




































 

 

 
1 

† Corresponding author 
© 2016 Conscientia Beam. All Rights Reserved. 

 

HYDROSTATIC AND HYDRODYNAMIC CHARACTERISTICS OF SWIMMING 
ANIMALS-AN INSPIRATION FOR HYBRID BUOYANT AIRCRAFT 

 

Anwar Ul Haque1† --- Waqar Asrar2 --- Ashraf Ali Omar3 --- Erwin Sulaeman4 --- Jaffar 

Syed Mohamed Ali5 
1,2,4,5Department of Mechanical Engineering, International Islamic University Malaysia (IIUM), Kuala Lumpur, Malaysia 
3Department of Aeronautical Engineering, University of Tripoli, Tripoli, Libya 

 

ABSTRACT 

In today’s world, the biological sciences are mostly considered separate from the existing modern knowledge 

of various other fields of sciences and engineering; however, there are many properties of nature and known 

facts of biological sciences that can be proved in the other domains of science and technology as well. 

Correlation of the geometric and buoyant properties of the swimming animals with the hybrid buoyant 

aerial vehicles is an example of this hypothesis. In the present work, some experiments related to the 

geometric parameters of a California sea lion were carried out. It was found that the fineness ratio of this 

animal is of the same order as the optimum value of that for the condition of minimum drag and power 

required for the buoyant aerial vehicle. The role of multiple fins on the elongated bodies of shark is also 

discussed in its application for yaw stability as well as to shroud the antennas that are used in the aircraft 

for various communication systems. 

Keywords: Fineness ratio, California sea lion, Dorsal fin, minimum drag, Aerodynamic lift, Hybrid buoyant aircraft, 

Minimum drag, Buoyant independent drag.   

 

Received: 23 November 2015/ Revised: 23 December 2015/ Accepted: 29 December 2015/ Published: 4 January 2016 

 

Contribution/ Originality 

The paper's primary contribution is to show that the hydrodynamic and a few geometric 

parameters of a California Sea Lion resemble to that of the well-known facts of buoyant and 

hybrid buoyant aerial vehicles. 

 

 

 

Animal Review 
2016 Vol. 3, No. 1, pp. 1-9 
ISSN(e): 2409-6490 
ISSN(p): 2412-3382 
DOI: 10.18488/journal.ar/2016.3.1/101.1.1.9 
© 2016 Conscientia Beam. All Rights Reserved. 

 

 
 

http://crossmark.crossref.org/dialog/?doi=10.18488/journal.ar/2016.3.1/101.1.1.9


Animal Review, 2016, 3(1):1-9 
 

 
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© 2016 Conscientia Beam. All Rights Reserved. 

1. INTRODUCTION 

Research in the area of biological sciences being carried out in this era is as promising as in 

the other fields of science and engineering. Research in this area when coupled with that of other 

fields can do wonders and can make a significant contribution to the advancement of technology. 

Hydrodynamic characteristics of swimming animals are one such area that has recently been 

explored for its application in aviation industry i.e. for hybrid buoyant (HB) aircraft. It is a type of 

aircraft in which partial weight is balanced by the buoyant lift;  similar to a swimming animal like 

Steller sea lion, Haque et al. (2015a). For example, the body of California sea lion has a cambered 

profile because of having flippers attached at an anhedral angle to it; where this geometric feature 

is perhaps to decrease the coefficient of pitching moment and to increase the roll stability. Similar 

findings are there from the static longitudinal stability analysis of HB aircraft (Haque et al., 

2015b). In the present work, hydrodynamic and geometric variables of California sea lion are 

explored in comparison with the known knowledge of the same for hybrid aerial vehicles. In this 

regard, a series of experiments were conducted on a 16 years old California sea lion present at the 

Zoo Negara, Kula Lumpur. The geometric parameters so obtained were further utilized for the 

purpose of comparison with what is known in the field of hybrid buoyant aerial vehicles. 

Moreover, the dorsal fin of Shark is discussed regarding biological sciences and its further 

application for HB aircraft. 

 

2. RESEMBLANCE OF CALIFORNIA SEA LION WITH BUOYANT AND 

HYBRID BUOYANT VEHICLES 

Some experiments were conducted recently on California sea lion, Fig. 1. These tests include 

the flow visualization on these animals while swimming in water. Also, major geometric 

parameters were estimated with the help of  four trainers; that took them a week to train the 

animal for these measurements. The maximum length (including the rear flippers), maximum 

width, span of  the front flippers was 116 inch (2.93 m), 10.5 inch (0.266 m) and 21.0 inch(0.53 m), 

respectively. To be able to find the correlation of  this marine animal in the water to a flying 

buoyant vehicle in the air, the fluid dynamic conditions, as well as the geometrical shape, have to 

be similar. But the body of  the animal is quite flexible, and it is quite hard to take measurements 

while the animal is inside the water. Therefore, all the measurements are done on the ground and 

hence are approximate values. 

 



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© 2016 Conscientia Beam. All Rights Reserved. 

