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1 UNCLASSIFIEDAD NUMBERADB240994 NEW LIMITATION CHANGETOA pproved for public release, distributionunlimitedFROMD istribution: DTIC users document coverTHIS PAGE IS UNCLASSIFIEDA TRIDENT SCHOLARPROJECT REPORTNO. 254 THE PREDICTION OF PORPOISING INCEPTIONFOR MODERN PLANING CRAFTUNITED STATES NAVAL ACADEMYANNAPOLIS, MARYLANDThis document has been approved for publicrelease and sale; its distribution is DOCUMENTATION PAGE Form ApprovedI O 0MB no. 0704-01881. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED4. TITLE AND SUBTITLE 5. FUNDING NUMBERSThe prediction of porpoising inception for modernplaning craft6. AUTHOR(S)Tullio Celano, III7. PERFORMING ORGANIZATIONS NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER Naval Academy, Annapolis, MDTrident report; (1998)9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING AGENCYREPORT NUMBER11. SUPPLEMENTARY NOTESA ccepted by the Trident Scholar Committee12a.

2 DISTRIBUTION/AVAILABILITY STATEMENT 12n. DISTRIBUTION CODEThis document has been approved for publicrelease; its distribution is ABSTRACT (Maximum 200 woras) The purpose of this project was to study porpoising, one of the most common forms ofdynamic instability found in planing boats. In descriptive terms, it is a coupled oscillation in pitch and heave that occurs inrelatively calm water. These oscillations can be divergent in amplitude, leading to loss of control, injury to occupants or damageto the craft. The mechanics of porpoising have been studied sporadically from theoretical and experimental perspectives for manyyears. Studies have shown that the inception of porpoising is influenced by displacement, center of gravity location, and varioushull characteristics such as deadrise and beam. Until now, Day & Haag's thesis provided the only systematic test resultsconcerning the porpoising stability limits of planing craft.

3 Although the Day and Haag model tests were brilliantly executed andthoroughly reported, many users of these data are not aware of the size of the models tested. The average beam of the three tinyprismatic hulls was only inches . As a starting point, these tests were recreated using a series of three hard-chined prismaticplanning hullforms approximately five times larger. The tests included hulls with higher deadrise angles, more typical of craftnow employed for high-speed military purposes. Two models of actual full-scale craft, complete with performance-enhancingfeatures including lifting strakes, trim tabs, and variable drive angle were tested. These additions were found to have a profoundeffect upon the conditions at the inception of porpoising. Established planing hull analysis methods were augmented withtechniques developed during the course of the study to provide a basis from which to design and outfit high-speed, heavily ladenplaning hulls with respect to porpoising SUBJECT TERMS 15.

4 NUMBER OF PAGESP orpoising, Planing, Stability, V-Hull, High-speed16. PRICE CODE17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION OF 19. SECURITY CLASSIFICATION OF 20. LIMITATATION OFOF REPORT THIS PAGE ABSTRACT ABSTRACTNSN 7540-01-280-5500 Standard Form 298 ( ) --- Trident Scholar project report; no 254 (1998)THE PREDICTION OF PORPOISING INCEPTIONFOR MODERN PLANING CRAFTbyMidshipman Tullio Celano HI, Class of 1998 United States Naval AcademyAnnapolis, Maryland(signature)Certification of Adviser's ApprovalProfessor Roger H. ComptonChair, Department of N Architecture, cean and Marine EngineeringS ..(signature)(date)Acceptance for the Trident Scholar CommitteeProfessor Joyce E. ShadeChair, Trident Scholar Committee(signature)(date)USNA-1531-2 Table of ContentsA bstract .. 1 List of Symbols and Nomenclature .. 2B ackground .. 4 Preliminary Analysis .. 7D iscussion .. 9 Testing Part I .. 14 Testing Part 1I .. 21 Models of Real Scale Hulls.

5 28 Summary & Analysis .. 39C onclusions .. 43R eferences .. 45A ppendix A .. 48A ppendix B .. 63 Appendix C .. 67A ppendix D .. 71 AbstractThe purpose of this project was to study porpoising, one of the most commonforms of dynamic instability found in planing boats. In descriptive terms, it is a coupledoscillation in pitch and heave that occurs in relatively calm water. These oscillations canbe divergent in amplitude, leading to loss of control, injury to occupants or damage to mechanics of porpoising have been studied sporadically from theoretical andexperimental perspectives for many years. Studies by Perring (1933), Savitsky (1950through 1976), Day and Haag (1952), Martin (1978), and others have shown that theinception of porpoising is influenced by displacement, center of gravity location, andvarious hull characteristics such as deadrise and now, Day & Haag's thesis provided the only systematic test resultsconcerning the porpoising stability limits of planing craft.

