IBEX 2012 Session 301 Dynamic Roll Stability. Richard Akers Maine Marine Composites LLC Portland, ME 04101, USA

Transcription

1 IBEX 2012 Session 301 Dynamic Roll Stability Richard Akers Maine Marine Composites LLC Portland, ME 04101, USA

2 Standards and the Law Hard to write standards to test system performance Easier to write a test for each component or function. Standards/CFR tend to emphasize components rather than missions. Standards/CFR System/Mission Tests Is the kill switch lanyard the Is there a better technology to stop right length? an engine than a lanyard and switch? How do you guarantee that a How do you prevent a boat from boat will stay afloat if it capsizes? capsizing? (No mention of distance from shore) Are there differences between operating in the ocean versus in a fresh water lake? Oct. 2, 2012 IBEX Session Dynamic Roll Stability 2

3 Standards and the Law Code of Federal Regulations 33 CFR 183, Subparts F, G and H American Boat and Yacht Council, Annapolis, MD ABYC H-5, H-8 International Organization for Standardization ISO Small Craft Stability and buoyancy assessment and categorization Part 1, Part 2, Part 3 Oct. 2, 2012 IBEX Session Dynamic Roll Stability 3

4 Standards and the Law: ABYC Standards and CFR Static Roll Stability: The purpose of the flotation standard (33 CFR 183, Subparts F, G and H) is to provide a suitable platform for the rescue of a boat s occupants in the event of capsizing or swamping and, in some cases, to deaths due to hypothermia. Proceedings of the Marine Safety Council, U.S. Department of Homeland Security, USCG, July Sept. 2003, Vol. 60, Number 3. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 4

5 Static Stability vs. Dynamic Stability Photo reprinted with permission from Hog Island Boat Works, th Street/POB , Steamboat Springs, CO Proc. of the Marine Safety Council, USCG, op. cit. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 5

6 Torque The Mystery of Dynamic Righting Moments A Work Heel Angle B Torque = Moment You can hold the boat at Point A or Point B with the same amount of torque Work (=Torque * angle) To heel to A takes a little work To heel to B takes a lot of work Oct. 2, 2012 IBEX Session Dynamic Roll Stability 6

7 Dynamic Righting Moments Torque (also called Moment) Work Righting Moment Work= (torque * angle) [*100] NOTE: Righting Arm is just the Righting Moment divided by the weight of the boat. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 7

8 ISO : Categories of Operation ISO includes recognition of operating conditions Individual rules vary according to the design category Design Category A B C D Wave height up to (meters) approx (feet) significant significant significant maximum Typical Beaufort wind force (up to) Calculation wind speed (m/s) (knots) (miles/hr) Oct. 2, 2012 IBEX Session Dynamic Roll Stability 8

9 Dynamic Stability In ISO Resistance to waves and wind Applies to Non-sailing boats of hull length greater than or equal to 6m [19.7 ft] Only applies to Design Category A and B Significant wave height > 6.6 feet Mean/Ave. wind speed > 38 mph Typ. Beaufort wind force > 6 Oct. 2, 2012 IBEX Session Dynamic Roll Stability 9

10 Dynamic Stability In ISO Heeling Moment Due to Wind: M W = 0.3*A LV *(A LV /L WL + T M )v W^2 T M = draft amidships A LV = windage area ( 0.55*L H *B H ) v W = speed (Cat. A: 28 m/s, Cat. B: 21 m/s) Assumed Roll Angle: Category A: Φ R = /V D Category B: Φ R = /V D Key 1 Righting moment 2 Heeling moment due to wind 3 Angle of downflooding Φ D or vanishing stability Φ V if less, or 50 if less ISO Figure 5 Roll resistance to waves and wind Oct. 2, 2012 IBEX Session Dynamic Roll Stability 10

11 Acknowledgement Much of the following material was derived from the final project report: Capsizing, Swamping and Sinking of Small Boats USCG, Office of Boating Safety: Grant Authors: Richard H. Akers, NA, PE; Gregory T. Davis; John D. Flaig, MSME Date: April 19, 2012 The project was administered by: American Boat & Yacht Council Oct. 2, 2012 IBEX Session Dynamic Roll Stability 11

12 Factors in Dynamic Roll Stability 1. Mass and motion of human beings 2. Loading conditions 3. Weather Wind Waves, standing waves, wakes 4. Water in boat Sloshing (resonant) Reduction in freeboard 5. Shape of hull Pitch and Roll Resonance Oct. 2, 2012 IBEX Session Dynamic Roll Stability 12

