10 – Shape 83

Section 10

Sail Shape

Aspect Ratio. In general usage, the aspect ratio of a sail is Lift. A force generated on the leeward side of a sail by air taken to be the length of the luff to the length of the foot passing over the airfoil shape. It is the force that drives a sailboat. Also, when close-hauled, a wind that shifts Attached Flow. The movement of particles along a sur- aft to allow a change of course toward the wind direction. face, such as the flow of air particles along the leeward side of a sail. Separation. Detachment of air in a smooth laminar flow from the leeward side of a sail. See also “Stall.” Backwinded. A sail is backwinded when another sail to leeward of it is trimmed so closely as to cause increased Stall. The condition that arises when the airfoil (sail) or pressure on the lee side of the windward sail thus causing hydrofoil (rudder) is turned too far off and the flow of air or the windward sail to bulge to windward. water separates from the foil; there is no longer an attached flow.

1 convert wind energy into energy to drive the boat. 5 With an airfoil shape and air flow angles like those shown This section examines the factors making this conversion in Figure 10–1, air velocities on the leeward (suction) smooth and efficient: the area of the sails, the direction side of the sail will be higher than air velocities on the of the apparent wind relative to the sail, and the course windward side of the sail. This difference in speeds cre- to be made good. ates a pressure difference that is a force similar to the lift and drag forces on an airplane wing in flight. Streamlines 2 Wind force on the sails generates boat propulsion either by smooth air flow creating aerodynamic lift, Leeward Side, High Speed, Low Pressure or by a simple push force when a boat sails directly downwind.

Air Flow 3 The flow of air across the sail develops forces on the sail. The forces on the sail vary as the square of the velocity of the wind. Therefore, when wind speed doubles, the forces caused by the air flow around the sail increase four- Windward Side, Low Speed, High Pressure fold.

4 To picture the flow of air across a sail, imag- Point of Maximum Draft ine smoke traces in the air passing the sail close to its surface. Picture the airfoil shape of the sail viewed from above with the air Draft flow passing along the sail. Chord

Figure 10–1 Air Flow Around a Sail

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Chord

Angle of Attack

Apparent Wind

Chord to Wind – Angle of Attack Figure 10–2 Angle of Attack Angle of Attack maintain this best angle of the sail to the wind. Typically, 6 The angle of attack of a sail is the angle between the this angle is approximately 20 degrees, Figure 10–3. apparent wind direction and the chord line of the sail. The chord is the straight line length of a sail measured 8 If a sail is slowly untrimmed (the is eased), the from the luff (leading edge) to the leech (trailing edge), angle of attack decreases. The region of the sail just Figure 10–2. behind the luff will begin to tremble, and, if the process is continued, the sail will begin to luff (flap) as it does 7 Angle of attack is controlled by sail trim angle. The when the boat is headed to wind. If, from the best trim angle of attack for maximum force should be the same position, the sail is slowly trimmed, the angle of attack whether close-hauled or on a reach. As boat angle to the will increase. The flow on the leeward (bulged) side of apparent wind changes, main sheet trim is changed to the sail will no longer be able to follow the more curved Apparent Apparent Apparent Wind Wind Wind

20 20 20

Close Hauled Reaching Beam Reaching Figure 10–3 Angle of Attack Held Constant

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contour and will separate from the sail. Although the sail will remain filled with no obvious change in appearance, the flow will separate from the leeward side of the sail. The force generated will be reduced Since the boat is slowing at this increased angle of attack, the sail is said to be stalled. This is easily visualized in Figure 10–4, which shows the air flow around a sail section for vary- ing angles of attack using streamlines.

9 Sailing directly downwind, even with the sail untrimmed so that the rests against the leeward shroud, the angle between the apparent wind and the sail is much greater than 20 degrees. On this point of sail, the air flow is separated from the leeward side of the sail. The force generated on the sail is primarily a function of the sail area exposed square to the wind. The sail is ineffi- cient while running compared to other points of sailing, Figure 10–5.

