842 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.21 



(a) Decrease resistance at planing speeds 



(b) Deflect spray roots and random water sharply 

 off the sides 



(c) Reduce spray throwing 



(d) Provide a positive lift in the forward part of 

 the boat which helps counteract any diving 

 moment 



(e) Reduce, in round-bottom boats, the large 

 bow wave which at full speed sometimes climbs 

 high up the side toward the deck edge. The spray 

 root and spray accompanying this wave can be 

 troublesome at times. 



Fig. 77.Ja Model of Large V-Bottom Craft 



Running Without Sprat Strips 



The displacement-length quotient A/(0.010L)' for this 



hull is SO. It is being towed here at a Tq of 3.5. The spray 



root climbs up the side and the spraj' rises higher than 



the deck. 



Fig. 77.Jb Model op Fig. 77. Ja Running With 

 Spray Strips 



The spray roots and the spray are thrown to either 

 side, well clear of the hull. The speed-length quotient T, 

 is the same as for Fig. 77.Ja but the bow is lifted slightly 

 higher. 



The photographs. Figs. 77.Ja and 77. Jb, repro- 

 duced from the Ashton report referenced at the 

 beginning of this section by the permission of the 

 Experimental Towing Tank, Stevens Institute of 

 Technology, offer an excellent pictorial means of 

 comparing spray formation on a planing-huU 

 model, mth and without spray strips. There are 

 many other pairs of photographs in the Ashton 

 report which give similar comparisons for both 

 models and full-scale motorboats, and which 

 show the beneficial effect of the spray strips. 



The Appendix to the referenced ETT report 

 by R. Ashton contains design comments from 



three "highly capable and well-known designers," 

 supplemented by design sketches on page 27. 

 Fortunately for the designer, spray strips are in 

 the nature of appendages which can be adjusted 

 in form and position to produce the best effect 

 without either major or minor changes in the hull. 



77.21 Stem Shape. Up to a speed-length quo- 

 tient T, of 2.0, F„ of O.GO, the stem of a displace- 

 ment or semi-planing type of boat should be 

 nearly plumb. This takes advantage of all the 

 waterline length possible, without increasing the 

 hull weight to provide an overhang. For a higher 

 T, and F„ , the forefoot should be cut away, 

 starting above the waterline, to get the bow 

 wave under the boat as much as possible and to 

 lessen sheering in a seaway [Spooner, C. W., Jr., 

 "Speed and Power of Motorboats," unpublished 

 manuscript dated Oct 1950 (in TMB library)]. 



For pleasure craft of all three types discussed 

 in this chapter the matter of appearance is not 

 to be overlooked. There is some objection, from 

 this standpoint, to a stem which rakes downward 

 and forward under any trim or running condition. 

 To prevent this, the stem must have a rake 

 downward and aft, when at rest, of at least 6 

 deg, and possibly as much as 7 or 8 deg. 



77.22 Deep Keel and Skeg ; Other Appendages. 

 Most motorboats carry a centerhne keel that is 

 rather deep, compared to the flatness of the 

 bottom. This keel terminates aft in a skeg through 

 which passes the shaft tube and shaft for an engine 

 and propeller that happen to be located on the 

 centerline. Examples are the small planing 

 motorboat of Fig. 30.A and the larger round- 

 bottom utility boat of Sec. 77.33 and Fig. 77.T. 

 Besides acting as a support and fairing for the 

 centerline shaft the skeg serves as a deep vertical 

 fin which gives the boat dynamic stability of route 

 and facilitates steering by serving as a sort of 

 fulcrum about which the rudder moment is 

 applied. For high-speed racing motorboats this 

 skeg is reduced to a small, thin metal fin, generally 

 forward of midlength, which also serves as a 

 fulcrum about which the swinging moment 

 exerted by the rudder is applied. 



There are no known rules for positioning and 

 shaping this skeg, unless it is desired to have it 

 extend far enough below the keel to serve as 

 mechanical protection ahead of the propeller. 

 Many such skegs are included on published 

 drawings of motorboats but a designer consulting 

 these drawings rarely knows whether the skegs 

 shown in them are good or othermse. 



