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Propulsive Lift and You
By Dr. Jolie Bookspan

Unless you have very big feet (and webbed toes) you need fins. Since modern fins were introduced in 1930, we've been gliding around in a medium 800 times denser than air. But how? It's not equal and opposite forces. If Newton's third law were the reason, then kicking directly backward would push you forward. But you really don't get far that way. You have to kick downward to go forward, so another force must be involved. It's the same force that flies a frisbee and lofts a tennis ball when you put spin on it-- Propulsive Lift. Why read about propulsive lift? You'll be a safer, faster, more efficient diver. You'll use less air. You'll keep your joints happy, and you'll know what the dimples on golf balls do.

The Force That's With Us
Lift is the force that moves you forward through the water when you kick. It's also called propulsive force or motive force. Lift doesn't just mean "up". It's created in any direction that is at right angles (perpendicular) to the flow of water around you. Imagine water flowing past a ball that is moving forward in the water. Lift forces push equally because the ball is symmetrical relative to the flow of water around it. Since all lift forces are equal, there's no net push in any one direction from lift. In this case there's plenty of lift, but no propulsive lift.

How Propulsive Lift Works
The net amount of lift generated by a moving object can be in any direction. If you're not perfectly round, the direction is determined by changes in pressure, as the velocity of water flowing past you changes. Velocity is low when your shape or position creates a barrier, causing water to slow down. When water slows, the pressure rises by the same amount. (Velocity is inversely related to pressure.)
Where the water is not slowed, velocity stays high and pressure drops. Because things move from high to low pressure, propulsive lift is created and you move toward the low pressure zone. This is Bernoulli's Principle.

Kick Down, Go Forward
Now you can relate propulsive lift to fins. You kick downward and somewhat backward in an arc. At the same time, you're moving forward. The perpendicular lift force created is upward and forward. Your body acts as a boat keel against upward lift and the net thrust propels you forward. Similar flow patterns are generated on the tail flukes of whales and dolphins. Sharks use it too, only sideways. Watch for their graceful side-to-side tail motion, and think of propulsive lift.

The longer the fin, the greater the pressure differential, so more lift is created. If you go overboard with monster fins, however, the energy cost of using them exceeds the benefit. The main purpose of fins is efficiency. Efficiency means that the energy cost at any given finning speed is far lower than for kicking with bare feet at the same speed, unless your feet are often mistaken for pontoons.  An efficient kick is a slow kick. A slow kick with a wide range of motion will move you fast enough for most diving situations. Fast kicking can double and even triple your air consumption rate.

How to Kick
Kick with your whole leg, not just from the knee, and keep your knees and ankles loose. You'll increase range of motion, keep your fins at a good lift angle, NS reduce resistance which increases fatigue.  But, kicking only from the knees directs the push backwards, and parallel to flow. No lift is created, which is why knee-kicking is inefficient (more about this later). If you hold your ankles tightly so that they do not move, your fins will not meet the direction of flow at an optimal angle for forward lift. Tight ankles also are a major factor in leg and foot cramps. Keep your ankles loose and floppy for better kicking.

Flexible fins bend, which reduces resistance. Less resistance means reduced muscular and joint strain, and a comfortable kick. But you also present less area perpendicular to flow to create lift.  Rigid fins require more work, and are difficult to maintain perpendicular to flow during the downstroke. If the fin is too stiff, you'll wind up kicking feebly backward with distressed ankles, getting little lift force to help out. Torque developed from bad joint angles transmits to the knee and hip, making unhappy joints. Fin engineers have their hands full designing reasonably flexible, strong, comfortable, propulsive lift devices.

Three Forces Are Against Us
Forces opposing motion are called resistive force or drag. Drag is the force parallel to the direction of motion, but in the opposite direction. Three kinds of drag are important to scuba diving.

Form Drag. Form Drag, also called profile drag, is influenced by your shape and the amount of cross sectional area you present to the water. Gear up and your frontal resistance area is nearly double that of a skin diver, because according to the Law of Squares, twice the surface area creates four times the resistance. Forget your fins and you'll find it hard to get around. Divers taking off their fins before getting hold of the boat ladder, or who forget fins and jump in overweighted, quickly earn the "Law of Squares."

