through the water, are not exactly com- 

 parable to the towhne tension on a tug or to the 

 wind force on a sailboat. Nevertheless, the drag- 

 pipe resistances and scraper forces caU for 

 increased propeller thrust and they may interfere 

 with steering. 



A self-propelled dredge usually scrapes or 

 dredges in shallow water, with a small bed clear- 

 ance. Furthermore, it necessarily scrapes at slow 

 speed, so that more than the usual rudder effect 

 is required to keep it under control and moving in 

 the desired lanes. This means that the rudder(s) 

 must be of greater-than-normal blade area and 

 that they must be placed in the outflow jets of 

 propellers, to provide the necessary lateral forces 

 at low speeds. 



Aside from the ground and hydrodynamic 

 resistances of the drag pipes and their gear, the 

 principal resistance components pecuUar to self- 

 propelled dredges are the pressure drags developed 



by the discontinuities in the bottom, in the form 

 of large recesses under the hopper doors. If the 

 doors, opening downward, are not to swing below 

 the baseplane the recesses are unavoidably deep. 

 The greater the hopper capacity of the dredge the 

 more recesses there are. 



Hopper-door recesses in the bottoms of self- 

 propelled dredges of a half-century ago are illus- 

 trated by T. M. Cornbrooks [SNAME, 1908, PI. 

 140], where one port and one starboard row is 

 indicated. Each recess is 4.42 ft wide and about 

 3 ft deep vertically. There are 6-in by 6-in by 

 5/8-in angles all around the lower edge of each 

 recess, projecting both into the recess and below 

 the bottom of the ship. The two rows of recesses 

 are shghtly over 6 ft apart, measured to their 

 inboard edges. Recesses in later designs of dredges 

 are similar, indicated by the hnes drawings of 

 SNAME RD sheet 103 and of ETT Stevens 

 Technical Memorandum 100. 



The whole subject of hopper-door recesses in 

 the bottom is discussed in Chap. VIII, pages 

 213-250, of the Scheffauer reference hsted near 

 the end of this section. Several conical dump 

 valves of a new type are illustrated in this refer- 

 ence. They practically ehminate the large hopper- 

 door recesses and the hydrodynamic resistance 

 generated by them. 



Most self-propelled suction dredges, with their 

 limited draft and large underwater volume, have 

 large B/H ratios. That of the 247-ft hopper dredge 

 described in SNAME RD sheet 103 is 3.08, at 

 load draft. For other vessels of this type it is 

 4.5 or more. At reduced draft, as when returning 

 from the dumping ground to the dredging area, 

 the B/H ratios are considerably larger. 



This type of hull definitely calls for a twin- or 

 multiple-skeg stern [SNAME, 1947, pp. 97-169]. 

 It is much easier to shape a twin-screw stern of 

 large beam and small draft with double skegs than 

 with a normal form. Further, there is every 

 reason to anticipate a better flow around the 

 twin skegs, although the resistance may suffer 

 because of the increased wetted area. Certainly it 

 is far simpler to hang twin rudders abaft twin 

 skegs carrying propellers than to mount them 

 under a normal-form stern of generally V-shape. 



As an indication of the percentage of time during 

 which a self-propelled seagoing suction dredge is 

 running to and from the dumping grounds, T. M. 

 Cornbrooks shows that for a working period 

 of about 7.5 days, during which time the dredge 

 made 44 complete round trips, it was outward 



