Volume 5, Physics 208
last updated on May 16, 1999
Air effects are used to drive sailboats. In combination with water effects, sailboats can be made to head partly toward the direction of the wind. Golf balls are hit with a low angle of launch but with backspin so that the ball rises to a great height and with long hang time to get a large distance. The curve in baseball is a tactical weapon to keep the batters guessing and less able to hit the ball. We will study the air and water effects on various sports. We will start with the curve ball in baseball.
The range of a ball launched at 40 degrees elevation and a speed of 90 mi/hr in air is about 1/2 the range of a similar ball launched in vacuum. Air resistance is a drag force which acts opposite to the direction of the ball's flight. For a thrown ball, the direction of the ball's velocity and hence the vector force changes continuously.The air resistance also changes it's magnitude. A ball standing still in the air has no air resistance. At high speed, the air resistance is quite large [think of the force on your hand in the airstream when you are driving fast]. The precise calculation of air resistance affects is beyond the scope of this course but we will look at some approximate caculations.
We have studied collisions in Volume 2. We can think of air resistance as the collision between air molecules and the ball. The collision transfers momentum to the air molecules and the reaction force acts on the ball to slow it down. Since there are so many air molecules, the molecules collide amongst themselves as well as with the ball so that a rather large scale coherent motion of the air results from this interaction.
We will describe the lift phenomenom in the familar terms of momentum transfer and Newton's laws of motion. The only new thing is the idea that air and water are viscous. They will stick to a surface and drag the fluid along with its motion. You can see that old fan blades have dust stuck to the blades when intuition tells you that the rushing air should have blown it away. It won't because the air next to the blade does not move.
There are further complications in the air-ball interaction. The air will stick to the surface of the ball and also to itself so that a ball moving through air will drag air along with it. This effect is called viscosity. Viscosity of air is the primary reason for streamlined designs which allow objects with large surface areas to efficiently pass through the air.
Air effects are used as part of the games of baseball and volleyball or golf. The balls are going fast so that the air effects are large. These air effects not only slow the balls down, but can also cause sideways forces which produce curve or drop balls, or rising balls. From our knowledge of Newton's third law that for every action, there is an equal and opposite reaction, we can deduce that the sideways force on the ball must be accompanied by a sideways force on the air. For a curve ball, the air will be forced sideways.
The figure above shows the deflection of air for a sideways force on a ball. While the air deflection is subtle, the physics behind it is the same as for the deflection of air by the blades of a helicopter. The air and ball start with zero momentum in the y direction. The ball obtains momentumm in +y and the air gains momentum in -y so that the net y momentum is zero as at the beginning.
To produce a sideways force, the pitcher must throw the ball with a spin. For a right-handed pitcher, the curve ball curves away from a right-handed hitter. Following the coordinate system above, the ball curves towards +y while the ball spins with a counter-clockwise (CCW) spin. The question is why does the ball feel a force in the +y direction with a counter-clockwise spin while being thrown in the direction of +x? The rules work for spins with the clockwise spin as well, the ball will be curving toward -y in this case. Why does the combination of translational motion and spin produce sideways forces? There is no obvious mechanism like a propeller driving the air toward -y or even an air scoop to do so. In fact, the curve ball will work perfectly well with a very smooth ball with no seams. Nevertheless, basic physics implies that the curve ball MUST be pumping air toward -y.
Consider a ball with zero translational velocity but spinning rapidly CCW. Since air has viscosity, the air next to the ball's surface will stick to the ball and be spinning with the ball. This air in turn will drag the air next to it so that the spinning ball will create a swirl of air moving in a circle around the ball. The volume of air which is set in motion is quite large and exceeds the diameter of the ball. Since the air had no motion before the ball started spinning, the ball has given the air momentum. This produces a equal and opposite change in momentum of the ball and will slow the ball's spin over time. There is no sideways force on the ball but there is a torque which slows the spin down. The air to the right of the ball is driven to +y (We'll call this upwash) and the air to the left of the ball is driven to -y (downwash) with equal and opposite momentum. [ditto for +x and -x]. How is this symmetry broken?
