Wind. Section 9. Sea Breeze. Dead Air. Land Breeze. Dead Air. 9 Wind 69

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1 9 69 Section 9 Apparent. The vector sum of the true wind and the wind created by the movement of the boat. direction and speed as they appear to an observer on a moving boat. Wake. Water surface turbulence left by a moving boat. Attached low. The movement of particles along a surface, such as the flow of air particles along the leeward side of a sail. Back. Change in the direction of the wind in a counterclockwise direction in the northern hemisphere and a clockwise direction in the southern hemisphere. See Veer. Boat. The so-called wind produced by the movement of the boat through the water. Bear Away. To change course so thay the boat alters course away from the wind. Sea Breeze ead Air Header. A wind that shifts ahead of a boat sailing close-hauled, forcing it to change course to avoid pinching or luffing. Usually a good time to tack. Puff. A short gust of wind. Speed Gradient. A marked difference in speed between two adjacent winds. Vector. A quantity that has both magnitude and direction, commonly represented by an arrow. The length of the arrow represents the magnitude; the direction in which the arrow flies represents the direction in which the quantity acts. Veer. Change in direction of the wind in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. See Back. 1 is a sailboat s source of power, and understanding it is important. earn the wind s behavior as it interacts with a boat s sails. Anticipate shifts in strength and direction. The goal of this section is to help gain this understanding. 2 is derived from pressure differences induced by thermal changes. The sailor needs to understand these basic changes; they are the principal causes for the direction and the speed of the surface wind, igure 9 1. and Breeze ead Air igure 9 1 Thermally-induced Breezes

2 The wind felt on the boat has three characteristics: speed, direction, and steadiness. These three wind characteristics are influenced by the natural movement of the air itself, the movement of the boat, and nearby terrain. 4 speed may be stated in knots (nautical miles per hour), in mph (statute miles per hour), or using the Beaufort Scale, igure 9 3. All speeds used in this section for wind, current, and boat speed are in knots. One knot equals approximately 1.15 statute miles per hour. 5 Note the difference in the designation of direction for wind and current. direction is the true direction from which it blows. Current direction is the direction toward which it flows. A north wind blows from the north; a northerly current flows toward the north. True 6 The velocity and direction of the true wind is that felt by a stationary observer. These elements of true wind are controlled by three major factors: (1) movement of the air in the prevailing general weather system, (2) the influence of heating or cooling of the earth s surface in the local area, and (3) local terrain. wind at the masthead is stronger than at the water s surface or at deck level. inally, local terrain affects true wind speed and direction. A shoreline deflects wind crossing it at an angle causing it to cross the shoreline more nearly at a right angle, igure A steep, high shoreline creates a wind-shadow zone for an offshore wind. The wind s speed is reduced because an offshore wind blowing off the top of a bluff does not reach the water s surface until a distance of approximately five times the bluff height. See igure An onshore wind may trap a pocket of stagnant air against the bluff with the main wind rising over it. Hilly terrain broken by prominent valleys running down to the shore may provide unexpectedly strong gusts in the valley. funnels down the valleys and accelerates. You will find knowledge of these phenomena valuable when hunting either more wind or shelter. The CPS and USPS Weather course materials describe the forecasting of these phenomena in greater detail. 11 An interesting weather phenomenon encountered by those sailing coastal waters is the movement of land and sea breezes. These are caused by the alternating differential heating and cooling of coastal land and sea areas. The land, particularly in summer, is warmer than 7 With temperature comparatively constant over water offshore, variations are very small. However, along shore and inshore where most of us sail, understanding the daily variations in wind flow caused by local land masses is important. ors must apply this knowledge of daily variation to the particular weather system air flow to be able to predict the surface wind flow direction at a particular time on a particular day. 8 The velocity of the wind is decreased by friction with the surface of the earth or water. Therefore, winds at higher altitudes blow at a higher speed than those on the surface. True Shore igure 9 2 as it Approaches Shore Water

