Millions of years before the Greek poet Oppian so beautifully proclaimed the eminence of the dolphin, the beginning of all creatures took place in the sea. From these creatures evolved fish and then animals, who left their home in the sea to venture onto land, where they developed feet and the ability to bear their young alive. These animals were the mammals.
The dolphin, contrary to the belief of Oppian, is not a fish but a mammal, having come initially from the sea to develop on land. The habit of eating fish drew certain mammals, our dolphin among them, back to the sea where gradually their shape evolved into that of a fish. The body was transformed for adaptation to life in the sea, and the front limbs became flippers, used only for balancing and steering. The hind legs disappeared, and the tail developed into a propulsive element which was much unlike that of a fish in that it moved in a vertical rather than a horizontal plane. The animal retained a basic lung system, which prevented its breathing in the natural water environment, and it continued to produce its young as babies requiring milk from their mothers.
The legend depicting the creation of the dolphin is attributed to the Greeks, particularly to Apollodorus, who wrote about Dionysos, God of Wine and Frenzy. On a voyage from the island Ikaria to Naxos, Dionysos was set upon by the piratical crew who, not aware that Dionysos was a god, attempted to abduct him. Dionysos used his powers to change the oars to snakes and to cause the sailors to dive into the sea, where they became dolphins. Prior to this occurrence, the dolphin did not exist, and from that day hence dolphins have been symbols of kindness and virtue in the sea.
Herodotus recorded the story of the poet and musician, Arion, who lived 600 years before Christ. Arion acquired great fame and wealth with his song and his harp. As in the tale of Dionysos, Arion was attacked by pirates who caused him to jump from the ship, presumably to his death. The pirates then sailed on to Corinth, where they were confronted by their victim, who related the story of his rescue and delivery to safety by a dolphin.
Shortly after Arion’s death, the Greek city of Taras minted coins bearing the imprint of a man riding a dolphin. Whether this coin was designed to depict Arion’s rescue is not known; however, the coin serves to substantiate to a degree the story told by Herodotus.
The first story attributed to the Romans was told by Pliny, in his Natural History, and is considered to have occurred during the early life of Christ. In a shallow sea inlet, the Lucrine Lake, a dolphin and the young son of a poor man in the district of Baiae developed a strong mutual affection. Each morning the boy would stand by the lake and call, “Simo, Simo,” at which sound the dolphin would draw near and take the boy upon his back to the distant shore to school. When the boy died, the dolphin continued to come in search of him until it died of sorrow.
Each of the tales developed thus far indicated man’s close relations and possible kinship with the dolphin. Apollodorus would have us believe through the legend of Dionysos that the dolphin was created from man. The stories of Arion and the boy from Baiae relate events which tend to corroborate the close affinity between man and dolphin. But these are legends and tales which are unsupported by sufficient evidence to warrant our unfaltering belief in their veracity.
The initial scientific approaches to a study of the dolphin are attributed to Aristotle around 400 B.C. Aristotle appears to have disregarded the lengendary writings about the animal and to have concentrated primarily on facts such as the distinction between dolphin and fish, the classification of the dolphin as a mammal, and descriptions of the birth processes of the dolphin. Actual observations substantiate to a great extent the validity of the writings of Aristotle, particularly in regard to the dolphin’s birth process.
During the spring of her fifth year, the female dolphin may be expected to bear the first of her young, and each year thereafter a maximum of one baby may be born. Gestation requires approximately 12 months as the young dolphin must be well-developed and self-reliant at birth. Gestation is culminated as the baby emerges tail first from its mother and rises quickly to the surface for its first breath. At this instant, the most controversial and mysterious of the dolphin’s abilities are manifested with the first stroke of its tail and the first few inches of motion. Researchers consider the sea animal locomotion technique employed by the dolphin to be the threshold of crossing the speed barrier presented by the dense medium in which it dwells.
Following World War II, naval research was directed toward developing improved undersea characteristics of submarines, and a study was conducted to ascertain the applicability of sea animal shapes to submarine hull design. The USS Albacore (AGSS- 569), first undersea craft constructed in the teardrop configuration of a whale, thus became the forerunner of the nuclear-powered attack submarine and finally of the Polaris weapons delivery system.
