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Basic Sailing Course 2-Krona Dinghies Fluid Dynamics, forces and basic boat handling by Torgrim Log / Prof. T. Log

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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

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Wind and wind angles Prof. T. Log

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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

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Prof. T. Log 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) The resulting Apparent Wind is 7.2 knots and at 60º angle AW = 7.2 knots 6 knots 4 knots 60º 6 knots

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

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Prof. T. Log 6 knots Wind fra the Nord We are jogging westbound 4 knots

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Prof. T. Log 6 knots Wind from the North We sail eastbound, speed 4 knots

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Prof. T. Log 6 knots wind from the North We sail westbound, speed 4 knots

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Prof. T. Log 6 knots Wind from North 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.) 6 knots

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

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BSW TW Apparent Wind (AW) (”Sailing wind”) Reaching 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) BSW TW Prof. T. Log

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BSW TW Reaching Apparent Wind Angle (AWA) 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.) AW Prof. T. Log

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BSW TW Reaching Apparent Wind Angle (AWA) AW 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

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BSW TW AW Apparent Wind Angle (AWA) 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. Beating Prof. T. Log

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

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BSW TWS AWS Apparent Wind Angle (AWA) 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 ) Dead Down Wind Prof. T. Log

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

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Aero Dynamics and Forces Prof. T. Log

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On the outside of the sail there is more diversion of the air flow, i.e. stronger forces involved. 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 Prof. T. Log

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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

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Lift Drag Total Force 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 Prof. T. Log

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Total Force Forward Force Component Sideways Force Component 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 Prof. T. Log

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

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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

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

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

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On a beat Prof. T. Log

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The two sails should work together Prof. T. Log

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Approximate forward forces for 2-Krona dinghies: At TWS = 10 knots and TWA = 45°: - a single fore sail 15 N - a single main sail 45 N - SUM: 60 N Putting them together properly gives: - fore sail 30 N (!) - main sail 40 N (-) - SUM: 70 N (!), i.e. more than 60 N !!! Magics, or Aero Dynamics ??? Main sail & fore sail: A complicated synergy Prof. T. Log

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

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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

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Good main sail / fore sail synergy is and 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

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

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

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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

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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

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Sail depth Prof. T. Log

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Hoisting the main sail Main halyard Tack point (inhaul) Outhaul Cunningham 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 for reefed main Kick NOR 1030 Prof. T. Log

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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

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Main sail depth ~ 40%~ 60% ~ 15% ~ 10% ~ 5% Low wind Medium wind Strong wind* ~ 35%~ 65% ~ 30% ~ 70% OuthaulMast bend *) Large depth in strong wind usually results in BIG TROUBLE !

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Simplified model of forces / wind speed Sail Force : F s = C dl A ps l (AWS) = = = = = 256 etc - 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) ! Prof. T. Log

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

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There is much wind. Imagine a stagnant boat. Does this work well ? Prof. T. Log 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!

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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 ! Any solution to get in charge again? The fore sail forces work far in front of the keel pushing the bow away from the wind Prof. T. Log

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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

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

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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

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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 s is all that is needed to get on to the new course Prof. T. Log

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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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Reef the main and take it down. Strap the fore sail with an elastic (from the blue seat bag or the inhouse box). *) You are not familiar with the tiller extension? Then leave the fore sail strapped by the elastic ! Prof. T. Log On the water, hoist only the reefed main: - Much forces work behind the keel - Easy to steer towards the wind - Easy to control the boat in tacks - May be sailed by one trained person - Wait to hoist the fore sail - Do you trust the fore sail trimmer? - OK, hoist the fore sail ! - Use the tiller extension* - Hike out to flatten the boat and have a nice trip !

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The tiller extension Get used to steering with the tiller extension - In low winds, the fore sail trimmer to leeward (counter balance) - In strong winds, hike out to balance the boat - Place the tiller extension in front of you on your leg (thigh) - That stabilizes the steering when hiking in/out - The forward arm sheets the main sail - Exception: When reaching, move into the boat Prof. T. Log

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Doing a tack Prof. T. Log

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Let us study a tack from port to starboard close hauled (The reference point is port reaching) Prof. T. Log

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We loose speed during the tack: - The Apparent Wind then enters at a larger angle to the bow - Slightly released sheets are therefore best afterwards - This releaves the sideways forces - The forces are more forwardly directed - The boat heels less - Better acceleration - When speed is OK, sheet in and head up to close hauled (or release the sheets and fall off if you want STB reaching) Prof. T. Log Did you loose all speed and starts moving backwards? Release the sheets, steer backwards, tighten carefully to build speed again !

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Tacking (helmsman) - Point the knees forward - Hold the main sheet - Push the rudder (away) 30º (the boat turns into the wind) - Bend the tiller extension upwards and behind you - Move the aft leg forward and over to the other side of the boat - Follow with the whole body - Sit down on the flanks of the boat - Change hand for tiller and main sheet - Release just a little main sheet ”just in case” - Go a few degrees low to gain speed and then up to close hauled - After tacking you shall have a course change of about º Prof. T. Log

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Problem in low winds Prof. T. Log

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You are sailing close hauled, but your speed is low and the others pull away. Proper sheeting: - The main must get more depth - Then you get more sail force - Go a little lower and faster - Release the fore sail sheet some - Gives more force in the fore sail - When speed is OK, tighten the sheets some and head up just a little Prof. T. Log

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Weight balance Prof. T. Log

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Why is this very slow ? Prof. T. Log

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Topics not treated in the beginner course Depth and shape of the fore sail Flow separation and Gentry tufts Adjusting sail twist (top vs. lower telltales, etc.) Roll tacks and roll gybes Spinnaker handling Wind profiles and ”wind shear” Polar diagrams (best VMG course, …) Meteorology and wind stability (expected shifts…) Wally (given repeated wind shifts) Regatta rules exceeding RRS Travellers, sheeting point, etc (on other boats) Current, wind at obstructions, … Wind field around the boat Racing strategy, tactics, starts, etc Prof. T. Log

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Have a nice trip ! Best regards Torgrim /

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