Normal Flow

Luffing Figure 10–5 Air Flow, Sailing Directly Downwind Draft Too Far Forward

Average Wind Stalled Leech Too Tight and Draft Too Far Aft

Figure 10–4 Angle of Attack and Streamlines Light WInd Draft Fresh WInd 10 Airfoil draft is the depth of a sail, that is, the maximum distance measured from the chord line to the sail at a Leech Too Loose particular section. Draft, of course, varies along the Figure 10–6 Sail Shape, Draft height of the sail. Sail draft is a second important factor determining the amount of lift generated by a sail. Sail the depth of the draft and move it aft to one-half the efficiency is governed by its ability to maintain strong distance between luff and leech, Figure 10–6. attached flow on the leeward side, and over as much of the sail area as possible. Draft depth and its fore and 12 Sail draft is controlled mainly by the outhaul. Draft aft location are among the key variables that determine position is controlled by halyard and cunningham. On whether the airflow will stay attached to the sail. racing rigs sail draft is controlled by bend. Leech and foot tension also provide minimum draft control. 11 In moderate conditions, keep sail draft fairly large, with Zippers along the length of the foot near the boom on maximum draft located about one-third to one-half the some either gather or release fabric to change distance from sail luff to leech. In higher winds, reduce draft. A flattening reef, a reef of 8 to 12 inches/ 20 to 30 draft and move it forward to approximately one-third of centimeters just above the boom, will achieve the same the distance from luff to leech. In lighter winds, increase effect. To move draft forward, increase luff tension. To move draft aft, decrease luff tension.

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ficiency obtained, the more critical the trim. With a high aspect ratio , it is more difficult to obtain and Aspect Ratio maintain this efficiency. A low aspect ratio mainsail will 13 In general usage, the aspect ratio of a sail is taken to be have a broader range over which trim efficiency can be the length of the luff to the length of the foot. Applied to maintained. mainsails, aspect ratio is the quotient of the luff length of a sail, divided by the foot of the sail. For a luff length of 30 feet and a foot length of 10 feet, the aspect ratio is Twist three. High aspect ratio rigs are beneficial when sailing 15 Sail twist is the change in the angle between the sail close to the wind. They are inherently more efficient on chords as sail height above the deck increases. Mainsail upwind points of sail and the taller masts reach heights twist is controlled by sheet tension, tension, where the breeze may be stronger, Figures 10–7 and and mainsheet traveller setting. It is impossible to sheet 10–8. a sail tightly enough to eliminate all sail twist even if it High Aspect were desired. Twist is used to advantage. Ratio Low Aspect 16 Friction between the wind and the sea causes wind speed Ratio to be reduced near the surface of the sea. The wind veloc- ] ity at the masthead can be much greater than the velocity at deck level, requiring that the sail be set at different angles of attack along its height.

17 Control of the twist in the sail helps establish the best angle of attack in each sail section from deck to mast- head. Figure 10–8 shows the percentage of the true wind speed felt at various heights above the water in a range of wind conditions. This information can be used to set sail twist. The change in apparent wind direction with height above the water determines the best angle of at- Figure 10–7 Low And High Aspect Ratio Rigs tack on that section of the sail. A fair amount of twist is needed in light winds, but very little twist is required as the wind freshens. Masthead 50 Feet Above Sea 40' a e S th Control and Trim o o 18 To optimize boat performance, monitor wind direction m a 30' S e S d constantly, trim the sails carefully, and retrim them n d a n promptly as conditions change. A wind vane on the mast- d a n i d W n head, clear of the sails, shows apparent wind direction t i 20' h g W i at that level, Figure 10–9. Yarns placed on shrouds and L e t a r stays indicate wind direction closer to the deck level. e d 10' o M 19 telltales should be placed about six to twelve inches/

Gusty / 30 centimtres aft of the luff at three different heights: Deck 10 Feet Deck Above Sea one relatively near the foot, one about a third of the way 20% 40% 60% 60% 100% to the head, and the last one about two-thirds of the way Local Wind: % of Masthead Wind to the head. Tape yarn to the sail on each side, or pass Figure 10–8 Wind Variation With Height it through the sail with a sail needle and knot the yarn close to the sail’s surface on each side, Figure 10–10.