If the ball has velocity toward +x, the air behind the ball with downwash continues downward. The air ahead of the ball with upwash is intercepted by the ball and does not continue upward. The moving ball has created an up/down asymmetry such that downwash is favored. The reaction to this is a force on the ball toward +y. If the ball has a CW spin, the roles of upwash and downwash are exchanged so that the downwash is intercepted by the moving ball and the ball is deflected toward -y. A left-handed pitcher throws this kind of curve.
The quantitiy of the air pumped depends on the speed of the downwash and consequently depends on the magnitude of the spin of the ball. More spin produces more downwash. Our figures above were shown with the air stationary for the observer and the ball is moving to the right. Tests of ball deflection are made in wind tunnels where the ball is stationary while the air moves from right to left. A photograph of the downwash of the air is shown in the figure to the left. If the downwash is actually going down, the ball is lifted. The effect is called lift. This is the basis for the flight of airplanes. For a golf ball hit with backspin (spin around a horizontal axis) the ball is lifted and so has a longer hang time and hence a longer flight. In tennis, the ball is hit with topspin so that it produces upwash producing a downward acceleration of the ball.
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| Figure of backspin golf ball. The air flow is as seen by an observer at rest with respect to the CM of the ball, not as seen by an observer at rest with the undisturbed air. |
To summarize, the ball with translational velocity and spin will pump air toward -y when it has CCW spin so that net momentum is transferred to the air in the -y direction. The ball is deflected toward +y. For a ball with CW spin, air is pumped toward +y and the ball is deflected toward -y. A right-handed pitcher normally throws with a three quarter arm motion while releasing the ball over the index finger giving the ball a spin around a tilted axis. the ball usually curves down and away from a right-handed batter. A fastball is released below the 2nd finger and with the CW spin so that the ball tends to bend up and toward the batter. This is sometimes called the Magnus effect.
The spin of the ball produces deflections of only 5-10". The gravitational force is much larger and produces a deflection of 39" or more. However, since the gravitational deflection is a constant, the batter can easily correct for it. Even if the batter knows that the ball has spin [by observing the motion of the seams of the ball], he may not be able to predict the flight of the ball to within the 1/2" accuracy needed to obtain a fair hit. The rising fast ball does not actually rise, it still falls 39" by gravity but is deflected upwards by about 5" so the net downward deflection is still 34". One can easily see this on a slow motion video. However, in real time, the batter interprets the ball to be rising because he automatically corrects for the gravitational deflection.
It has been observed that a spinning smooth ball [not rough like a baseball] can have either a Magnus OR an Anti-Magnus effect! This occurs because the flow of air over surfaces sometimes follows the contour and sometimes it breaks away in a systematic way. A slightly rough surface will make a turbulent boundary layer that sticks to the surface better than a laminar layer flowing on a smooth surface. The smooth surface flow is a little more unpredictable, it could break into full turbulence or it could make a turbulent boundary layer. If a turbulent boundary layer is produced on the side going against the translational flow, this will break the upwash/downwash symmetry in the opposite way thereby giving the Anti-Magnus effect. The production of the turbulent boundary layer by the seams of the ball produces the effects of the knuckleball. The whole story is the story of the Magnus effect; the Magnus effect embraces turbulence and viscosity. The following figure shows the various regimes for laminar flow, turbulent boundary layer and full turbulence as characterized by the Reynolds number. Baseball is played just on the edge of the various kinds of turbulence so that diverse air effects can be obtained.
The flow of air or water past an object depends on the velocity, the density and the viscosity of the fluid as well as the size of an object. For a given boat moving through the water, at very small speeds, the flow is laminar [very smooth] and it gets more and more turbulent as the speed increases. A smaller boat would have less turbulence than a larger boat at the same speed. To scale the behaviour of different boats and in different fluids, it was discovered that the fluid behaviour is very similar when the same Reynolds number is obtained.
R = v r D/h where v is the fluid velocity, r is the fluid density, D is the characteristic size of the object and h is the viscosity. The more viscous the fluid, the more drag it exerts. Molasses is much more viscous than water. The viscosity varies with temperature. Here are some examples.
| fluid | temperature oC | h (Pa-s) |
| water | 0 | 1.8x10-3 |
| water | 20 | 1x10-3 |
| air | 20 | 1.8x10-5 |
| glycerine | 20 | 1.5 |
The lower temperature increases viscosity so that rowing in almost freezing water increases the drag forces by almost a factor of 2 compared to summer time water temperatures. A baseball operates in the regime of R ~ 1x105 which is just on the boundary of going from fully turbulent to the turbulent boundary layer condition. This makes the flight of balls quite interesting. Please see the chart for the characteristic R for various kinds of flying objects in lift.html.
