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Basic Sailing Course 2-Krona Dinghies

Basic Sailing Course 2-Krona Dinghies. Fluid Dynamics, forces and basic boat handling by Torgrim Log tlog @ statoilhydro.com / torgrim.log @ hsh.no. Prof. T. Log. As a professor of Fire Dynamics (physics, maths and chemistry) I know that the sailing theory may be hard to understand

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Basic Sailing Course 2-Krona Dinghies

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  1. Basic Sailing Course2-Krona Dinghies Fluid Dynamics, forces andbasic boat handlingbyTorgrim Logtlog@statoilhydro.com / torgrim.log@hsh.no Prof. T. Log

  2. As a professor of Fire Dynamics (physics, maths and chemistry) I know that the sailing theory may be hard to understand for those not educated in the physics of fluid flows. We do not actually see the wind. It may therefore be difficult to realize how it interacts with the sails and how a boat responds. I hope this introduction to sailing theory makes it easier for you to get a grip on the forces involved. Torgrim Log (National Judge) Haugesund, Norway Thanks to Dirk Jahelka for correcting the English language Prof. T. Log

  3. Wind and wind angles Prof. T. Log

  4. No wind today You are jogging eastbound: - Speed 4 knots - What do you feel while jogging? - A head wind of 4 knots - Apparent Wind = 4 knots Let us study the same situation in wind from the north Prof. T. Log

  5. 6 knots Wind from the North You are jogging eastbound (90º to the wind): - Jogging speed 4 knots - What do you feel now then? - A wind at a certain angle, agree? - The Apparent Wind) is the vector sum (arrow sum) of the True Wind from North (6 knots) + the ”head wind” you made by jogging eastbound (4 knots) 6 knots AW = 7.2 knots 6 knots 60º 4 knots The resulting Apparent Wind is 7.2 knots and at 60º angle Prof. T. Log

  6. 6 knots Wind from the NorthWe are jogging eastbound at 4 knots Prof. T. Log

  7. 6 knots Wind fra the NordWe are jogging westbound 4 knots Prof. T. Log

  8. 6 knots Wind from the NorthWe sail eastbound, speed 4 knots Prof. T. Log

  9. 6 knots wind from the NorthWe sail westbound, speed 4 knots Prof. T. Log

  10. 6 knots Wind from North 6 knots The True Wind was from North all the time… (In sailing, we move relative to the True Wind and have to adapt to the resulting Apparent Wind.) Prof. T. Log

  11. Two principal winds are True Wind (TW) True Wind Angle (TWA) Boat Speed Wind (BSW) Prof. T. Log

  12. Reaching BSW Apparent Wind (AW) (”Sailing wind”) TW TW BSW The Apparent Wind (AW) is the sum of the TW and BSW put after each other (i.e. the vector sum of TW og BSW). (The lenght of each arrow indicates its strenght) Prof. T. Log

  13. Reaching AW TW Apparent Wind Angle (AWA) BSW We always adjust the sails according to AW (AWS and AWA) (It is this ”sailing wind” (speed and angle) the boat, the sails and the sailors are exposed to while sailing.) Prof. T. Log

  14. Reaching AW TW Apparent Wind Angle (AWA) BSW While reaching, the Apparent Wind (AW) is always closer to the bow (less angle) and stronger than the True Wind. This generally results in great boat speed  Prof. T. Log

  15. Beating TW AW Apparent Wind Angle (AWA) BSW While beating (TWA ≈ 45°) we have AWA ~ 30°. The Apparent Wind is then much stronger than the True Wind and, as we see, at even closer angle to the bow. Prof. T. Log

  16. Running AWS Apparent Wind Angle (AWA) TWS BSW While running, the Apparent Wind is weaker than the True Wind, and at a closer angle to the bow Prof. T. Log