6 Although the Day and Haagmodel tests were brilliantly executed and thoroughly reported, many users of these dataare not aware of the size of the models tested. The average beam of the three tinyprismatic hulls was only inches . As a starting point, these tests were re-created usinga series of three hard-chined prismatic planing hullforms approximately five times tests included hulls with higher deadrise angles, more typical of craft now employedfor high-speed military purposes. Two models of actual full-scale craft, complete withperformance-enhancing features including lifting strakes, trim tabs and variable driveangle were tested. These additions were found to have a profound effect upon theconditions at the inception of planing hull analysis methods were augmented with techniquesdeveloped during the course of the study to provide a basis from which to design andoutfit high-speed, heavily laden planing hulls with respect to porpoising Porpoising: Coupled oscillations in pitch and heave of constant or Planing: Operation in which dynamic lifting forces provide the majority of therequired support.

7 A planing craft appears to ride on top of the Stability: The ability to operate at a steady state condition, and return to that samecondition after a V-Hull: A hull with reasonably straight buttock lines and a clearly defined "V-shaped section, measured by the deadrise High-Speed: Operation at speeds such that porpoising may and Symbolsa= trim tab deflectionb= beam overallbpx= maximum chine beam3= deadrise angleCG=- position of center of gravityLCG= longitudinal position of center of gravity measured forward of transomVCG=- vertical position of center of gravity, measured above keelA= displacementg= acceleration of gravity= ft/sec2 ACA = = load coefficientpgb3A -Tab LiftC = = load coefficient adjusted for trim tab liftpgb3 ACCo _ 1 = lift coefficient for a flat planing surface!pV2b22 ACI3 -= lift coefficient for a deadrise planing surfaceC pV2bdCL -Lift Curve SlopedocHSAC MKII= 40' High-Speed Assualt Craft, MKIILOA= length overallLBP= length between perpendicularsLc= chine wetted lengthLK= keel wetted length-LK -Lc -mean wetted length to beam ratio2b1 dCL 2 LTA = a-c L pV2 A = Trim Tab Lift2 dczVC = V= speed coefficientXDH= scale ratio for Day & Haag dataNAHL= Naval Academy Hydromechanics LaboratoryNSWC= Naval Surface Warfare Center3 PCC= 42', waterjet powered Patrol Craft, CoastalPVC= Polyvinyl Chloridep= water densityV= speed of craft4 BackgroundPlaning craft are high-speed marine vehicles that derive most of their supportfrom hydrodynamic pressures acting on their relatively flat, wide bottom surfaces.

8 Whilethe concept of planing was recognized in the late nineteenth century, the first practicalapplication of the concept can be traced to the development of seaplane hulls during thebeginning of the twentieth century. As power plants became light and powerful enoughto propel a small to medium size boat past its "hump speed," defined by the generatedwave patterns, into the planing speed regime, a whole new facet of marine transportationbegan. While the planing hull introduced the ability to operate at high speeds across thesurface of the water, it can easily fall victim to dynamic instabilities, which havemanifested themselves in both vertical and transverse responses. In mild cases, theseinstabilities can be a mere annoyance, but in the most extreme cases, they have led tocatastrophic structural failure, to capsizing and to serious personal injury. One of themost common instabilities, known as "porpoising," is a vertical plane, coupled oscillationin pitch and heave which occurs in calm water, and can be divergent in inception and the craft parameters that influence it are the subjects of theresearch described in this craft have taken various forms, dependent upon the design speed andintended operational profile.

9 The hull forms addressed in this paper are those designedfor high-speed operation under moderate to heavy loading conditions. The typicalbottom design for such craft include sharp comers at the chines and transom that ensuredistinct water flow separation to minimize hull side flow attachment and to maximizedynamic lift. Without these sharp comers, the flow paths would become unpredictable,planing resistance would increase, and dynamic lift and stability would suffer. Hullsbelonging to this family are known as "hard chine planing hulls."The simplest of these planing hull forms are "prismatic" in form, meaning thatfrom a point around amidships continuing aft, the planing surface has a constant deadriseand chine beam; , the afterbody is a prism from a hydrodynamic standpoint."Deadrise" is the transverse slope of the bottom of the boat, measured in degrees. Thus,a boat with zero degrees of deadrise would have a totally flat bottom.

10 The angle ofdeadrise of a planing surface has an effect on the calm water stability and performance ofthe craft. The deadrise angles analyzed in this study range from 15 to 25 degrees, whichspan the majority of high-speed offshore "V-hulled" boats. Hulls with more thanapproximately 22 degrees are referred to as "deep-V" forms and are renowned for theirability to perform at high speeds in rough water at the expense of increased resistance. A"non-prismatic" hullform can have one or more deviations from .the simple prismaticshape including variable deadrise along the hull length, typically decreasing toward thestem of the boat and tapering of the chine beam toward the after end of the hull. Reversechines, flat strips at the outboard edge of the planing bottom, and running strakes arelocated longitudinally along the planing surface to help improve lift on heavily ladenhulls. Both reverse chines and running strakes can extend forward and serve as sprayrails that deflect the spray away from the hull, keeping passengers and cargo features found on high-speed hulls are keel pads, transverse steps, transomnotches and trim tabs.