3 9 71 BEAUORT WIN SCAE World Beaufort Speed Meteorological Number Organization Estimating Speed Effects Observed or orce Knots Mph km/hr escription Effects Observed at Sea Effects Observed Near and on and 0 under 1 under 1 under 1 Calm Sea like a mirror Calm Calm; smoke rises vertically ight Air Ripples with appearance Small sailboat just has Smoke drift indicates of scales: no foam crests steerage way wind direction, vanes do not move ight Breeze Small wavelets: crests of fills the sails of small felt on face: leaves glassy appearance, not boats which then travel at rustle, vanes begin to breaking about 1-2 knots move Gentle Breeze arge wavelets, crests boats begin to heel and eaves, small twigs in begin to break, scattered travel at about 3-4 knots constant motion: light whitecaps flags extended Moderate Small waves Good working breeze, ust, leaves, and loose Breeze meters high, becoming sailboats carry all sail with paper raised up, small longer: numerous good heel branches move whitecaps resh Breeze Moderate waves of boats shorten sail Small trees in leaf begin meters taking longer form, to sway many whitecaps, some spray Strong Breeze arger waves meters boats have double arger branches of trees forming. whitecaps reefed mainsails in motion, whistling everywhere, more spray heard in wires Near Gale Sea heaps up, waves 4-6 Boats remain in harbor, Whole trees in motion, meters, white foam from those at sea heave-to resistance felt in walking breaking waves begins to be against wind blown in streaks Gale Moderately high (4-6 All boats make for harbor, Twigs and small meter) waves of greater if near branches broken off length, edges of crests begin trees, progress generally to break into spindrift, impaired foam is blown in wellmarked streaks Strong Gale High waves (6 meters): sea Slight structural damage begins to roll, dense streaks occurs, slate blown from of foam: spray may reduce roofs visibility Storm Very high waves (6-9 Seldom experienced on meters) with overhanging land, trees broken or crests, sea takes a white uprooted, considerable appearance as foam is structural damage occurs blown in very dense streaks, rolling is heavy and visibility is reduced Violent Storm Exceptionally high (9-14 Very rarely experienced meters) waves, sea covered on land, usually with white foam patches, accompanied by visibility still more reduced widespread damage & 73 & 118 & Hurricane Air filled with foam, waves over over over 14 meters, sea completely white with driving spray, visibility greatly reduced igure 9-3 Beaufort Scale

4 72 9 the sea during the day and cooler than the sea at night. The sea breeze (wind blowing onto shore) and land breeze (wind blowing off the shore) usually extend to a distance of about thirty miles, both on and off shore, and extend to a height of a few hundred feet. On a sunny day, land warms much faster than water. As the air over land becomes warmer than air over the water, thermal expansion causes it to become lighter and rise. Cooler, denser air over water moves in to fill the space left by the rising warm air, generating an onshore or sea breeze, igure 9 4. The sea breeze begins in the late morning hours, usually not until 1100 or later as the land warms. In the late afternoon it dies away. 12 The sea breeze becomes the dominant wind of the afternoon if there is not an overpowering wind brought on by a strong weather system. After sunset, the land cools faster than the water, causing the reverse effect, an offshore or land breeze, igure 9 5, which blows gently out to sea until morning. 13 This daily cycle causes a reliable sailing wind in some localities. The system of sea and land breezes is superimposed on the general wind pattern created by a more powerful weather system. If the weather system also causes an onshore breeze, the addition of the thermal sea breeze in late afternoon may create more wind than small boat sailors desire. ay 14 Also, as cloud cover burns off, rapidly increasing thermal generation, the sea breeze will increase. The cool- surface air flow will move further and further offshore. reed of the surface friction and veered by the Coriolis effect (a deflecting force acting on the wind and caused by the earth s rotation), the wind pattern shifts from a light, perpendicular-to-the-shore flow at noon to a strong veered flow at 1600 (in the northern hemisphere). Under cloud cover, boats are sailing in a light, shallow, local breeze. In the sunshine, boats are sailing in a strong, deep, veered flow from aloft. 15 Conversely, if the weather system s pattern creates a light offshore wind, the onset of a sea breeze will cancel the offshore wind, resulting in a calm along the shore. Plan to take advantage of these thermal effects during a day of sailing. Boat 16 When a boat moves through the water on a calm day, it also moves through the air above the water, creating a wind effect felt on the boat. This wind is called boat wind (BW). Boat wind has a speed exactly equal to that of the boat and a direction opposite to the boat s course, igure 9 6. Night Sun Warms and Mass Heated Air Rises Air Over Warm Water Rises igure 9 4 Sea Breeze igure 9 5 and Breeze

