Scr. 72.13 



DESIGN FOR CONFINED WATERS 



669 



stern, with its lower surface lying very slightly 

 below the DWL, and with a small tip clearance 

 under this surface. N. G. Herreshoff and others 

 built many successful craft to this design. 



(c) Use surface propellers for extremely small 

 draftSi This scheme, of course, does not increase 

 the thrust-producing area in proportion to the 

 increase in diameter. 



(d) Employ a tunnel-stern design, with at-rest 

 tip submergences ranging from a small positive 

 to a large negative value. 



72.13 The Design of a Tunnel Stern. The 

 tops or roofs of the tunnels described in Sec. 25.20 

 and diagrammed in Fig. 25. M may lie below the 

 designed waterline or extend above it. The 

 design rules given here apply generally to a tunnel 

 whose roof extends above the DWL. 



When laying out a tunnel stern, whether for 

 one or for multiple screw propellers, it is first 

 decided how much of the propeller disc and the 

 upper blades can be out of water. It is believed 

 that a tunnel system can be designed to function 

 properly even if the shaft axis has above the DWL, 

 as for a surface propeller. This extreme may be 

 considered necessary to permit removing a 

 propeller through an access hatch above, provided 

 the vessel can not be trimmed by the bow for 

 this purpose. However, it is preferable to place 

 the axis at least O.IOD below the DWL, where D 

 is the propeller diameter. This keeps the propeller 

 bearing always lubricated (if this is a practical 

 item), keeps the tunnel entirely full of water, 

 and prevents cutting too much out of the hull 

 for the tunnel slopes forward and aft. 



The hull tip clearances need be only large 

 enough to insure against mechanical rubbing 

 under all conceivable circumstances and to pass 

 any foreign material that may be in the water 

 without jamming it between the propeller tips 

 and the hull. A tip clearance of 0.04 times the 

 propeller diameter appears to be ample, both 

 mechanically and hydrodynamically. The small 

 tip clearances necessary to insure that the highest 

 part of the tunnel runs full of water may be a 

 partial insurance against excessive tip-vortex 

 losses, particularly when the shp ratio is large. 



The next major step is to determine the maxi- 

 mum permissible fore-and-aft slope of the tunnel 

 top, forward of the wheel. Although described in 

 Sec. 25.20, it is well to emphasize here the effect 

 on propulsion of the roof slopes, both forward of 

 and abaft the wheel. In the words of a renowned 



designer and builder of tliose craft, A. F. Yarrow 

 ["The Screw as a Means of Propulsion for Shallow 

 Draught Vessels," INA, 1903, p. 107]: 



"There will be an increased resistance to the forward 

 motion of the vessel, due to the action of the screw in 

 reducing the pressure of water at the inclined part of 

 the tunnel forward of the propeller, and this increased 

 resistance is common, more or less, to all screw ships, 

 but it is probably proportionately greater in this class 

 of vessel than in those where the propeller in in the usual 

 position. There is also a loss of efficiency due to the resist- 

 ance of the inclined surface of the tunnel aft of the pro- 

 peller." 



The region of maximum slope, at about half 

 height of the tunnel, is usually not far below the 

 DWL, where the hydrostatic pressure is small. 

 Good design to prevent separation calls for a 

 maximum roof slope, in a vertical plane through 

 the shaft, not exceeding 14 or 15 deg. A. R. 

 Mitchell recommends a limit of 12 deg in fast 

 vessels and 15 deg in slow ones [INA, Jul 1952, 

 p. 148]. This is on the basis of a negligible change 

 of trim when underway. 



However, limiting the roof slope to avoid 

 separation is only part of the story, on the basis 

 that propulsive efficiency is a design factor of 

 sufficient importance so that it can not be dis- 

 regarded completely. A value of i7p(eta) superior 

 to those achieved in the past, even though it is 

 not comparable to that of a large deep-water 

 vessel, is possible only by rather drastic levehng 

 of the tunnel-roof slopes, both forward of and 

 abaft the propeller position. These may have to 

 be of the order of 6 to 10 deg, instead of 12 or 15 

 to 18 deg. The Hillman design of Figs. 72.E 

 and 72. F achieves this and more. Moving the 

 tunnel boundaries farther forward of and abaft 

 the propeller position reduces the displacement 

 volume aft so that the stern portion of the hull 

 becomes not much more than a cover over the 

 propeller inflow and outflow jets. 



There are major structural problems involved in 

 stiffening and supporting a long stern overhang 

 with a buoyancy that is small compared with its 

 size and weight. One solution is to raise the deck 

 aft and increase the girder depth, as was done by 

 the Dravo Corporation for the 200-ft pushboats 

 A. D. Haynes II and Valley Transporter [Mari- 

 time Reporter, 15 Dec 1955, p. 11]. 



Another important reason for small fore-and-aft 

 slopes in the roof of a tunnel oyer a screw pro- 

 peller is to provide as great an astern thrust as 

 possible with a given shaft power and a given 