The knucle ball is devastating to the batter because of its unpredictable path. It may go left by 12" and then to the right by the same amount all in the same pitch in an unpredictable way. Even catchers of knuckleballs have trouble putting their mitt in the right place and they KNEW that a knuckleball was coming. The ideal knuckleball is thrown with hardly any spin. If the ball rotates by 1/4 turn, the slowly moving seams make the ball "look" different, and air is pumped in different directions. The unpredictable forces cause displacements of the ball away from the parabolic gravitational arc. Since the knuckleball has such an unpredictable flight, the pitcher can throw balls with relatively small velocities with a good chance that the ball cannot be hit. By pitching with vastly different velocities, the timing of the swing is affected to further trouble the would-be hitter. Good knuckleball pitchers are rare. It seems that the technique for throwing a ball such that it rotates only 1 or 1/2 times on its flight is very difficult to develop.
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Lift was first discovered by passing a rigid plane through the air. A wing consisting of a barn door tilted 5 degrees upward and with translational velocity toward +x will produce a circulating pattern of air as well as a spinning ball. The wing gets out of the way of the downwash and intercepts the upwash thereby producing lift. In order to produce lift with the least amount of undesireable drag effects over a wide range of angles, the wing is made to have a streamlined shape. The figure on the left shows the flow of the air over a wing with a positive angle of attack producing a downwash and hence lift. |
| Figure of wing producing lift upwards with motion of wing to the left. The wing has a positive angle of attack. The flow is as seen by an observer at rest with the wing. |
To verify the existence of the circulating air around a plane, try this excercise. Take an index card or a business card and give it a little shove forward. It will fly forward and will spin with a backspin. You can improve this by making index card cut to 1" in cord and 4" in span. This will fly quite well and it always will have backspin. If you try to give it topspin, the card won't fly. It will stall and then begin backspin on its own.
The circulation produced by a spinning ball occurs because the air being viscous is dragged along by the surface of the ball. The viscosity of the air is also at the heart of the circulation around a wing. With a positive angle of attack, the latter part of the wing is headed downwards. From the point of view of the observer in still air, the air adjacent to the wing sticks to the top of the wing and is pulled downward. As the wing passes, the downward motion continues producing the downwash. The air at the bottom of the wing, likewise is accelerated downward. Since the air in this circumstance acts like an incompressible fluid in a large volume, the downward flow of air must be accompanied by an upward flow elsewhere. In front of the wing an upwash is produced. The wing gets out of the way of the downwash but tends to intercept the upwash so that a net downward momentum of air is produced. The wing pumps air downwards to produce the lift. This widespread motion of the air occurs because the pressure wave caused by the wing travels at the speed of sound and so affects the air's motion way ahead of the wing. The following is a figure of the air flow pattern around a wing approaching with a 2 degree angle of attack from the point of view of an observer in the still air. The wing is moving from left to right in this picture. [This view of the velocity field of the air was generated by a computer simulation at the U.W. aeronautics department.]
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| The vector field of the air by an observer at rest with the undisturbed air. The wing has an angle of attack of 2 degrees and is moving from right to left. |
To dramatically illustrate this effect, you can hold a wine bottle and direct tap water on one half of the bottle. The water will curve around the bottle and be directed sideways. This pumping of the water sidways, of course, produces a opposite sideways force which is lift. Another demonstration of this is to run a vacuum cleaner hose blowing air upwards. Put a light ball in the air stream. It will float above the hose in a fairly stable position instead of being blown away as you might first think. If the ball drifts to the outside of the stream, the larger flow of air on the inside gets bent by the ball and deflects the ball toward the center. This is a condition for stable equilibrium.
Since the bottle will be rather heavy compared to the lift force, you may not feel the lift. Try another experiment. Hold a piece of paper in front of your lips such that the paper droops downwards.