5 If a boat is motoring dead downwind at five knots in a five knot true wind, a sailor standing on deck would feel no wind at all. The apparent wind would be zero. If the boat turned and motored directly into the wind, the boat wind and true wind would reinforce each other and the apparent wind would be on the nose at 10 knots. or these simple situations, the direction and speed of the apparent wind can be calculated intuitively. 19 diagrams offer a relatively simple method of calculating the apparent wind from the boat wind and the true wind. Vectors 20 Any quantity having both magnitude and direction is a vector. Vectors can be represented graphically by a length of line with an arrowhead to show the direction in which the force acts. Examples include wind force vectors drawn through centers of effort of sails, and water force vectors drawn through centers of lateral resistance of hulls. 21 Velocity can also be represented by a vector. A velocity vector can be drawn as an arrow pointing in the direction of motion. Its length is scaled to represent speed. igure 9 6 Boat. Apparent 17 A crew member on the deck of a boat underway feels neither the true wind nor the boat wind, but a combination of the two, the apparent wind. The boat s wind indicator shows apparent wind, and sails are trimmed to apparent wind. 22 The wind velocity vector affecting the sails is the result of the interaction of the true wind and boat wind. velocity vector diagrams are used to illustrate this interaction. diagrams are presented and explained on the next page.

6 74 9 True 180, 4.0 KN Boat, 090, 3.0 KN Apparent 143, 5.0 KN 24 igure 9 7 is a wind diagram for a boat headed east at three knots with a true wind of four knots from the south. In constructing the diagram, north was assigned to the top of the page. A scale of one-knot-equals-oneinch / 2.5 centimeters was chosen for simplicity, but has been reduced in the illustration. The true wind and boat wind vectors were plotted to scale with the arrowheads at the boat and labeled True and Boat, respectively. The parallel construction lines were plotted. The diagonal from the intersection of the construction lines to the origin was then drawn, an arrowhead added and the vector labeled Apparent. The vector s length was measured and found to be five inches / 12.5 centimeters. At one inch / 2.5 centimeters per knot, five inches / 12.5 centimeters equals five knots. The direction of the Apparent vector was measured at 143 degrees. igure 9 7 Apparent iagrams 23 diagrams offer a simple way to calculate the apparent wind. The wind diagram may be constructed using a course plotter and dividers with any convenient scale such as one inch or 2.5 centimeters equals one knot. The apparent wind force acts through the combined center of effort (CE) of the sail plan. or ease in constructing the diagrams below, the vectors are drawn as though they act on the mast at the deck level. a) raw an arrow to the mast in the direction in which the boat wind is blowing. raw an arrowhead at the mast and scale the arrow to the speed of the boat wind. abel the vector Boat. b) raw a second vector to the mast in the direction of the true wind. Add an arrowhead at the mast and scale the arrow to the speed of the true wind. abel the vector True. 25 igures 9 8 through 9 11 illustrate four wind diagrams, each for a boat wind of three knots and a true wind of six knots from the south. The figures show the variation of apparent wind with boat direction relative to the wind. igure 9 8 is for a sailboat going to windward close-hauled. igure 9 9 is for a sailboat sailing on a close reach with the true wind abeam. igure 9 10 is for a sailboat sailing downwind on a broad reach. igure Boat 225, 3.0 KN True c) rom the tail ends of the arrows, add lightly penciled construction lines, one from the boat wind tail parallel to the true wind vector and the other from the true wind tail parallel to the boat wind vector. The intersection of these construction lines is the location of the tail end of the apparent wind vector. raw the vector from this point to the mast. abel the vector Apparent. Its direction is the direction of the apparent wind. Its length is proportional to the apparent wind speed. igure 9 8 Close-hauled Apparent 195, 8.4 KN