14 The concept of aspect ratio can be applied to underwater 20 When a sail is properly trimmed, yarns on both sides of blades (keel, rudder, etc.) as well as to the mainsail. It the sail will stream aft. When sailing too high (close to is a measure of flow efficiency. However, the higher ef- the wind) or if the sail is under-trimmed, even before the

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luff breaks, the weather-side telltales will flutter and fly up. Sailing too low (off the wind) or overtrimming will cause the lee flow to separate and the lee telltales will flutter and gyrate in the stall zone eddies, Figure 10–11.

21 Yarns placed at other points on sails are also used. The yarns indicate the direction of flow at the points attached. Yarns may be placed along a mainsail leech at the batten pockets to indicate proper mainsail trim. Overtrimming the mainsail will cause a yarn to stream outboard and gyrate.

22 The sheet is the primary control of sail shape. It adjusts the angle formed between sail chord and apparent wind. The direction and amount of tension on the sheet controls the flatness and twist of the sail. The direction of sheet tension is in turn controlled by the location of the turning block through which the sheet runs. On , the turning block is often on a track mounted along the deck or along the gunwale. The mainsail sheet usually is attached to a traveller that runs athwartships under the boom. Use of the sheets, traveller, and boom vang for controlling sail shape during different conditions is covered in another section.

Figure 10–9 Wind Vanes

Too Loose Trim or Too Close to Wind

Yarn

Yarn Correct Trim

Yarn

Too Tight Trim or Angle of Attack Too High Figure 10–10 Telltales Figure 10–11 How Telltales React to Sail Trim

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Summary 23 Wind force on sails powers sailboats either by smooth air flow creating aerodynamic lift or by a simple push when a boat sails downwind. Air flow across a sail creates a pressure difference that is a force similar to the lift and drag forces on an airplane wing in flight. Angle of attack is the biggest single variable in determining the force generated by a sail. Sail draft is a second important factor determining the amount of lift gener- ated by a sail. High aspect ratio rigs (tall mast and short boom) are beneficial when sailing close to the wind. A fair amount of twist is needed in light winds, but very little twist is required as the wind freshens. When a sail is lifting properly, telltales on both sides of the sail will stream aft.

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Homework: Section 10: Sail Shape Name ______

1. How does wind speed change affect the wind force? The wind force: a) varies directly as the wind speed. b) varies as the cube of the wind speed. c) varies as the square of the wind speed. d) does not relate directly to wind speed.

2. Which of the following is a prime concern when adjusting sails? a) True wind. b) Boat wind. c) Sea Breeze. d) Apparent wind.

3. Which side of the sail has lower pressure? a) Leeward. b) Windward.

4. The angle of attack is the angle between the sail chord line and the: a) true wind. b) sail draft. c) apparent wind. d) boat’s centerline.

5. The angle of attack of a sail: a) is unimportant to sail performance. b) is not affected by sail trim. c) should be the same on all points of sail. d) should be the same on a close reach or beating.

6. Draft, when referring to sail shape, is defined as the: a) height of the sail centered above the water. b) length of the sail from the head to the tack. c) maximum distance measured from the chord line to the sail. d) length of the sail section from the luff to the leech.

7. The location of the maximum sail draft is controlled principally by: a) luff tension. b) leech and foot tension. c) the force of the true wind. d) a line from the center of the sail to the deck.

8. In strong winds, draft should be: a) increased and moved aft. b) reduced and moved forward. c) increased and moved forward. d) changed only if the boat heels excessively.

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9. Aspect ratio is the quotient of sail: a) area to luff length. b) luff length to sail foot length. c) luff length squared to sail area. d) luff length to foot length squared.

10. High aspect ratio sails are beneficial when: a) jibing b) running. c) sailing close to the wind. d) reaching with apparent wind abaft the beam.

11. The twist in the sail is defined as: a) the heavy thread used to patch the sail. b) a result of improper bending on of the sail. c) the change of apparent wind direction from the deck to the truck. d) the change in angles between sail chords as sail height above deck increases.

12. Pieces of yarn are attached to the sail to indicate trim. A sail is correctly trimmed when: a) the windward yarn is twirling. b) both leeward and windward yarns point forward. c) the leeward and windward yarns are streaming aft. d) the leeward yarn points forward, the windward yarn points backward.

Sail