836 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.16 



Some Nominal Rise-of-Roor Annie j£ For All 



-Hr^Holf-Sidinq 



'^Possible Modification 



Fig. 77. G Four Types of Typical Forebody 

 Sections for Planing Craft 



rough water there is heavy pounding and slam- 

 ming, accompanied by either a reduction in speed 

 or a risk of damage to the boat and injury to the 

 personnel. 



Better performance in rough water is obtained 

 by raising the chine forward and increasing the 

 rise of floor but this change increases the resistance 

 and involves a sacrifice of smooth-water speed. 

 Running into a wind, the concave forward section 

 keeps the boat dry because the water is thrown 

 to the side with little spray and disturbance. In 

 heavy weather, entrance sections which are fine 

 as well as concave result in heavy pitching. They 

 give little reserve buoyancy and they are still 

 subject to pounding and slamming. 



Boats with convex sections, sketched at 4 in 

 Fig. 77. G, and of a type to be found in developable 

 bottoms, have higher smooth-water resistances 

 than those with concave sections. The water 

 climbs up the side more easily and does not 

 separate cleanly from the chine, resulting in 

 greater wetted area. This type of section is much 

 better for wavegoing because, like the round- 

 bottom form, it presents a constantly decreasing 

 flare to the oncoming (and upcoming) water as 

 the bow pitches down. Pounding and slamming are 

 reduced considerably or avoided altogether. The 

 convex section builds up reserve buoyancy and 

 acts to ease the pitching motion. As might be 

 expected, boats with convex sections will not 

 plane as rapidly or as readily as those with concave 

 sections. 



Convex sections throw considerably more spray 

 and make the boat wetter in a wind. The fitting 

 of spray strips, described in Sec. 77.20, largely 

 overcomes these disadvantages. Convex sections 

 forward give more internal volume than concave 



sections. This is often an important design 

 consideration. 



Straight entrance sections, depicted at 1 in 

 Fig. 77. F, have many of the disadvantages of both 

 the concave and convex types with few of their 

 advantages. Straight segments are often used in 

 sections near the stern where the rise of floor is a 

 minimum and there is little difference between 

 the various section types. In regions of high 

 rise-of-floor angle, they are used only for ease of 

 construction. 



The yoke or inverted-bell section, drawn at 3 

 in the figure, offers a good compromise. It gives 

 the excellent wavegoing performance of the 

 round-bottom entrance but retains the sharp 

 angle at the chine to throw the water off cleanly, 

 thus holding down the wetted area. It provides 

 adequate reserve buoyancy, an ample rise of 

 floor, and a constantly diminishing flare with 

 vertical distance above the base, to prevent 

 slamming and pounding. By using a high chine 

 forward, the entrance sections are kept fine 

 enough to avoid too great a hook at the chine 

 corner where water could be trapped in a slam. 

 The rounded portion near the centerline acts to 

 deflect the water under the bottom and give 

 reasonably quick planing with good flow lines. 

 Bell-shaped sections produce less spray than the 

 pure convex type. They have the disadvantage 

 that they are more difficult to construct, they 

 involve slightly higher building costs, and they 

 can not be worked into developable surfaces. 

 They have slightly greater resistance in smooth 

 water than the low-chine concave sections, due 

 to the greater wetted surface, but probably less 

 resistance than pure convex sections. 



77.16 Rise-of -Floor Magnitude and Variation. 

 The rise of floor or deadrise, expressed as a section- 

 slope angle ;8(beta), is an important characteristic 

 of a planing boat. In fact, its choice influences 

 many other features of the underwater and above- 

 water hulls. From pure planing and dynamic-lift 

 considerations the smaller the rise-of-floor angle 

 the better; in other words, the barn-door type of 

 inchned flat surface is the best load-carrying 

 device. As the rise-of-floor angle of the bottom 

 increases, the resistance increases and the dynamic 

 lift diminishes. 



In the case of racing motorboats of the stepless 

 type, havmg a continuous bottom terminating in 

 the transom edge, this bottom is often made 

 practically flat. At other times it has a shape 

 approximating that of an inverted deck with a 