7 9 75 Boat, 270, 3.0 KN Boat 351, 3.0 KN True 180, 6.0 KN Apparent 207, 6.7 KN Apparent 189, 3.0 KN True 180, 6.0 KN igure 9 9 True Abeam Boat 315, 3.0 KN igure 9 11 Running 9 11 is for a sailboat sailing nearly dead downwind. These diagrams reveal interesting relationships: a) The apparent wind direction always lies between the direction of true wind and boat wind. igure 9 10 Broad Reach True 180, 6.0 KN Apparent 209, 4.4 KN b) The apparent wind direction always lies forward of the true wind. The apparent wind speed may be greater or less than that of the true wind speed. c) The direction and the speed of the apparent wind changes if the speed or the direction of either the true wind or of the boat wind changes. 26 These relationships have important practical implications. a) The greater the boat speed, the farther forward the apparent wind. Since it is the apparent wind to which a boat must point when sailing close-hauled, a fast boat may not point as high as a slow one. b) A sudden puff (increase in the speed of the true wind) causes the apparent wind to shift aft. While this effect is temporary, it allows an alert helmsman to point up in a puff and thereby gain toward wind-

8 76 9 ward. This is called a velocity lift. If a sustained true wind shifts aft (not merely a puff) this too enables the helmsman to point higher and is also a lift. c) A lull is a temporary sudden drop in the wind velocity. A lull causes the apparent wind to shift forward, temporarily forcing the helmsman to bear away or risk losing headway. This temporary change in apparent wind speed and, therefore, direction is called a velocity header. If a sustained true wind shifts forward, it also requires the helmsman to bear away and is also a header. Apparent irection d) When coming about, the apparent wind always shifts to the disadvantage of the sailor trying to work to windward. e) riction between the wind and the water s surface makes the speed of the true wind at the surface less than that aloft. Therefore, the apparent wind direction at the top of the mast will be further aft than at deck level requiring the set of the sail to change from masthead to foot. Trim the sail to retain some twist to maintain attached flow on the sail uniformly from the masthead to the foot. f) In light winds, some sailors use the engine, if so equipped, to increase boat speed and consequently, apparent wind speed and bring the wind direction forward. A broad reach where the genoa will not fill can be changed to a beam reach filling the sails by adding a modest push from the engine. Such engine use can make cruising under sail more pleasant. Resolving orces 27 The forward motion of a boat under sail results from the ability of the wind force () acting on the sails to propel the boat to overcome the resisting forces of hull friction and waves. This propelling wind force on the sails () is applied approximately perpendicular to the boom. To understand the propelling effect, resolve the force into two component forces, one perpendicular to the centerline of the boat and one parallel to the centerline of the boat. As an alternative, components parallel and perpendicular to the boat s course made good, considering leeway angle, could be used. The difference is small, igure igure 9 12 Resolving orces 28 The force perpendicular to the centerline of the boat is the lateral force () causing heeling and leeway: sideways drift. The force parallel to the centerline of the boat is the driving force () producing forward motion, igure boat hulls are designed with hull lines to provide minimum resistance to forward motion, and maximum resistance to lateral motion or leeway. Boat speed depends on the wind force on the sail driving the boat forward. 29 The lateral force () acts to cause leeway and to heel the boat. The stability of the hull acts to resist heeling as discussed in Section 7, Stability. The hull and a keel (or centerboard) minimize leeway. The area of the underwater hull and keel are made large enough by design to permit only a small amount of leeway, creating a lateral resistance force on the boat equal and opposite to the lateral force () on the sail.

