670 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 72.13 



This Portion Transverse and Vertical, 

 Main Decki^.. 1 / |ii,e q Ironsom 



Fig. 72. G Afterbody Plan of Twin-Screw 

 Vessel with Oblique Tunnels 



overall disc area of the propeller (s). All shallow- 

 draft self-propelled vessels are required to do a 

 great deal of maneuvering. Furthermore, they 

 maneuver under the handicap of sluggish response 

 because of the limited bed clearance and the 

 obstructions to under-the-bottom flow within 

 that clearance. A configuration of the bottom of 

 the hull and the tunnel roof which facilitates 

 flow from aft is therefore almost as important as a 

 configuration providing good flow from ahead. 

 The slope of the roof abaft the wheel may have 

 to be reduced to the order of 5 deg, if the backing 

 characteristics are sufficiently important. 



Contraction of the inflow jet to the propeller 

 is allowed for by flaring the sides and widening 

 the tunnel from aft forward, in much the same 

 manner as the tunnel between twin skegs is 

 widened. The same effect is achieved by pro- 

 gressively increasing the radius of the tunnel 

 roof, also from aft forward. 



It is customary to drop the elevated tunnel 

 roof abaft the propeller position until at the 

 extreme stern it meets or falls just below the 

 at-rest water level. This prevents air from being 

 drawn into the tunnel when the propeller rotates 

 astern. It also keeps debris from floating into 

 the tunnel from aft when the vessel is at rest. 

 Unfortunately, this shape also gives rise to an 

 added thrust-deduction force which detracts from 

 the propelling power, described in Sec. 25.20 and 

 earlier in the present section. The length required 

 for an easy slope abaft the propeller positions 

 may be as much as 0.15 or 0.18L; that for a 

 14-deg maximum slope forward as much as 0.23 

 to 0.25L. 



For vessels having two or more screw propellers, 

 it is customary to provide a separate tunnel for 

 each propeller. If the vessels are to run in regions 

 where there is enough bed clearance to permit a 

 good flow of water to the propellers fro7n under 

 the bottom, it appears best for the tunnel recesses 

 ahead of the propeller positions to be parallel to 



the ship centerline. The tunnels may, with 

 propeller shaft axes, even diverge slightly with 

 distance aft. It often happens, on the other 

 hand, that the bed clearance is extremely small, 

 so small in fact that the propellers can not be 

 fed adequately with water from underneath. 

 One solution is to draw it in from the sides, from 

 the open regions abreast the hull, using oblique, 

 converging recesses that are tunnels in name only. 

 These oblique tunnels were developed at least 

 as early as 1938 and were said to have performed 

 well in service, when the conditions were favor- 

 able to their use. 



Figs. 72. G and 72. H, adapted from model fines 

 published by A. R. Mitchell ["Tunnel Type 

 Vessels," lESS, 1952-1953, Vol. 96, Fig. 9, p. 141], 

 illustrate one form of oblique tunnel for a twin- 

 screw shallow-draft vessel. This proved its 

 superiority in self-propelled model tests over a 

 vessel with parallel tunnels, when run in shallow 

 water. Fig. 24 on page 165 of the lESS reference 

 is a photograph of half the afterbody of a model 

 built to this design. The same photograph is 

 reproduced in INA, July 1952, Fig. 10, facing 

 page 148. Results of the tests are described 

 rather fully on pages 143 through 150 of the 

 Mitchell reference. 



For a tunnel-stern towboat, built without 

 oblique tunnels, on which the flow to the pro- 

 pellers was inadequate because of limited bed 

 clearance under the hull in shallow water, L. A. 

 Baier and J. Ormondroyd effected a solution by 

 building a large-diameter duct through the hull 

 for each propeller. This duct took in water for- 

 ward, above the baseplane, and discharged it 

 downward and aft through the tunnel roof ahead 

 of the propeller [Third Midwestern Conf. on 

 Fluid Mech., Univ. of Minn., Jun 1953, pp. 406, 

 411]. 



H. Waas shows a twin-screw shallow-draft 

 river craft with propellers far outboard, each 

 housed in a tunnel that fits the tip circles closely 

 but is of limited circumferential extent. A partly 

 underhung, balanced rudder is fitted abaft the 

 skeg endings just outboard of the wheels. The 

 propellers are carried by struts forward of them 

 [STG, 1952, Figs. 16, 17, p. 213]. 



If the exposed positions of the blade tips of 

 the propellers can be accepted, there is no reason 

 why the tunnel in which they work can not extend 

 abaft the propeller in a direction nearly hori- 

 zontal. The elevated portion of the outflow jet, 

 acted upon by gravity forces, falls at a certain 