The key to final capture of maximum underwater efficiency had been inserted in the lock with the consideration of sea animal shapes; however, the door was not completely opened until research was initiated to formulate theories based on the most nearly perfect example of underwater propulsive efficiency, the dolphin, more commonly referred to as the porpoise.
Today, the dolphin has become the object of massive and intense research by the U. S. Navy in the field of sea animal locomotion. The nuclear Navy of today is characterized by the ominous presence in strategic locations of Polaris and attack submarines. These elements of the Fleet represent the major deterrent to attack by an enemy nation, and their proficiency may be in part attributed to studies in sea animal locomotion.
Naval propulsive machinery design is governed by efforts directed toward system quietness, high propulsive speed, high endurance or long range, and high over-all efficiency. The zenith of design for operation with a maximum of each of these characteristics appears in the propulsive technique and locomotory apparatus of the dolphin.
Primary topics of interest to the Navy are speed and power relationships demonstrated by the dolphin. Analytical calculations based upon mathematical formulas indicate that the dolphin is incapable of swimming at speeds in excess of 11 knots, but actual observations of the animal refute these calculations, as it has been clocked at over twice this speed. The problem is to determine how the dolphin accomplishes this biologically impossible feat of speed swimming, and to apply the results to naval ship construction.
The dolphin species of primary interest in the study of sea animal locomotion is Tursiops truncatus, the bottle-nosed dolphin. This mammal is most common to the East Coast of the United States and is distinguished by a short, well-defined snout and a prominent vertical fin, the apex of which is so shaped as to point aft. Flipper appendages are moderate in size and are pigmented, as are the flukes. The back of Tursiops is black or dark grey- brown, and the belly is white with a pigmented area from vent to flukes. The head and snout are dark, but the lower jaw is white. Adults vary in maximum length between 11 and 12 feet, but the average is about eight feet. An eight-foot Tursiops weighs approximately 300 pounds. Tursiops truncatus is a docile and intelligent mammal and adapts with facility to naval research.
The dolphin, Tursiops, is capable of diving for a period of about seven minutes. An actual experiment to determine the depths to which a porpoise may descend is difficult to program; however, the results of observations by Robert L. Edwards of the U. S. Fish and Wildlife Service indicate that a depth of 200 feet is possible. Edwards used a sonar system to record the behavior of dolphins of the Delphinus delphis species and found that sonar tracings disclosed a 200-foot dive.
The depth to which a dolphin may dive and the length of stay at this depth is important in the considerations of muscle power, as it will be discussed in the context of this paper. In an inactive period, the animal usually breathes from one to four times per minute, the rate increasing with increased activity.
Studies of sea animal locomotion must be conducted by semi-empirical methods, as the art has not advanced to the point of resolution of the large number of variables and unknowns in a completely analytical and exact experimental analysis. Such an exact solution to the problems presented by sea animal locomotion could not include inherent capabilities of the animal under study and would yield results of doubtful accuracy. The theory of rigid- body hydrodynamics must be assumed in semi-empirical analysis, and it introduces a number of controversial predictions, which will be described.
Initial studies of sea animal locomotion were conducted by Professor Sir James Gray of Cambridge University. Gray based his ideas on calculations that the power output of a pound of porpoise muscle is equivalent to the similar output of an athlete. However, a comparison of the expected power output of the total weight of porpoise muscle and the actual speeds of the animal indicated that seven times the power calculated would actually be required. Gray’s final assumption was that the form or surface of an inert dolphin should have drag equivalent to that of a streamlined object. These assumptions were inadequate in the explanation of the differences encountered and led to “Gray’s Paradox: the discrepancy between the power requirement (calculated from rigid-body hydrodynamics) and the power available (calculated from biological data on muscle power).”
The resolution of Gray’s Paradox should prove invaluable in ship design, and efforts have been directed toward solution of the paradox through investigation of certain physical characteristics considered to be determining factors in the dolphin’s exaggerated speed. Among the characteristics of the dolphin investigated are: muscular power, length, weight, coefficient of drag, zone body temperatures, boundary layer control, and pressure accommodation.
A basic assumption employed in investigation of the paradox is that the dolphin is enabled to maintain a certain degree of flow free from turbulence through body chemistry or physics. Evidence of laminar flow has been found to be abundant.
