Basic Sailing Course 2-Krona Dinghies
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Transcript of Basic Sailing Course 2-Krona Dinghies
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Basic Sailing Course
2-Krona Dinghies Fluid Dynamics, forces and
basic boat handlingby
Torgrim [email protected] / [email protected]
Prof. T. Log
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Prof. T. Log
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 easierfor 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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Prof. T. Log
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 NorthYou 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 angleAW
= 7
.2 k
nots
6 knots
4 knots
60º
6 knots
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Prof. T. Log
6 knots Wind from the NorthWe are jogging eastbound at 4 knots
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Prof. T. Log
6 knots Wind fra the NordWe are jogging westbound 4 knots
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Prof. T. Log
6 knots Wind from the NorthWe sail eastbound, speed 4 knots
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Prof. T. Log
6 knots wind from the NorthWe 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 BSWput after each other (i.e. the vector sum of TW og BSW).
(The lenght of each arrow indicates its strenght)
BSW
TW
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BSW
TW
Reaching
Apparent WindAngle (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 WindAngle (AWA)
AW
While reaching, the Apparent Wind (AW) is always closerto 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
TWAW
Apparent WindAngle (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
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BSW
TWS
AWSApparent WindAngle (AWA)
While running, the Apparent Wind is weaker than the True Wind, and at a closer angle to the bow
Running
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BSW
TWSAWSApparent WindAngle (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
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Sailing off the windin 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
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On the outside of the sail there ismore 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 Newtona similar opposite counter force mustfor sure be acting upon the sail:
Force = Counter Force
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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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LiftDrag
Total Force
The total force is the arrow sum of these small forcearrows (vectors) and can principly be divided intolift 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 ForceComponent
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
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The limited ”pushing” on the wind-ward side accounts for only about20-30% of the forward forces
Pressure = Force / Area Force = Pressure • Area
Total Force
The leeward underpressure (similar tothe 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 niceand everything seems quite OK…) The outside air currentis, 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 laminærair 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 telltalesshall indicate laminær air current
Prof. T. Log
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On a beat
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The two sails should work together
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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 aheadof 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 sailat a larger angleto the bow due tomain sail ”upwash”(very beneficial !)
The fore sail efficiency increases.We can get more speed and point higherinto 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 20-30 cm ? - Then check the response - OK then?
Prof. T. Log
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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
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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 trimRemember that the boat has two sails …
Sheet just enough to avoid floggingKeep 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 sailMain halyard
Tack point (inhaul)Outhaul
Cunningham
Tack point (inhaul) + Cunninghamwhen main is reefed (to fix this tack-point on the reefed sail is done correctlyby less than half the sailors despite our teaching and trainig efforts. Usually theinhaul rope is forgotten…)
Outhaul forreefed main
Kick
NOR1030
Prof. T. Log
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Main sail depthTight 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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Prof. T. Log
Main sail depth
~ 40%~ 60%
~ 15%
~ 10%
~ 5%
Low wind
Medium wind
Strong wind*
~ 35%~ 65%
~ 30%~ 70%
Outhaul Mast bend
*) Large depth in strong wind usually results in BIG TROUBLE !
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Simplified model of forces / wind speed
Sail Force : Fs = Cdl Apsl(AWS)212
82 = 64
102 = 100
122 = 144
142 = 196
162 = 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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Problemin strong wind
(NB! The beginners course does not qualify for wind > 10(12) knots)
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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 easethe 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 forceswork far in front of thekeel pushing the bowaway from the wind
Prof. T. Log
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Make the forces work forwards! Sheet the sailsso 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 ?
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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 2 - 3 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 sailwith 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 extensionGet 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
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Let us study a tack from port to starboard close hauled(The reference point is port reaching)
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Prof. T. Log
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Prof. T. Log
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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 95-100º
Prof. T. Log
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Problem in low winds
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You are sailing close hauled, but yourspeed 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
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Why is this very slow ?
Prof. T. Log
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Prof. T. Log
![Page 67: Basic Sailing Course 2-Krona Dinghies](https://reader035.fdocuments.net/reader035/viewer/2022062305/5681603a550346895dcf5de6/html5/thumbnails/67.jpg)
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 1 - 12• Travellers, sheeting point, etc (on other boats)• Current, wind at obstructions, …• Wind field around the boat• Racing strategy, tactics, starts, etc
Prof. T. Log