Line of Top of Roof or 

 Crown of Tunnel 



Line, of Bottom of Hull 



Outboard of Tunnel 



OUTBOARD PROFILE 



The Afterbody Plon of This Vessel is Shown on Fio. 72.& 



FISH-EYE VIEW, STARBOARD SIDE 



L<£, 



II 10.5 10 9.5 9 Stations 



Fig. 72.H Outboard Profile and Fish-Eye View of Vessel with an Oblique Tunnel 



rate; in addition, the jet contracts aft. The top 

 of the tunnel can be bent downward shghtly to 

 take care of these two effects. It is easy for the 

 propeller, when rotating ahead and accelerating 

 the vessel in that direction, to sweep the air out 

 of the tunnel and to fill it completely with water. 

 However, keeping debris out of the propeller and 

 keeping air out of the disc when going astern, as 

 mentioned previously, require a closure for the 

 after end. Indeed, if the after end is not closed 

 in some way the craft may not go astern at all. 

 Making this closure by dropping the tunnel roof 

 is not always the best solution, especially when 

 there are large changes of draft aft. This difficulty 

 is overcome by a simple yet effective hinged flap, 

 introduced by A. F. Yarrow in the early 1900's, 

 which may be lowered to close the after end of 

 the tunnel. Fig. 72.1 shows schematically the 

 arrangement of a device of this kind. 



The flap forms the upper part of the tunnel 

 ending, either close abaft the propeller or at a 

 short distance from it. The raising and lowering 

 may be done mechanically or automatically; in 

 the latter case the force exerted by the outflow 

 jet holds the flap at the proper angle. The flap 

 is sealed along its sides. Its lower edge is a fraction 

 of a foot below the at-rest waterline, with the 

 vessel in the light condition, so as to exclude air 



when the propeller is starting. When underway 

 the flap levels out; its after end usually rises above 

 the waterline. A good tunnel seal is maintained 

 in all conditions of loading. The resistance is 

 lowered because of the reduced inclination of the 

 tunnel roof abaft the propeller. Going astern, the 

 automatic flap is forced down onto a sill, which 

 is set at the lowest point of flap travel. This 

 maintains the seal in the tunnel and the propeller 

 continues to work in solid water [Mitchell, A. R., 

 lESS, 1952-1953, Vol. 96, p. 183]. 



When the change in draft aft is small, for 

 various service conditions, tunnel endings with 



5tern of Vessel 



-Flop in Roise^-^Position, as when Underway I 



-Ap proximate Woterline L 



Lme of Hull, Ootboard \ 

 of Tunnel 



Vertical Curtoin Plates 

 on Each Side of I 



Tunnel, Between i 



which the Flop Propeller ' 

 F,ts Neatly ^— H 



Flop in Lowered Position 

 03 when Stortinq or 

 Backinq 



Fig. 72.1 Arrangement Sketch op a Hinged Flap 

 Closing the After End of a Propeller Tunnel 