13 Factors: People Oct. 2, 2012 IBEX Session Dynamic Roll Stability 13

14 Example: Jon Boat Capsizes News Report: Two men were rescued after high winds caused their boat to capsize. The passenger lost his balance and the boat capsized. The occupants were able to hang onto the boat until rescue crews were able to pull them from the water. Interview: in the time since we left the dock (about 1 hour) there were 2 foot waves with some good troughs. We were getting splashed from the waves.... There seemed to be a larger wave coming from the left side at an angle to the bow. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 14

15 Example: Jon Boat Capsizes Interview (con td): [The passenger] seeing the wave coming in tried to avoid getting wet by leaning forward and to the right. As he did this we went over a rise and the bow started to go down and went under as it caught the edge of the wave. This took us right over. Question: So what you are saying is the bow went under water and the boat rolled to the right and capsized? Answer: That is correct. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 15

16 Accident Case Study Oct. 2, 2012 IBEX Session Dynamic Roll Stability 16

17 Accident Case Study: Critical Roll Angle Boat is swamped to rail per ABYC H-8 Level Flotation condition. (People shown for illustration purposes only) Interior water barely reaches the chine on the port side. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 17

18 Freeboard of accident boat, relative to heel angle. Boat loaded as it was at the accident. Accident Case Study: Freeboard vs. Heel Angle Aft, Port Rail: Height (in) Heel Angle (degrees) Boat downfloods at heel angle = 22 degrees. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 18

19 Dynamic Righting Moment Boat will downflood at heel angle = 22 degrees (dashed line) How much energy (work) to roll boat to critical heel angle? Max dynamic righting moment = 1,451 inch-pound-radians = 121 foot-pound-radians. Equivalent to energy to raise 50 pound block (e.g. cooler full of ice) 2.4 feet. Energy could be applied to boat by wave, occupant standing or lurching, tug on fishing pole boat will downflood. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 19

20 Example: Factor: Instinctive Reaction Assume 200 pound person moves 1 foot in ½ second VCG of person is 1-foot above Center of Gravity of boat Mass of 200 pound person is 6.22 slugs Offset= ½ * cos( 2 * p *t) Acceleration = -2 * p * cos(2 * p *t) Peak acceleration (maximum force) at beginning and end of lurch, about ft/sec Requires pounds of force Newton s 2 nd Law: Roll moment = 1 ft * lb = foot-pounds F = M * A = Moment needed to roll boat to critical downflood angle. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 20

21 Factor: Instinctive Reaction pound person moving horizontally by 1 foot. Instinctive motion so the entire event might occur in ½ second Acceleration (ft/sec^2) R-M, ft-lb Distance Velocity Acceleration and roll moment associated with example conditions. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 21

22 Factor: Instinctive Motion What if the 200 pound person was standing up? Casting a fishing line Walking to get an adult beverage Changing places with another passenger, etc. Then the person s VCG could be higher than 1-foott above boat s VCG If Person s VCG is 2-feet above boats VCG then lurch acceleration would be double, maximum of ft-lbs Oct. 2, 2012 IBEX Session Dynamic Roll Stability 22

23 Loading conditions According to the USCG (Boating Safety Circular #68) USCG uses 160 pounds based on: Studies of average weights in accidents Comparison with State and International Laws Typical boating group is two adults and one or two children Use straight-line interpolation for all circumstances Result: Max Persons = ( Persons Capacity + 32 ) / 141 According to ISO , average person has mass of 75 kg (corresponds to pounds) Oct. 2, 2012 IBEX Session Dynamic Roll Stability 23

24 1000% Significant of People Weight Person Weight as % of Boat Weight In this region the movement of the person is as important as the movement of the boat 100% % 1% Oct. 2, 2012 IBEX Session Dynamic Roll Stability 24

25 Many capsizing accidents occur in standing waves at mouth of river Static tests don t allow for sloping water Falls of Lora Just near mid ebb-tide. Tidal streams run 6 knots Waves / Wake-Wash Copyright Gordon Brown ( and licensed for reuse under this Creative Commons License ( Oct. 2, 2012 IBEX Session Dynamic Roll Stability 25