While serving in the Royal Navy during World War II, G. A. Steven, a biologist attached to the marine biological station in Plymouth, England, one evening saw a number of seals and dolphins swimming about in the phosphorescent sea. Countless phosphorescent unicellular organisms gave him a clear picture of their motion just as aluminum powder sprinkled on water gives a clear picture of currents in a laboratory tank. Steven saw that the dolphins produced two straight glowing lines as they swam through the sea, while the seals caused a great deal of turbulence.
The flow of a fluid past a streamlined body is not absolutely uniform, as viscous effects of the body surface retard the fluid particles in its immediate vicinity. Basic boundary layer theory dictates decreasing retardation with increasing distance from the surface until a region is reached where flow becomes steady. Laminar flow results when drag force exerted on particles in close proximity to the body is small enough to allow the outer layers to execute a gliding motion over one another. Turbulent flow results when drag is increased to the critical velocity at which the innermost particles cease to glide within the outer layers. It has been found that turbulent flow greatly impedes the motion of a body with increasingly adverse effects as the source of turbulence is moved nearer to the front of the body.
Considering, first, the theory of increased muscle power in the dolphin, the problems of heat and oxygen might be employed in refutation. Dolphins have a thick layer of blubber which serves as natural insulation; therefore, the heat generated by excess muscle power would tend to accumulate, rather than dissipate, in the body of the mammal. Further substantiation of disproof exists in the oxygen content required for muscle function. Although the dolphin’s respiratory and circulatory systems are highly efficient, they are not sufficient to supply the oxygen required for the muscle power needed for observed speed. The conclusions are that the power calculated is an impossible figure and that another solution to the problem must exist.
Consideration is now directed to the coefficient of drag. Theory of probability and statistics dictates that randomly selected families of elements will yield relatively accurate average data. Considering this statistical theory, a sample space of four groups of dolphins was observed to maintain an average sustained speed of 14 to 18 knots. The implication here is that the apparent coefficient of surface friction may be considered approximately constant for dolphins ranging in length from six to 12 feet. The lengths determined are based on knowledge that a group of dolphins will normally contain a relatively full range of sizes and ages.
Calculations of drag coefficient were made by using a rigid scale model of the dolphin in a tow tank. This experiment produced a coefficient much too high to be considered permissible and led researchers toward a test involving a flexible scale model. A foam rubber dolphin was constructed over mechanically flexed rods designed to imitate the actual phases of locomotion employed by the swimming dolphin. Results of this experiment proved to be enlightening, in that the drag coefficient was considerably less than for the rigid model, a fact which motion pictures of the experiment led researchers to attribute to pure laminar flow. Flow about the rigid model was determined to be turbulent, which explains the increased drag. This flexible-model experiment tends to substantiate the theory that the “undulating swimming motion of the living porpoise, like the flexible model, reduces turbulence and thus reduces drag.”
The reduction of drag appears to be of paramount importance in the solution of Gray’s Paradox. Drag and temperature are related through the Reynolds Number (scale velocity times length over Kinematic viscosity), which prompts investigation of the heat generated by the dolphin. As the Reynolds Number is increased beyond a critical value, flow is transformed from laminar classification to turbulence and drag is increased.
The dolphin is endowed with a rich vascular system near the surface of the skin which might be considered a heat regulating device. Should this prove true, the dolphin has a mechanism with which it can control the Reynolds Number, the boundary layer and, therefore, the classification of flow. Microscopic slides have illustrated the greater supply of blood to areas of maximum turbulence, and a chart illustrating various point temperatures shows increased values in the turbulent areas, a phenomenon which tends to corroborate the heat control theory.
Observations of rapidly swimming dolphins have produced the theory of natural pressure accommodation. When a dolphin is moving fast or decelerating suddenly, transverse ripples appear on its lower and lateral surfaces (see photo at right) and remain in a stationary position for a period lasting from a fraction of a second to two or more seconds before they disappear. To some researchers, these ripples or folds are merely caused by the rapid motion, but a more scientific deduction is that the folds are actually used to accommodate unequal water pressures resulting from the rapid motion of the animal. Should the latter theory be true, the dolphin has another drag reduction device.