The sail of a sailboat can be thought of as a wing mounted vertically. The lift produced by the sail is horizontal. A sailboat can sail at 45 degrees to the upper right of the picture below with the sail set in an orientation similar to that of the wing shown above. This is called sailing close hauled. In order for the boat to have constant velocity in this direction, we must cancel out the sideways force on the boat from the sail. The keel will have a negative angle of attack relative to the water thereby producing upwash in the water. The combinationn of the keel force and the sail force produces a net force in the direction of motion. This net force in turn is cancelled by the drag force acting opposite to the motion. The equilibrium of the sail lift, keel lift and the drag force on the hull produces a steady velocity. We can think of steady state sailing as the sailboat "falling" at it's terminal velocity.
In order to go faster in a sailboat, you must either reduce the drag or increase the lift of the sail. Sailboat designers try to do both. The drag can be reduced by better shapes and less whetted area. The whetted area is reduced by reducing weight. The lift of the sail can be increased by larger area. However, too large a sail is not practical. The sail can be made more efficient so that it produces a lot of downwash without increasing the drag. This is accomplished by shaping the sail so that the flow of air is smooth with little turbulence. Large scale turbulence is a killer of lift. If the air is randomly swirling around, it cannot also be the circulation around the wing. Since the wind velocity varies with height [again because of the viscosity of air], the angle of attack of the sail must be systemtically adjusted so that the sale produces maximum lift at every part. The fastest sailboats use a sail that is an almost rigid wing mounted vertically on hull that is very light to minimize drag. The catamaran design gives a very light hull which has stability against rolling by virtue of the geometry.
A sail that is led by a mast is very inefficient because the airflow is greatly disturbed. The foresail, the jib doesn't suffer from mast disturbance and so has a much higher efficiency. Recognizing this, many 2-sail boats (sloops) have been made with very large jibs and very small mainsails behind the mast. These boats, unfortunately, are much more difficult to manage because the jib requires very big forces and must be manhandled with every tack. The main sail rig is self-tacking. The wing sail of the C class catamaran has the mast enclosed by the wing surfaces thereby eliminating the spoilage of the air flow of the mast. There are simple remedies for the flow problem. A small boat, the Laser threads the mast in an enlarged pocket in the front of the sail giving a pretty good flow pattern. This boat also has paid proper attention to the dagger board and rudder shapes to give excellent underwater lift characteristics.
Wind surfers sail at speeds that exceed the best dingys by 50-100%. How is this done? By using the athletism of the sailor, the wind surfer obtains a big thrust from the sail and a small drag from the hull. First of all, the windsurfer is very light so that the whetted area is small. This is further reduced in two ways:
The sails are fully battened and the mast is threaded in a deep pocket at the sail's leading edge so that the aerodynamics are excellent for producing lift.
Early Chinese sailing ships had full battens and multiple sheets on the battens' trailing edges. This gave the sailors excellent control of the sail shape over the entire height and so produced very efficient sailing. The sails of these Junks are of a balanced rig where the leading edge of the sail does not have mast interference so that the sails have quite good aerodynamics. Having about 1/3 of the sail ahead of the mast makes the sail controls simpler as well because the forces for rotating the sail are much reduced.
When sailing downwind, the sail acts merely as a resistance device so that the force becomes zero when the boat is at the same speed as the wind. Modern sailing tactics use tacking downwind. That is, the boat never goes directly downwind even though that may be the destination. Instead, the boat will sail partly sideways thereby producing lift with the sails and getting much bigger forces than just going directly downwind.
In a race between two boats the leading boat will try to keep the trailing boat in its downwash. The turbulent air produces very poor lift so that the trailing boat will have difficulty catching up.
Unlike the calculations for a pair of colliding objects, exact analytical calculations for a continuous medium like a fluid are not yet possible. Boeing aircraft designers will use numerical approximations which are then tested in wind tunnels. In spite of these difficulties, we can make pretty good approximate calculations using the idea of a dynamic pressure produced by the relative motion of a fluid. A head on collision with the flow produces a pressure which is just the dynamic pressure. Deflecting the air by a spinning ball or by a wing produces lift which is a function of the dynamic pressure. These calculations have proven to be quite successful but they must be used with some tabulated fudge factors which are obtained from measurements.