  17. Dead Down Wind Apparent Wind Angle (AWA) TWS AWS BSW The Apparent Wind is now significantly weaker than the True Wind. (In a weak breeze, the sailing wind (AWS) is so small that the boat speed turns out to be quite disappointing ) Prof. T. Log

  18. Sailing off the windin Hetlandsvågen The Apparent Wind is now significantly weaker than the True Wind. (and the corresponding speed is rather disappointing ) Prof. T. Log

  19. Aero Dynamics and Forces Prof. T. Log

  20. The sail diverts the air current. Hence, a force is acting on the air current. According to the great Isac Newton a similar opposite counter force must for sure be acting upon the sail: Force=Counter Force On the outside of the sail there is more diversion of the air flow, i.e. stronger forces involved. Prof. T. Log

  21. The resulting lower pressure on the sail / air plane wing (lift) sucks the sail leewards. (Parts of this lift is made even before the wind reaches the sail.) Prof. T. Log

  22. The total force is the arrow sum of these small force arrows (vectors) and can principly be divided into lift 90º to the AW and drag along the AW. Towards the apparent wind, the sail trim is optimized for maximum lift and minimum drag Drag Lift Total Force Prof. T. Log

  23. The keel and rudder limit sideways drifting. (At speed, any sideways drifting results in lower pressure (lift) on the opposite side of these foils. Good boat speed gives great keel lift and the rudder gets steering lift.) AW Forward Force Component Total Force Sideways Force Component Prof. T. Log

  24. The limited ”pushing” on the wind- ward side accounts for only about 20-30% of the forward forces Total Force The leeward underpressure (similar to the top of an aircraft wing) accounts for about 70-80% of the forward forces Pressure = Force / Area  Force = Pressure • Area Prof. T. Log

  25. Too tight sheeting is common. (The sail shape looks nice and everything seems quite OK…) The outside air current is, however, turbulent with a minimum lift. Prof. T. Log

  26. Too tight sheeting (cont.) The inside telltale is OK The outside telltale indicates chaos Very important to have laminær air current on the outside ! Prof. T. Log

  27. While reaching and beating both telltales shall indicate laminær air current Windward telltale OK Leeward telltale OK Prof. T. Log

  28. On a beat Prof. T. Log

  29. The two sails should work together Prof. T. Log

  30. Main sail & fore sail: A complicated synergy Approximate forward forces for 2-Krona dinghies: At TWS = 10 knots and TWA = 45°: - a single fore sail 15 N - a single main sail45 N - SUM: 60N Putting them together properly gives: - fore sail30 N (!) - main sail40 N (-) - SUM: 70 N (!), i.e. more than 60 N !!! Magics, or Aero Dynamics ??? Prof. T. Log

  31. Bent air current ahead of the main sail (”upwash”) Main sail Prof. T. Log

  32. The main sail gives beneficial AWA for the fore sail: - Significantly increased fore sail efficiency - Can beat higher into the wind - Gives balance in the steering AW at the fore sail at a larger angle to the bow due to main sail ”upwash” (very beneficial !) The fore sail efficiency increases. We can get more speed and point higher into the wind on a beat. Prof. T. Log

  33. Good main sail / fore sail synergy is a and w for great boat speed and windward pointing ability. Main sail sheeting: - Any flogging at the mast - Leech telltale(s) shall freely stream backwards - Shall look / feel correct (constant adjustments) - Heeling in strong winds Fore sail sheeting: - Fore leech telltales - Stream backwards - Avoid turbulence Prof. T. Log

  34. Problem now ? - A too tight main sheet - Release 20-30 cm ? - Then check the response - OK then? Prof. T. Log

  35. Problem now ? - Too loose main sheet - Tighten 20-30 cm ? - Then check the response - Or was it sheeted loose on pur- pose to avoid excess heeling? Prof. T. Log

  36. Problem now ? - Fore sail sheeted on the wrong side - Think aero dynamics and lift - Do not set up a wind break - Keep focus all the time Prof. T. Log

