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Boat Speed in Small Boats: The Physics of Going Faster

Boat Speed in Small Boats: The Physics of Going Faster. Paul Miller Naval Arch & Ocean Engineering Dept. US Naval Academy. The Big Picture in Winning Races. Boatspeed: A useful application of what you learned (?) in physics!. Background:.

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Boat Speed in Small Boats: The Physics of Going Faster

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  1. Boat Speed in Small Boats:The Physics of Going Faster Paul Miller Naval Arch & Ocean Engineering Dept. US Naval Academy

  2. The Big Picture in Winning Races Boatspeed: A useful application of what you learned (?) in physics!

  3. Background: Fact:Dinghy sailors win in more types of boats than big boat sailors? Why? Assuming you learned something more than starts and tactics in college!

  4. Warning: “Boatspeed Blindness” can be detrimental to your racing success!

  5. 1996 Int’l Canoe WorldsLemon Tree Passage, Australia The Start! The Finish...

  6. The Key Measurement of Racing Boatspeed • V = Velocity of boat • Vmg = Velocity of boat made good to the next mark (sometimes V “to windward”) Which wins boat races? They are related by: Vmg = V * cos(f)

  7. Example: Two Boats Beating In “Point Mode”; V = 5.24 knots, f = 37 degrees In “Foot Mode”; V = 5.40 knots, f = 40 degrees Which gets to the weather mark first?

  8. Solution In “Point Mode” Vmg = 4.18 knots In “Foot Mode” Vmg = 4.13 knots On a 1/2 mile beat, the “pointer” is 6 seconds (3 boatlengths) ahead!

  9. How do you find the optimum V and pointing angle, f? 1. Experiment, measure and record (could be “seat of the pants”) 2. Two-boat-test for relative improvement (race experience or practice) 3. Predict using a Velocity Prediction Program (VPP) (IMS and IRM use a VPP to get ratings) 4. Switch to Naval Architecture as a major... (My chance to put in a plug!)

  10. VPP “Polar” Provides predicted speeds for all points of sail for common wind strengths. VPP’s are often customized for different boats types (ex. IACC, IMS, 12m)

  11. Basic Physics of Boat Speed • F=m*a ! • The sum of the forces equals zero F=0 • The sum of the moments equals zero M=0 or, “For every action there is an equal and opposite reaction.”

  12. Sail Force Where does the force in the sails come from and where does it go? Lift Drag Wind Note the wind is deflected by the sail!

  13. In Detail: Force Generated by the Sails = Mass of Wind x the amount the wind is decelerated by the sails versus Force Generated by the Sails = Mass of the boat x the amount the boat is accelerated, (“Thrust”) plus the mass of the water x the amount the water is accelerated, plus the mass of air x the amount the air is accelerated (“Drag”)

  14. Why is “Acceleration” Important? Velocity = Acceleration dt Distance = Velocity dt And the one that goes the farthest in a given amount of time, or covers the same amount of distance in the shortest time wins the race!

  15. The Goal’s From Physics Are: • Take as much from the wind as you can • Reduce the mass of the boat as much as possible • Disturb the water and wind as little as possible • All the while making sure you are maximizing Vmg rather than V!

  16. It isn’t quite that simple(but it’s close)!Quiz 1:Which is faster? Boat “A” 22 feet 4200 lbs 300 sq ft Boat “B” 24.5 feet 4200 lbs 300 sq ft Waterline Length Weight Sail Area 177 PHRF Rating 129 Boat B is 48 seconds per mile faster!

  17. Boat A and Boat B J/24 Express 27 If everything else is equal, the longer boat is faster!

  18. The Big Picture in Winning Races

  19. Sail Force Recall that “For every action…” As the fluid is deflected past the sail, the sail is deflected the opposite way.

  20. Sail Force The Magnitude of the force is approximated by Bernoulli’s Equation: F=½(air density)(wind velocity)2(Sail Area)(Coef. of Lift) To get more sail force you can increase any of these terms! 1. Sail for the puff, or put up more sails... 2. For most sailors the only “legal” option is to adjust the Coefficient of Lift… This is accomplished through “sail trim”.

  21. Sail Trim • The Direction of the Sail Force depends on how much Lift and Drag the sail is producing. • Lift is the force produced perpendicular to the wind • Drag is the force parallel to the wind.