9 9 77 orce riving orce ateral orce 31 The effect of hull and keel (or centerboard) in reducing the leeway angle may be demonstrated in a centerboard boat. the boat on a constant heading on a close reach with the centerboard full down. The angle between the wake and the hull shows an almost undetectable leeway angle. The angle may be observed by trailing a long line astern and seeing the angle between the line and the centerline of the boat. Maintain the same heading in similar wind and sea conditions, but pull the board up and watch the angle of leeway increase. igure igure 9 13 orce 30 esigners attempt to minimize leeway to enable a boat to sail to windward easily. Unfortunately, the designer cannot add more hull and keel to reduce leeway without increasing frictional resistance to the forward motion of the hull. Therefore, designers must compromise in hull design to reach balances between leeway, frictional resistance, speed, and comfort. 32 The boat s motion through the water also affects the lateral force on the hull. Just as air passing along the leeward side of the sail creates reduced pressure or lift on a boat that is underway, water passing along the windward side of the hull and keel creates hydrodynamic lift. Therefore, a boat sailing close-hauled develops less leeway than one just starting to accelerate after a tack. eeway Angle Course ed Boat Heading irection of ateral orce, irection of ateral Resistance igure 9 14 eeway

10 Total wind force can be resolved into drive and leeway components. The sailor s primary objective on all points of sail is to make the forward driving (drive) component of the wind () as large as possible and the lateral (leeway) component of the wind () as small as possible. In order to obtain maximum driving force the following two conditions must be achieved: a) The angle between the apparent wind and the sail must be correct to provide maximum overall force. When the angle is too small, the sail aligns with the wind, reducing force, and luffs, igure 9 15(a). An angle too large between the sail and the apparent wind cannot be detected as easily as an angle too small. Too large an angle reduces force and causes air to break away from its smooth flow around the lee side of the sail, resulting in less lift, igure 9 15(c). The flow of luff yarns is the best way to detect reduced lift and stalling of the sail. Trimming to achieve maximum force involves only the sail and the apparent wind, igure 9 15(b). Boat heading is not involved, except when the wind is abaft the beam. b) When reaching, and within reasonable limits, easing the sheet to permit the boom to swing out causes the force to point further forward, tending to increase the drive component and to reduce the leeway component. Conversely, trimming the sheet to hold the boom at a smaller angle with the boat s centerline causes force to point more nearly abeam, thereby increasing leeway and reducing drive, igure 9 15(a). Sheets should be eased as far as possible short of luffing to get the greatest drive. When in doubt, let it out. 34 When bearing away from sailing upwind to sailing downwind, from reaching to running, the apparent wind speed is reduced and the resulting driving force is also reduced. As a consequence of all these effects, the maximum drive and boat speed are obtained with the apparent wind approximately abeam. 35 Study igure The sail can be trimmed for maximum force when sailing close-hauled, but it is impossible to get large drive forces because the boom must form a narrow angle with the centerline of the boat in order to sail close to the wind. 36 If the boat is permitted to bear away from the wind, ease the sheets to maintain maximum force. Otherwise, the smooth flow of air over the lee side of the sail will break down, resulting in loss of force and speed. As the boat continues to bear away from the wind and the sheets are eased to maintain maximum force, the drive component of force continues to increase. Apparent irection (a) uffing orce Reduced igure 9 15 Effect of Trim (b) Maximum orce (c) Stall orce Reduced