Swimming in a head up position makes you unstreamlined because of form drag. A common reason for head up swimming is overinflating the BC to offset an overloaded weightbelt. Unstreamlined profile creates turbulence. The turbulence zone behind you has high velocity eddies which drop the pressure, while the pressure in front of you builds in the area of low velocity. You're pushed from high to low pressure, and are resisted against forward motion. Form drag tires you and increases air consumption.

Streamline reduces water (and air) resistance by reducing form drag. Look on the back of bicycle racing helmets and at the front of tractor trailers for tapered air deflectors that reduce form drag. The tapered shape of swim fins also reduces drag. Keep your body parallel to your direction of motion to promote streamline. The less drag you create, the less work to move through the water.

An interesting method to reduce form drag on round objects is to dimple the surface. Dimples on a golf ball pull air around and reduce the trailing low pressure area. Dimpling would probably not reduce form drag on fins because of their tapered shape, but who knows what the manufacturers are working on next. There is an optimum amount and depth of dimple to reduce form drag (manufacturers' secret). Further "roughing up" of the surface will only increase another kind of drag. This drag, less of consequence to diving than other sports, is Surface Drag.

Surface drag, also called friction drag, results when water flowing past you is slowed by contact with your surface material. Fins aren't made with rough uneven surfaces since it would increase surface drag, and slow forward motion. On the other hand, fuzz on tennis balls is crucial to beginning players for the same reason. Barnacles must be routinely cleaned from boat bottoms, dirt must be removed from supersonic jets, and speed racers wear shiny lycra suits, all to reduce surface drag.

Wave Drag. The third opposing force is Wave Drag. Wave drag opposes you only when you swim at or within a few feet of the surface of the water and cause waves. Any water skier who has ever fallen knows you decelerate in a hurry when you contact the water. Wave drag is action-reaction. You push the water; the water pushes back. The faster you move, the greater the resistance of the bow wave created. Resistance increases as the square of the velocity so if you double your speed, wave drag quadruples. If you swim three times faster, the resistance is nine times as high. Wave drag may be the single factor that finally limits how fast the fastest Olympic swimmer may ever swim. Hydrofoils use propulsive lift to lift boats above the water's surface. With wave drag no longer a factor, hydrofoil speed far exceeds speeds of conventional craft. Beat wave drag by swimming leisurely when at the surface and keep your scuba diving well beneath the surface (also less likely to be run over by a boat).

Propulsive Drag
Not all drag forces are against us. Pushing directly backward on the water in order to go forward does create a small amount of propulsive drag (a very small amount.) If you push your hand directly backward through the water, the push builds pressure on the palm side while turbulence drops pressure at the back of the hand. Pressure differential moves you from high to low and creates propulsive drag.  All of propulsive drag is directed forward, but only contributes a small amount to your forward progress. Kicking only from the knees down results in a backward directed motion and propulsive drag. That's why arm or leg movements push backward are inefficient.

All bodies experience drag, but not all experience lift. You must create lift yourself. Proper kicking for lift while diving was described above. For arm stroking while swimming, use a large "S" shape sculling movement as you stroke backward, to generate lift. Competition swimmers use this "S-stroke". Try it when you're swimming. You'll be amazed how much faster you go. When snorkeling or diving, arm movements are tiring and can't compete with the lift generated by fins. Keep arms at your sides to minimize fatigue, air consumption, and form drag.

Summary
Bernoulli's propulsive lift, not Newton's equal reactions, is your major motive force in a fluid system. When you move through the water you create both lift and drag. To move forward, the lift force you generate must be greater than all drag forces against you.  Fins create more lift than even the biggest bare feet, by increasing the length available to generate lift. The fins' tapered shape reduces form drag. Their smooth surface reduces surface drag. Fins must be flexible enough to optimize the angle of push to create lift, and keep energy cost of use low. Kicking slowly from the hip with freely moving knees and ankles, and stay more than several feet under the surface of the water to reduce wave drag and being hit by a boat - which can be a very big drag.
 

The Author
Dr. Bookspan's feet are sometimes mistaken for pontoons.

Former competition swimmer, and military researcher.


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 Ernest Campbell, MD, FACS All Rights Reserved.


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