  22. Quiz #2Which contributes more to boatspeed; Lift or Drag? Answer: Both! Upwind Goal: High Lift & Low Drag Downwind Goal: High Lift & High Drag

  23. Upwind Sail Trim • High Lift • Full sail • High Angle of Attack • Even twist • Low Drag • Flat sail • Low Angle of Attack • Even twist Highest Lift

  24. Downwind Sail Trim • High Drag and Lift • Full sail • High Angle of Attack (near stall on reach, stalled on run) • Even twist Highest Drag

  25. Tell-Tales(Results from Wind Tunnel Tests) High Lift/ Low Drag High Lift/ High Drag

  26. Other Sail Controls • Vang (twist, forestay tension, mast and boom bend) • Outhaul (lower part of the main lift/drag control) • Luff adjustment (flow attachment and lift coefficient control) • Mast bend (spreaders, shroud tension)

  27. How do you know when to adjust the controls? • Is the twist even? • Boom and top batten roughly parallel • Is the boat overpowered? • Can’t keep it flat, luffing sails • What are the faster boats doing? • If they are going faster than you, find out why!

  28. The Ultimate Sail? Cogito Current holder of the “Little America’s Cup” Routine speeds of 20 knots in 15 knots of breeze! “World’s Fastest Raceboat”

  29. IACC/Int’l Canoe Mast Project

  30. The Big Picture in Winning Races

  31. Foil Basics • F=0 • So Side Force generated by the sails is balanced by the side force (Lift) of the Foils (Centerboard and Rudder)

  32. Foil Lift and DragCenterboard and Rudder • The same concept as sails • Bernoulli’s Eqn for force (Lift or Drag) magnitude • Vector addition of lift and drag components for direction • Goal is high efficiency • (High Lift/Drag ratio) Lift and Drag on Foils

  33. Foil Drag Components • Friction (Viscosity) • Pressure (Lift induced, eddies) • Aspect Ratio (Span2/Area) • Planform The Drag Equation from Bernoulli’s is: Fdrag=½(water density)(boat speed)2(Foil Area)(Coef. of Drag) The two easily-changed variables are area and Cd!

  34. Foil Frictional Drag Two things for sailors to think about: • Smoothness (1/c Huffman: EN245A) • Smoother the better • Laminar vs Turbulent • Min sand w/400 grit • All coatings were worse • Area Polished Sanded with 180 grit Cl Angle of Attack

  35. Example of Area Reduction Centerboard area is approximately 10% of the total wetted surface. In light air “wetted surface drag” is approximately 80% of total drag. A 420 Running: Raising the board 90% of the way will reduce drag 7%! Giving 0.14 kt! This assumes you don’t increase rudder drag due to loss of steering control!

  36. Foil Pressure Drag • Keep angles of attack small so as to stay in low drag area of foil performance. (High Lift/Drag ratio) High Drag 8o In a 420, increasing the rudder angle from 2o to 6o will cost 0.1 kt! Low Drag 0-2o

  37. Example of How to Minimize Angle of Attack “Steer with your weight” “Steer with the sails” This minimizes the foil drag. Think of the rudder as a brake.

  38. The Big Picture in Winning Races

  39. Hull Resistance • Friction • Pressure (eddies) • Wave Making • Spray

  40. Typical Dinghy Resistance Curve 420 Int’l Canoe

  41. Hull Friction Drag • Like foils, make it as smooth as possible! (Min 400)(Benefit is not as great as foils) • Reduce area by heel or trim (flat areas out, round sections in)

  42. Hull Pressure Drag • Reduce eddies by not letting transom drag (look for “clean” flow off stern) • Move forward if possible

  43. Hull Wave-Making Drag Vmg • To make waves takes a lot of energy! • Energy used in making waves is based on: • Wave length • Volume of water displaced When beating in a 420 in light air, the lighter crew (~50 lbs) is 0.15 knots faster!

  44. Example of Weight/Length Effect Cal 20 and Moore 24 (originally) Same Weight and Sail Area: Different Length Moore 24 is 1.5 minutes a mile faster! Moral is, “Think Light!”

  45. Research in Length

  46. “New” Navy 44 Research

  47. The Big Picture in Winning Races Saving the best for last!

  48. Effect of heel on drag Increased yaw moment Increased leeway Increased rig drag Increased wave making StabilityThe most important factor in speed? Except in light air and flat bottomed boats, heel is slow!

  49. StabilityThe most important factor in speed? Effect of heel on thrust • Reduced sail area • Reduced rig efficiency

  50. How stability fits with physics F=0,  M=0 h x SF = weight x t Thrust=SF x sin(B) B=sail trim angle So, Thrust =(w x t x sin(B))/h There will not be a quiz at the end!

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