11 9 79 Apparent irection (a) Beam Reach igure 9 16 Effect of Heading (b) Close Reach (c) Close Hauled 1. Boom angle to the apparent wind is constant 2. Boat is rotated while boom is held still 3. "" remains at the same magnitude and direction in each picture 4. Other vectors ("" and "") rotate relative to the axis of the boat) 37 The speed of the apparent wind decreases as the true wind direction moves farther aft. This results in a decrease in force, partially offset by an increase in the drive component of the total force. As apparent wind moves abaft the beam, another problem emerges. Shrouds and spreaders may prevent the sail from being trimmed at the optimum angle to maximize force. Thus the use of a sail as a true airfoil is limited to points of sailing on which the apparent wind is on or forward of the beam. 38 The direction of the apparent wind will change with height because the speed of the true wind increases with height. shape is allowed to twist slightly to adjust for this change in apparent wind. This twist is controlled by the position of the sheet lead, sheet tension, and boom vang. Summary 39 direction refers to the true wind and the direction from which it blows. Both the sea breeze and the land breeze result from local heating and cooling of land and water. The velocity and direction of the true wind is that felt by a stationary observer. Boat wind is the wind felt by an observer on a boat moving through the water on a calm day. When the wind is blowing, a crew member on the deck of a moving boat feels neither the true wind nor the boat wind, but a combination of the two, the apparent wind. The boat s wind indicator points to the apparent wind, and sails are trimmed to the apparent wind. force and, consequently, boat speed are highest when the apparent wind is abeam. or sails to develop maximum driving force, they must be trimmed just enough to avoid luffing. When in doubt,

12 80 9

13 9 81 let it out. Homework: Section 9: Name 1. Which of the following is characteristic of the true wind? It is: a) unaffected by local terrain. b) stronger at deck level than at masthead. c) unaffected by friction with the surface of the water. d) the speed and direction of a body of moving air as noted by a stationary observer. 2. When air flows from the sea to the land, it: a) will fall because of the more rapid heating of water than land. b) will continue even if there is a strong weather system ashore. c) will be deflected so as to flow more nearly at right angles to the shore. d) usually occurs in the early morning before the sun has risen very high. 3. An offshore breeze coming off a high bluff: a) will create a wind shadow just above the bluff. b) will create a very strong breeze below the bluff. c) will result in a pocket of stagnant air just above the bluff. d) may not touch the surface of the water for a distance of approximately five times the height of the bluff. 4. The sails must be trimmed to the: a) boat wind. b) true wind. c) apparent wind. d) combination of boat wind and apparent wind. 5. A sea breeze is: a) intensified at night when the sun is not a factor. b) displacement of solar heated land air by water cooled sea air. c) stimulated by frontal activity, particularly near thunderstorms. d) encountered only off shore some distance from large land masses. 6. A boat is on a true course of 45 at 5 knots with a north wind of 10 knots. Which of the following diagrams correctly shows the apparent wind? True a) Apparent Boat True Boat c) Apparent True Apparent True Apparent b) Boat d) Boat

14 If the true wind speed increases while beating, the: a) helmsman should bear off. b) apparent wind moves forward. c) apparent wind speed decreases. d) helmsman can point up more and gain ground to windward. 8. Experiencing a header means the: a) boat wind falls off. b) true wind moves aft. c) apparent wind moves forward. d) wind velocity suddenly increases. 9. Apparent wind direction is: a) an illusion. b) shown by the luff yarns. c) usually aft of the true wind. d) the wind for which the sails should be trimmed. 10. The sideways lateral force of the wind on a sailboat: a) is unbalanced when the boat is underway. b) minimizes the unbalancing heeling moment. c) is resisted by water forces on the freeboard. d) is resisted by forces on underwater surfaces of the boat. 11. The forward driving force of the apparent wind on a sailboat is greatest when: a) true wind is abeam. b) the boat is beating. c) apparent wind is abeam. d) the boat is going directly downwind. 12. To provide maximum overall drive force except when the true wind is abaft the beam, the sail should be trimmed: a) 2 to 5 to the boat wind. b) 30 to 40 to the true wind. c) 2 to 5 to the apparent wind. d) to the optimum angle as detected by the luff yarns.