Dynamic Pressure = F/A = 1/2 CDr v2
where r is the density of the fluid and v is the flow velocity. These are the main determinants of the dynamic pressure. CD is the coefficient of drag. The density, r, of air is 1.3 kg/m3 and for water it's r = 1000 kg/m3. Because water has such a large density, water effects are much larger. Try running in a pool compared to through air! As a result, wings for water craft can be much smaller than the wings of an airplane.
Since Power = F*v, we have Power = 1/2 ACDr v3. Power requirements go up VERY fast with speed.
Here's a rule of thumb for estimating when air effects are significant. In the table on the properties of balls, the terminal velocity is listed. Terminal velocity is the velocity reached by a falling ball when the forces of air resistance upwards and the gravitational force downwards are balanced so the ball is no longer being accelerated. The ball continues to fall with a constant velocity which is called the terminal velocity. This varies from 325 mi/hr for a 16 pound shot and 95 mi/hr for a baseball to 20 mi/hr for a pingpong ball. Comparing a ball's speed to its terminal velocity is a good indication of how important air effects will be. A lauch velocity of 50 mi/hr for a shot is only 15% effect but it would be a 52% effect for a baseball. In baseball, the speeds of thrown and batted balls are a fairly large fraction of the terminal velocity so the air effects are big in baseball.
Like a baseball pitcher, the volleyball server can dish out a variety of spins or knuckleballs which will reduce the ability of the opponents to predict the flight and prepare for the attack. The job of the server is to deliver a ball which ends up in the fair area [target zone] and is fast and unpredictable. The curves and knuckleballs that are used increase the scope of the offensive serve.
If the server stands at the service line and must get the ball over the net, a parabolic trajectory will end up at the fair line on the other side. This does not allow much flexibility for aiming the serve. In high levels games of volleyball, the server may stand 10 m or more back away from the service line so that the flight of the ball takes a long time. The advantage of this serve is that the ball reaches it's peak well before the net and is heading downward into a big target area. You can have a high speed serve AND a big target. Since the trajectory of the ball is made unpredictable by air effects, the server can have it all!.... speed, unpredictability and a high percentage for hitting the target area.
Other tactics include the jump serve. The higher the launch point, the bigger the target area. If you have a good jump and hang time, then you should serve from the jump.
You can run much faster on a hard surface than in loose sand. The push with your feet on the sand transfers kinetic energy to the sand and this energy is lost for propulsion. The push on a hard track is transferred to a very large mass (the earth) and so has little kinetic energy. You save the energy for your body. In the water, the blades push against an even more elusive medium than sand: the water. When you push, the water is given kinetic energy which is lost. However, there is a way of decreasing the amount of kinetic energy that is lost by stirring up the water. This method is by using the lift from the rowing or paddling blade. In this case, lift is the in the horizontal direction in the direction of the velocity. Recall the basic physics laws:
To get big thrust, you need big momentum transfer to the water. This of course puts energy into the water. The momentum can be obtained with either big mass or big velocity ... it doesn't matter. However, for the KE, it costs much more energy to put a large velocity into the water [because of the v2]. To keep energy expenditure small, it's best to get the momentum by putting a lot of mass into motion with a small velocity. For a propellor, the most efficient blades would be very large blades that push large masses of water with small velocities. The limitation in the propellor size is that the propellor tips will reach a velocity that is too high and the water will cavitate [produce bubbles] so that the thrust is compromised.
This is the principle of Fan Jets on Boeing planes as well as big blades for rowing or paddling. However, one can even have a much bigger effective blade by using the principle of lift. The airplane wing affects the mass of air which is all along its path and not just the air around the wing at any given time. It gives a very large mass of air a smallish velocity. In contrast, a helicopter hovering in one spot is giving a rather small mass a big velocity. The helicopter needs much more power to fly than an airplane of the same weight. The mass of air affected by the plane per unt time is proportional to the wingspan times the air speed. The helicopter's air mass is proportional to the rotor radius squared.
The normal paddling stroke for UW students with rental canoes in the arboretum is to stick the blade in the water and then to pull back. This is the classical method which works by resistance. No lift is created. It's rather like going downwind in a sailboat. Only a smallish amount of water is propelled with a large velocity for a big force. Twin vortices are created at the edge of the paddle with no larger scale vortex motion as found in lift. The blade is taken out of the water aft of the entry position. This motion puts a lot of kinetic energy into the water for a given thrust force.
