794 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.26 



high waves made by another vessel [Naut. Gaz., 

 Feb 1951, p. 28; MESR, Feb 1952, p. 61; MESR, 

 Dec 1952, p. 85; Mar. Eng'g., 1954, p. 12]. In 

 this case the encountered wave is superposed on 

 the ship's bow wave. Running over shoal spots 

 and meetuig the unexpected waves which often 

 come along at inopportune times can greatly 

 reduce the nominal main-hull freeboard above 

 these crests. Solid water over the large deck areas 

 at the ends can be disastrous. 



Double-ended and double-direction craft are 

 invariably fitted with double steering rudders. 

 Because of the excessive torque imposed on a 

 bow rudder if it is allowed to SAving and to take 

 part in the steeling action, to say nothing of its 

 inefficiency as a steering device, for the reasons 

 explained and illustrated in Sec. 37.11 and Fig. 

 37. G, it is preferable to lock the rudder mechanic- 

 ally at the end which happens to be the bow. 

 The steering control to that rudder is discon- 

 nected or de-energized, a centering pin is dropped 

 into the top of the rudder, and the vessel is steered 

 only with the rudder at the after or trailing end. 

 For this reason, each rudder is required to provide 

 the entire control necessary for maneuverability 

 of the vessel. 



Even though located in a propeller outflow jet, 

 as it should be, the rudder(s) of a ferryboat 

 should be relatively large, to enable it to dodge 

 traffic, to maneuver promptly in a fog, to enter 

 and leave its berth in a cross tidal current, and 

 to turn around, if need be. 



Ice guards and rope guards are often fitted 

 ahead of bow rudders and abaft stern ones. If 

 these extend continuously around the outer 

 rudder profile, from the hull above to the rudder 

 shoe below, they form effective rope guards 

 [Graemer, L., Schiffbau, 11 Oct 1911, Fig. 4, 

 p. 4, and Pis. 1-2; WRH, 15 Dec 1939, p. 381]. 

 However, if they are bent inward accidentally, 

 even only shghtly, they foul the rudder and 

 prevent its swinging. They have one advantage 

 that they create a separation zone of sorts in 

 which the larger part of the rudder blade lies, 

 so that excessive torque is not apphed continually 

 on a rudder at the leading end of the vessel. 

 Conversely, they may vibrate transversely be- 

 cause of alternate eddies shed abaft them. It is 

 therefore best to make such a guard, if fitted, a 

 sort of prolongation of the sides of the rudder 

 blade. 



There is much to be said in favor of supporting 

 a long, wide deck overhang on each side of a 



ferryboat mth a plated-in sponson. It affords 

 additional buoyancy and righting moment in an 

 emergency, it eliminates fouling of a long row 

 of strut supports by waves and foreign objects, 

 and it gives a far cleaner appearance to the 

 craft as a whole. Some notes and sketches relative 

 to these sponsons are found in Sec. 68.12 and 

 Fig. 68. K. An admirable view of the hull of the 

 ferryboat Evergreen Stale, embodying this feature, 

 is published in Diesel Times, November 1955, 

 page 3. 



The hulls of many ferryboats lend themselves 

 to the use of straight-element forms. A few of 

 this kind, to be found on ferryboats in service, 

 are illustrated in SNAME, HT, 1943, Figs. 9, 11, 

 and 20 on pages 172, 174, and 179, respectively. 



76.26 Special Problems of Icebreakers and 

 Iceships. An icebreaker is a special-service vessel 

 capable of breakmg up and making its way 

 through heavy floe ice, pack ice, and soM sea ice. 

 It makes navigable lanes for other vessels as well 

 as for itself. Indeed, it may be called upon to 

 tow other vessels through these lanes, or to push 

 on another icebreaker ahead of it when the going 

 gets particularly rough. Its primary duty, 

 involving great power on a limited length and 

 exceptional sturdiness, is such that it can carry 

 very fit tie useful load, either in weight or volume, 

 other than that required for its own services. 

 Like a tug, it is often called upon to deliver 

 maximum forward (and astern) thrust at or near 

 zero speed. This thrust, furthermore, is required 

 to overcome forces other than its own hydro- 

 dynamic resistance, just as its structure is required 

 to withstand forces other than those imposed 

 upon it in wavegoing. 



An iceship is a vessel designed and constructed 

 for, or adapted to traveling in heavily iced waters 

 without the necessity for breaking solid ice and 

 making its own water lane. It is intended to be 

 capable only of withstanding ice impact and 

 traversing an ice field which has previously been 

 broken up by an icebreaker or by natural causes 

 such as wind and swell. As a rule, the iceship is 

 of more-or-less normal form, although it may 

 have a Maier bow, intended to ride up on and 

 break through not-too-tliick ice. Its hull plating 

 is thick at the waterline belt, at least, and its 

 framing is heavily reinforced. It is, in fact, 

 designed and constructed for carrying cargo or 

 for some other primary mission. Its abiUty to 

 make its way through and to withstand not-too- 

 heavy ice is purely secondary ["Ships for Arctic 