An explanation of the relation of pressure accommodation to drag is taken from the basic theory of fluid mechanics. Pressure variations exist just prior to transition from laminar to turbulent flow, a phenomenon which leads researchers to investigate the possibility of delaying transition by obviating these pressure variations. Two possibilities exist in regard to pressure absorption and accommodation—passive control and active control. The surface of the sea animal which is presented to fluid flow might naturally conform to pressure oscillations through anatomical structure (passive control) and thus delay transition indefinitely. The sea animal might actively engage in controlling pressure instabilities through muscular action (active control). Both of these theories are developed in dolphin research.
Evidence in support of the accommodation theory has been produced by Frank Essapian and Dr. Max Kramer of the Scripps Oceanographic Institute. Essapian photographed the dolphin skin-folds and illustrated the suppleness and pliability of dolphin skin, which hangs in wrinkles when the dolphin is removed from the water. Kramer followed Essapian’s theory and determined that dolphin skin is essentially composed of two major layers, “a soft water-logged outer layer overlaying a fat but harder inner layer.” Further investigation produced evidence of a reduction of the outer skin into two sublayers, a layer of vertical spongy ducts supporting a water-logged outer layer. The nature of the pressure-sensitive ducts allows the outer skin to take up minute water pressure variations through deformation into microscopic wrinkles.
The theory of pressure accommodation thus has taken two forms—skin-folds and microscopic wrinkles. A preponderance of evidence substantiates both forms and indicates a complementary action between them. Hydrodynamic pressure fluctuations are controlled by skin-folds, and micro-turbulence in the boundary layer is manipulated by the nature of the skin composition and structure.
A misconception among laymen is that fins and appendages of the dolphin contribute to its fast locomotion capability. Although this is not true, the fins and appendages serve a necessary function in the performance of quick maneuvers, turns and stops, and in maintaining stability and hydrodynamic lift control. Speed characteristics must be attributed to a combination of body motions, functions and structure.
When swimming at high speeds, the dolphin displays a motion which has been designated as “anguilliforin” and which appears to be the generation of a traveling, almost sinusoidal wave which is characterized by increasing amplitude with increasing distance from the animal’s snout.
“In its simplest conception this action would “energize the boundary layer” at all but a few nonstationary points. It seems conceivable that such action would provide thrust at or in the proximity of points where drag would occur. This thrust would cancel drag as it tended to build up, thus also postponing the transition to turbulent flow indefinitely. This form of motion may also achieve a form of boundary layer control through effectively blowing and sucking and a rhythmic change in all aspects.”
Another interesting aspect of dolphin swimming was introduced in 1948 by Alfred H. Woodcock of the Woods Hole Oceanographic Institution. Woodcock observed dolphins riding just in front of a research ship’s bow with no apparent body motion. The dolphins appeared to be riding the bow wave, an accomplishment which mocks the laws of hydrodynamics and which has fostered controversy for more than a decade.
Woodcock and A. F. McBridge developed the assumption that the dolphin is actually planing on the bow wave and overcoming drag through laminar flow. In effect, they claimed, the dolphin was actually falling continuously on the down slope of the wave. This explanation was refuted somewhat by the fairly small weight of the dolphin in sea water, which yields gravitational effects adverse to the conditions required for planing.
In 1953, W. D. Hayes of the U. S. Office of Naval Research presented evidence that the “buoyancy of a dolphin in a pressure gradient determines the amount of hydro- dynamic lift required but has no effect on drag.” Through application of the fact that surface buoyancy acts not in a vertical plane, but rather is angled forward, Hayes illustrated clearly that the total weight, not just the submerged weight, of the dolphin is the significant factor in the planing process. Hayes’s findings once again gave rise to the planing theory.
The third of four acceptable explanations was contributed by P. F. Scholander of the Scripps Institution of Oceanography in 1959. Scholander concluded that the dolphins do not actually lie in the wave slope but forward of it. From this position, he stated, the dolphins place their tail flukes in the incline of the bow wave at a certain angle and thus convert energy of the bow wave into forward thrust. This concept was refuted by Hayes, who explained that such action would destroy the dolphin’s equilibrium.
