556 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 6S.10 



its whole length. Only one such impact is necessary 

 to tear loose a breakwater that is not sufficiently 

 strong or sturdy. It appears unwise, therefore, 

 to use for the design any striking velocity less than 

 the sustained speed V plus the orbital velocity 



f/o.b. 



The force on the breakwater, exerted parallel 

 to the deck, may be taken as that developed by 

 a uniformly distributed ram pressure q = 0.5p 

 (F + Uorh)' acting over the whole area of the 

 face. When an obstruction of this kind deflects 

 onrushing water back upon itself there is usually 

 a doubling of the impact load. This is compensated 

 for in the present instance by the fact that a 

 stream of water only half as high as the break- 

 water can be reversed in direction by the action 

 of the deflectors shown in Fig. 68. E. A deeper 

 layer of water is only deflected upward, with a 

 ram pressure corresponding to (F + f/orb) and a 

 force on the breakwater that is not doubled. 

 Fitting the breakwater at an angle of from 45 

 to 60 deg to the ship centerline, indicated in the 

 figure, deflects water outward and helps to 

 reduce the impact load on the structure for seas 

 coming from directly ahead. 



For the ABC design, it may be assumed that 

 the ship can maintain 18.7 kt in a sea made up 

 of regular waves 800 ft long with an angle of 

 encounter a of 180 deg. If these waves have a 

 steepness ratio as great as 1/20 the orbital velocity 

 in the crests is, from Table 48. e, approximately 

 10 ft per sec. The nominal striking velocity is 

 then 31.6 ft per sec (equivalent to 18.7 kt) plus 

 10 ft per sec or say 42 ft per sec. Taking a round 

 value of 1.00 for 0.5p in salt water, the ram pres- 

 sure is approximately 1.00(42)" or 1,764 lb perft^. 

 This is just over 12 lb per in'. The ultimate-load 

 factor for an installation of this kind, where it is 

 particularly important that it not be torn loose or 

 that leaks should not be started in the forecastle 

 deck, should be at least 5 times the calculated load. 



If it is desired to expend only a moderate 

 amount of weight on a breakwater, the generation 

 of excessive ram and dynamic pressures on it is 

 prevented by cutting holes in it. These are of 

 moderate size, at about midheight. The holes 

 permit the breakwater to catch and shed small 

 amounts of water but relieve the load on it when 

 large quantities of solid water come rushing 

 against the breakwater structure [SBMEB, Jan 

 1954, p. 43], In this respect it resembles the dive 

 brakes of certain airplanes, in the form of flaps 

 rather well perforated with holes. 



68.10 Design of Anchor Recesses. Protrud- 

 ing stockless anchors are unsightly and they are 

 abominable spray-throwers. They represent defi- 

 nite collision hazards in that such an anchor can 

 tear open the whole side of another ship, when 

 the bumping or sliding damage might otherwise 

 be slight. 



It is reported that on the German World 

 War II battleships Bismarck and Tirpitz the 

 anchors were hauled, not into orthodox external 

 hawsepipes or anchor recesses but up onto the 

 main deck. There they lay flat, suitably secured. 

 When it was desired to anchor, they were appar- 

 ently pushed over the side by mechanical gear. 

 This left the sides of the bow entirely clear of 

 any major projections or recesses, and eliminated 

 the throwing of spray from that cause. 



On the German World War II cruiser Prinz 



Fig. 68.F Anchor Recess as Used on Great 

 Lakes Freighters 



