10 – Sail 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 sailing 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 Sails 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 Sail 84 10 – Sail Shape 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 sheet 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 Sail 10 – Sail Shape 85 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 boom 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 mast 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 mainsails 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. Sail 86 10 – Sail Shape ficiency obtained, the more critical the trim. With a high aspect ratio mainsail, 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, boom vang 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.