For racing paddlers, the blade is plunged into the water with a positive angle of attack. This transverse (vertical) motion creates sufficient lift that the paddle is lifted forward during this phase of the stroke. A study of the blade position shows that the blade is taken out of the water forward of the entry position. This is similar to the well rowed racing stroke. Since a larger amount of water is pushed back and with a smaller velocity, the kinetic energy given to the water is much less than the arboretum type stroke.
See the web article on the Physics of Rowing from Oxford University.
Present day competitive swimming utilizes the principles of lift discussed for rowing and paddling. The competitive swimmer is taught to scull with his hands. This is particularly evident in the breast stroke and the butterfly stroke. The swimmers use their hands like paddle blades and scull their hands outward then inward to produce thrust with the most efficiency. Even in the freestyle and backstroke, the swimmers use a sculling motion with an out and in motion. The hand is angled to give a positive angle of attack for each segment of the stroke. If you only pull your hand straight back, then your hand is performing the arboretum canoe stroke with poor efficiency.
Please see the web link, the science of swimming.
A well made single racing shell with a 75 Kg sculler at a racing velocity of 4.5 m/s will have a drag force of about 15 pounds or 67 Newtons. This is a rather small force so why does it take such muscular effort to row fast? Recall that Power =work/time = F*distance/time = Force*velocity.
To row with a constant velocity we must apply a force that counters the drag force so the power required is P = 67 * 4.5 = 300 watts. The efficiency of the sculls for applying the force is much less than 100% and the drag of the boat is larger than 67 N because of the slow-fast cycle of the boat so that the actual power output required of the rower is expected to be at least 20% higher. This means that the required power is about 400 watts. The duration for a 2000 m race is about 7 minutes so it is an aerobic contest. The 400 watt output required is approximately at the edge of human performance for free rowing.
To go faster, one needs to have higher power output and to have excellent technique so that the efficiency of the rowing cycle is as high as possible. Beginner and intermediate rowers are much, much slower than their power output would predict because of the inefficiency of the rowing motion.
Since racing shells are very light, the mass of the boat-sculler system is dominated by the sculler. This means that the heavier the sculler, the deeper the boat sinks into the water and the higher the drag force. This argues for lighter scullers. However, the bigger the athlete, the greater the power that can be produced. Which wins?
Let's make the argument for a boat which has about zero mass but has all the properties of a shell. Racing shells are designed to be very slim and long so that the drag is quite small. The main drag comes from the skin resistance [viscosity again]. For the average racing shell design, the whetted area varies as m2/3. Since the drag increases with whetted area, the power required is m2/3. [Whetted area is the area of the shell's surface that is actually in contact with the water.]
For a well-developed athlete, the aerobic power that is produced is dependent on the cooling potential of the athlete. The cooling depends on the area of the cooling surface [the skin]. The surface area varies as l2 or m2/3.
Power required/Power produced = m2/3/m2/3 = 1.
So potential speed is INDEPENDENT of the mass of the rower [in the approximation of a zero mass shell]. A similar analysis applies to swimmers. For realistic shells with 15 kg mass, the physics calculations slightly favor more massive rowers but not by much. Speeds of lightweight rowers and open weight rowers at World Championship events differ only by about 1% on the average. It's also noted that the start and the finish of the race utilize anaerobic energy and so further favors larger athlete since anaerobic power depends directly on mass. For coxed boats, the rowers have to pull the deadweight of the coxain [55 Kg] and so the bigger athletes have a big advantage. The coxed heavyweights are about 5-10% faster than the coxed lightweights.
A contest between a world-class flat water kayak and a scull would show that the kayak would have a higher average speed for a distance of about 500 m. The efficiency obtained by paddlers with well-designed and executed strokes is very high. The paddler will paddle with 100 strokes per minute so that the kayak doesn't slow as much between strokes when compared with the single sculler rowing with 35 strokes/minute. This gives the paddler less average drag on the boat for a given average speed.