  37. Where does the wind come from? Think forces Think sail trim Remember that the boat has two sails … Sheet just enough to avoid flogging Keep the boat moving Keep trimming It is like in safty work: Always something that can be improved ! Prof. T. Log

  38. Sail depth Prof. T. Log

  39. Hoisting the main sail Main halyard NOR 1030 Outhaul for reefed main Tack point (inhaul) + Cunningham when main is reefed (to fix this tack- point on the reefed sail is done correctly by less than half the sailors despite our teaching and trainig efforts. Usually the inhaul rope is forgotten…) Outhaul Tack point (inhaul) Cunningham Kick Prof. T. Log

  40. Main sail depth Tight kick*(pulls the boom down and the mast top backwards)  Flattens the upper 2/3 of the main (strong wind) - When we need more pointing than force and speed - When excessive heeling (without reefing) - (The deepest point then moves backwards) Tight halyard / Cunningham - Gets the depth forward again Outhaul (influences the lower 1/3 of the main): - Low wind: Loose outhaul (deep sail) - Medium wind:Medium tight outhaul (medium depth) - Strong wind: Tight outhaul (flat sail) *) Efficient on fractional rigged boats (where the forestay does not reach to the mast top) (On larger keel boats we can additionally tighten the back stay and fractional stays.) Prof. T. Log

  41. Main sail depth Low wind Outhaul Mast bend ~ 60% ~ 40% ~ 15% Medium wind ~ 65% ~ 35% ~ 10% Strong wind* ~ 70% ~ 30% ~ 5% *) Large depth in strong wind usually results in BIG TROUBLE ! Prof. T. Log

  42. Simplified model of forces / wind speed Fs = CdlApsrl(AWS)2 1 Sail Force : 2 • Wind speed from 10 to 14 knots: Forces double ! • The strong wind is more turbulent • The puffs arrive faster and more violently • Gives you less time to adapt to increased forces • A small error  loss of control, swimming, etc ! • 14 knots is much tougher than 10 knots • Not to forget the sudden 20 knots puff • 2K sailing in strong wind is advanced and • requires experience (and is very exciting) ! 82 = 64 102 = 100 122 = 144 142 = 196 162 = 256 etc Prof. T. Log

  43. Problemin strong wind(NB! The beginners course does not qualify for wind > 10(12) knots) Prof. T. Log

  44. There is much wind. Imagine a stagnant boat. Does this work well ? Too tight sheeting: - Sideways forces - Minimum forward movement - Impossible to tack - The boat drifts sideways - It does not obey the rudder - Serious heeling or cap size! Prof. T. Log

  45. By instinct you release the main sail to ease the sideways forces. How does that go? Well, somewhat better: - Better forward movement, but - The bow is forced to leeward (not proper force balance) - Must use a lot of rudder to keep balance (= break) - Slow speed, ”steady state” - Not possible to tack ! The fore sail forces work far in front of the keel pushing the bow away from the wind Any solution to get in charge again? Prof. T. Log

  46. Make the forces work forwards! Sheet the sails so they are on the verge of starting to flog. Correct sheeting: - Forces directed forwards - The boat gains speed - Keel and rudder work again - Easy to maintain the steering - Limited heeling forces - The control is regained Prof. T. Log

  47. What happens when the speed increases ? Prof. T. Log

  48. Must trim the sails again: - The forces regained - The boat gains more speed - Easy to keep it going - Can tack if you want What should you do before you tack ? Prof. T. Log

  49. Head up to a beat and sheet the sails accordingly. (Regulate the pressure with the main, balance with your bodies, i.e. hang by the feet with the bodies balancing outside.) Proper sheeting: - Keeps the boat speed - Easy to keep the course - Easy to tack - Only 45º up to head-to-wind - 30º rudder in 2 - 3 s is all that is needed to get on to the new course Prof. T. Log

  50. How to get started in strong wind ?(NB! The beginner course do not qualify for wind > 5(6) m/s.) Prof. T. Log

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