Credit for the most logical explanation to date is given to A. A. Fejer and R. H. Backus of the Woods Hole Oceanographic Institution who state that the dolphin is not actually riding the visible wave, as has been concluded by the previously discussed researchers. A study of fluid mechanics in relation to wave formation yields the interesting concept of pressure field creation by the ship. The forward motion of the ship initiates a pressure field just ahead of the stem from which field the visible wave is a side effect. Although the pressure field is invisible to man, the dolphin might detect its presence through the tactile sense. The concept offered by Fejer and Backus is that dolphins utilize the energy of this pressure wave, which extends from the surface to the stem base, to propel them along with almost no expenditure of their energy.
In support of this final discussion are the observations made, by H. S. H. Yuen on the subject of dolphins riding the bow wave at various depths. Yuen reports that the dolphins would ride in a motionless position for some time and then weave laterally among each other exhibiting excellent control of hydro- dynamic lift. They appeared to use their pectorals and flukes by alternating their planes of force, and then appeared to shift their body conformations to maintain stability and to regulate lift.
Cetaceans, including the dolphin, have a specific gravity of one resulting from the large proportion of fat in the skeleton and the presence of large quantities of blubber. Consequently, dolphins exist in a state of neutra- stability, neither sinking nor rising naturally. Neutral stability indicates that the dolphin need exert no force to maintain its vertical position in relation to the wave. The fundamental concept supported by this analysis of bow-wave riding is that of laminar flow, the term that has become the key to dolphin research.
In conclusion, the applications of dolphin research to naval ship construction may assume many forms, the most feasible of which at the present state of the art are: shape, boundary layer control, undulating motions, and pressure accommodation. Research in these fields has proved the acceptability of sea animal locomotion as a governing factor in design, e.g., the fleet of nuclear submarines that are on station today.
The modern submarine is designed for maximum speed and efficiency in its natural environment, the darkened depths of the sea, and is characterized by the general shape of a curve of revolution thickened forward with a gentle taper aft. The lines are almost identical to the effective shape of the dolphin.
Boundary layer control may be effected by numerous methods, the most elementary of which is heat regulation. An increase in temperature, as described in the context of this paper, decreases the Reynolds Number and thus tends toward the formation of laminar flow. The employment of heat-generating devices at certain points on the submarine would reduce the conditions of turbulence and decrease the drag coefficient. Thus, heat generation is a feasible area for study.
The primary efforts of experimentation are directed toward reduction of the drag coefficient, as has been shown in the development of shape and of each of the other three applications mentioned. The problem appears to this writer to exist in defining the most efficient method of minimizing drag. The two final considerations are those of undulating motion and pressure accommodation.
Researchers developing theories coincident with undulating motion (anguilliform) have been confronted by the almost impossible task of distinguishing between resistance and propulsion generated by such motion. Although sinusoidal motion is considered to be a major factor in the wakeless and semisilent locomotion of the dolphin, the application of such motion in the construction of ships would be characterized by difficulties of control and required space. Perhaps research will lead to an acceptable solution to these problems; however, the conclusion appears evident that undulating motion may not be successfully applied in ship construction.
Dr. Max Kramer has apparently found the most acceptable explanation of dolphin locomotion efficiency in his experiments with pressure accommodation. Dr. Kramer produced a synthetic dolphin skin which imitates the surface flexion of the live dolphin so successfully that a small craft coated with the synthetic skin experienced a 60 per cent reduction in drag. Herein lies the answer. A coating of such skin on the nuclear-powered submarine, combined with the already employed streamlined shape of the dolphin, should decrease drag in a ratio similar to that of the experimental craft.
The results of dolphin research and the most awesome of natural powers, nuclear power, have produced America’s major deterrent to war. In legend, Apollodorus created the dolphin as the docile and peaceful symbol of virtue in the sea, and Herodotus amplified and expounded upon this peaceful coexistence with man. Aristotle recognized the intelligence and speed of the dolphin and initiated scientific studies of this fascinating mammal- This, then, was the first link in the chain of events which has led in many ways to the development of the nuclear submarine. Thus it is that in legend, in ancient history, and in the era of progress that exists today, the dolphin, whose intelligence approaches that of man, remains a symbol of peace.