However, over longer distances, the recruitment of more and larger muscles gives the aerobic advantage to the sculler. The racing paddler will use mostly the torso muscles for propulsion, but the sculler can also use the leg muscles. Having this extra muscle mass (and the fuel stored in it) available for propulsion will allow the sculler to win over the long haul.
Rowing with sliding seats evolved to the present form in the late 19th century. This is certainly not the most efficient of human-powered boats. There have been human powerd boats built which can go 2X faster. These boats use a bicycle chain type drive with a propeller. Drag is also reduced by having a hydrofoil system. However, these high tech boats race in a different category and tradition-bound boats dominate the rowing racing scene. Still, there have been some small but significant design changes that have improved boat speeds.
Reducing the average drag of the boats. The slim designs of the boats have reached their best and not much improvement has been obtained in the last few decades. However, the slow-fast cycles of the boats caused by the periodic rowing motion [30-45 strokes/minute] adds to the average drag. German boat designers in the 70s made a rowing rig where the rower's seat was fixed to the boat but the rowing rig [oarlocks] moved. This permitted the full use of the legs in the drive and at the same time reduced the slow-fast boat movement by about 50% thereby decreasing the average drag. These boats were used to win the world championships at that time. However the construction of these boats is expensive and the maintainance is difficult. At this time, this type of rig is banned from competition.
Improving the efficiency of the propulsion of the sculls. In the last decade the rowing community has switched from a slim blade design to a big blade [hatchet] design. The big blades have yielded higher speeds. The rowers like them because they are easier to row well. These big blades are especially good when used in conjunction with a rowing stroke which gives a large transverse velocity to the blade for a longish time. This means that the blade must enter the water at an angle of about 30 degrees from the bow. The combination of the blade size, aspect ratio and stroke mechanics produces much more LIFT in the stroke. This means that much less kinetic energy is given to the water thereby using the rowers power to propel the boat and less energy in stirring the water. Any propulsive action that stirs the water is energy lost to the propulsion of the boat. The lift of the blade is sufficiently large that it will actually produce a forward blade velocity during the stroke. You can see this at by checking out a study on the LIFT during the sculling stroke.
In the figures, shown, when the blade is at 90 degrees relative to the bow, the transverse blade velocity is zero and the propulsion is obtained by the blade slipping backwards where no lift is produced. We thank Dick Dreissigacker of Concept II oars for the pictures of his tests with the big blades in sculling.
That the blade is producing lift is apparent from the production of vortex motion of the water for a single vortex. A blade producing force only by drag will produce a double vortex that comes off the blade. In lift, a single vortex is left behind while the other vortex with opposite spin stays with the blade [circulation around the wing] until the blade is brought to rest.
In textbooks for flight and manuals for pilots, the convention is to describe the lift phenomenom in terms of the higher speed for air flow on top of the wing. The higher speed generates lower pressure in accordance with the Bernoulli Principle. Even physics publications on the curve ball suffer from using this conventional theory. This description suffers for several reasons.
The good things about the conventional theory include:
So, using conventional theory is a little like blindly following a recipe and so isn't a true theory from which one can obtain insights as to how things really work and can be applied to new and innovative situations. At the present good physics based calculations can be obtained using numerical methods which trace the evolution of finite elements of air when confronted with a moving object.
Questions.
1. If wings of a certain dimension can support a 700 kg airplane flying at 60 m/s, what is the relative area of the hydrofoils [underwater wings] to support a boat of 700 kg at the same speed. What is the relative wing area to support the same boat at 25 m/s?
2. If a cyclist can produce a sustained 300 watts output, he can propel a bike at 17 m/s. If he is to be able to propel the bike at 25 m/s, how much power must he produce?
3. If our 300 watt cyclist can propel a bike at 17 m/s , and you were to put him into a man-powered submarine, what speed could he sustain?
4. If a truck needs 200HP to travel at 60 mph in air, how much power does a submarine [of a similar size] need to travel at the same speed underwater?
5. A major league fastball is perceived to be rising from an otherwise straight trajectory. Is this really what happens?
6. A power boat can cruise at 20 mi/hr with the power set at 1/2 of the maximum power. Assuming that the boat is of a type that is a displacement hull and will not "plane " or otherwise change it's attitude, what is the expected speed at full power? What is the speed at 1/4 power? What is the ratio of energy expended to go 20 miles at each of the power settings?