26 Hull: Roll Restoring Force Robert F. Smiley, A STUDY OF WATER PRESSURE DISTRIBUTIONS DURING LANDINGS WITH SPECIAL REFERENCE TO A PRISMATIC MODEL HAVING A HEAVY BEAM LOADING AND A 30 ANGLE OF DEAD RISE, Langley Aeronautical Laboratory, Langley AFB, VA, NACA TECHNICAL NOTE 2111, July Oct. 2, 2012 IBEX Session Dynamic Roll Stability 26

27 Hull: Roll Restoring Force Robert F. Smiley, op. cit. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 27

28 Hull: Roll Restoring Force Boat will roll into a turn if sum of side forces are above axis of roll moment but will roll away from the turn if sum of side forces are below axis of roll moment E.g. high deadrise, low VCG Robert F. Smiley, A THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF YAW ON PRESSURES, FORCES, AND MOMENTS DURING SEAPLANE LANDINGS AND PLANING, Langley Aeronautical Laboratory, Langley AFB, VA, NACA TECHNICAL NOTE 2817, Nov Oct. 2, 2012 IBEX Session Dynamic Roll Stability 28

29 Why Boats Are Course-Stable Because the side force is at the back of the boat. You wouldn t put the feathers on the front of the arrow, would you?

30 Stepped Hulls and Trim Angles Sterndrive trimmed under (or tabs trimmed down) Forces stern UP Force bow DOWN NOTE: In a V-Hull it is common practice to trim UNDER to carve good turns Center of Lateral Pressure (side pressure) is FORWARD of Center of Gravity SPIN OUT!

31 Planing Boat Pitching Response depends on Wavelength Large response for wavelength = 2.5 x boat length Planing boats have both roll and pitch natural frequencies Depends on loading, hull form July 2012 POWERSEA User Training 31

32 Pitch Resonance Kelvin Diverging Wake Planing boats resonate in pitch If you cross a diverging wake pattern you will always hit resonant wavelength Boat will pitch strongly at some point in wake Oct. 2, 2012 IBEX Session Dynamic Roll Stability 32

33 Recommendations Human Factors Caution, Caution, Caution. Education (formal, On-the-job) Weather Planing boats resonate in roll and pitch Mfr. should test for resonant period Avoid waves with those periods? Hull Design: Increased freeboard Speed + extreme buttock curvature bow-down (unstable) trim Oct. 2, 2012 IBEX Session Dynamic Roll Stability 33

34 Coasties are good boat drivers because they practice They know how to cross a bar They know the limits of their vessels Recommendations: Training Top: "Sixth RB-M Delivered to Port Aransas, Texas" Bottom: "Coast Guard units train on Lake Pontchartrain" U.S. Coast Guard photo by Petty Officer 3rd Class Stephen Lehmann. Oct. 2, 2012 IBEX Session Dynamic Roll Stability 34

35 Recommendation: Freeboard Determines Mission Suitability Manufacturer can t predict how a boatowner will act, so assume that boat will roll or stuff in waves and design accordingly. See J0763 : Sand barge, Lough Neagh (6). The Lennie arriving with a full load of suctiondredged sand. There is a noticeable difference in the freeboard. Note: taken on private property by kind consent. Copyright Albert Bridge and licensed for reuse under this Creative Commons Licence Oct. 2, 2012 IBEX Session Dynamic Roll Stability 35

36 Recommendations Manufacturers: Find roll and pitch natural frequency for expected loading Make sure you have enough freeboard to avoid downflooding under design mission conditions Boat Operators: Watch for roll resonance go to safe harbor if necessary Don t overload (Obesity Epidemic) Sit down! Both: Mission, mission, mission Oct. 2, 2012 IBEX Session Dynamic Roll Stability 36

37 Recommendations Standard and/or CFR similar to ISO Section 6.3 Resistance to waves and wind Standard and/or CFR that takes into account human movement Add method similar to ISO Section Rolling in beam waves and wind Except use moment from lunge instead of moment from wind Lunge Moment = ½*Weight*1 foot Oct. 2, 2012 IBEX Session Dynamic Roll Stability 37

38 References Marr, Miller, Schultz, boats chapter 4 lecture, US Naval Academy, ( Mori Flapan, "What happens when you start getting wet feet? The revised standard for buoyancy and stability after flooding," National Marine Safety Committee Inc., Pacific 2010 Conference Sydney January 2010 ( Revised_standard_buoyancy.pdf). Oct. 2, 2012 IBEX Session Dynamic Roll Stability 38

39 Questions? Oct. 2, 2012 IBEX Session Dynamic Roll Stability 39