 
Fig-1. Measurements for the geometric parameters of  California sea lion 

 

2.1. Buoyant Lift 

Sea animals utilize buoyant lift to stay uplift to balance the effect of  gravity. This buoyant lift 

is independent of  the drag force exerted on the body of  a sea lion (Suzuki et al., 2014). Buoyant 

lift is independent of  the surface/wetted area of  the marine animal. But, similar to any 

aerodynamic and hydrodynamic coefficient, the drag coefficient is dependent on the reference area 

of  the body immersed in the fluid. In 1990, Alexander (1990) carried out a study related to the 

effect of  different reference areas for estimation of  the drag coefficient of  swimming animals. He 

found that there was a drastic reversal in the drag coefficient data obtained by using the wetted 

area as a reference area for two species of  Idotea; genus of  isopod crustaceans. He mentioned that 

that there is no powerful hydrodynamic basis for the selection of  reference area for the estimation 

of  drag coefficient and the same holds good for hybrid aerial vehicles as well. As far as buoyant 

lift is concerned, it is mainly linked with the criterion of  good performance for an animal that 

takes into account that how much energy it can process and store (guts, so to speak) and how 

many offspring it can produce (gonads, loosely put).  In the case of  Sealion, the maximum 

available buoyant lift is 50% of  the gross weight (Suzuki et al., 2014). This digit is consistent with 

the recent findings related to the optimum buoyancy ratio for hybrid airship (Raymer, 2006). 

 

2.2. Lifting Profile of  California Sea Loin 

Marine animal's body can generate additional lift; hydrodynamic profile of California sea lion 

is one of the examples of it. This marine animal has big guts and gonads to utilize the buoyant lift 



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to stay uplift to balance the effect of gravity; maximum value of which is of the same order as 

recommended for hybrid buoyant aircraft; an airplane that specialized as a volume maximizer. 

Based on our predicted similarity between California sea lion and man-made buoyant aerial 

vehicles, we have proposed that California sea lion’s body “can be tailored for aeronautical 

applications such as the lifting fuselage of a hybrid buoyant aircraft” (Haque et al., 2015c). Complete 

geometric details of the outer contour of the California sea lion are not yet available. Perhaps, due 

to the flexible body of sea lions, it is nearly impossible to measure the same; the only information 

available is the location of maximum thickness, which is about 34 % of body length (Suzuki et al., 

2014). In one of our studies related to the hybrid lifting hull, we selected Eppler-1200 airfoil for 

the hybrid hull, based on the visual judgment of geometric profile of California sea lion and 

requirement of takeoff ground roll angle to fulfill future certification requirement (Haque et al., 

2014). 

 

2.3. The Position of Maximum Thickness of Body 

The position of the maximum thickness is an important aerodynamic as well as 

hydrodynamic parameter to define the point where the transition of the boundary layer occurs. 

It’s  position further aft is desirable to have more laminar flow as well as less drag. During the 

measurements of California sea lion on a flat surface, it has been observed that the position of 

maximum thickness is about 36 percent of the length of the body. This value is slightly lower 

than that observed by Feldkamp (1987). Interestingly, this is the location of the shoulders and 

flippers of this animal as well. Eppler -1200 is one of the airfoils with the location of its maximum 

thickness close to that measured earlier for sea lion (Haque et al., 2015b). This airfoil has recently 

been used for the design of a hybrid lifting hull. The selection of this airfoil was based on the 

location of maximum thickness, visual judgment of geometric profile of California sea lion and 

requirement of takeoff ground roll angle to fulfill future certification requirement (Haque et al., 

2015c). 

 

2.4. Optimum Fineness Ratio 

It is well known that conventional non-rigid airships usually have fineness ratio value of 

about 2.3-2.8. However, a  fineness ratio between 5 and 6 was found to be optimal for maximum 

propulsion efficiency of a fixed maximum diameter buoyant vehicle (Ilieva et al., 2014). 

Interestingly,  as per the findings of Suzuki et al. (2014) this range of fineness ratio is found 

comparable to the California sea lion. This marine animal has a fineness ratio of 5.55 and location 

of maximum thickness of outer profile is at 34% of the overall length (Cheneval, 2005). This 

fineness ratio is also common with other marine mammals like dolphins, fishes, and whales (Fish, 

1994). To have optimum drag and the propulsion power, a fineness ratio between 5 and 6 can be 

used for the design of hybrid lifting hull 



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3. BIOLOGICAL SCIENCE OF DORSAL FIN AND ITS POTENTIAL 

APPLICATIONS FOR HB AIRCRAFT 

In the field of aerospace, the dorsal fin is usually referred as the extension of the vertical tail 

area at the leading edge. It is unlike of a dorsal fin that sticks up by itself (Nakamura et al., 2015) 

such as one can see on a dolphin or shark; located forward of the vertical tail, but not connected to 

it. The dorsal fin is one of the potential solutions to increase the yaw stability as the moment arm 

between the aerodynamic center of the dorsal fin and center of gravity is enough for an elongated 

shaped fuselage to generate the additional yawing moment. When an HB aircraft is subjected to 

side-slip angle, then it should exhibit static directional stability to return the nose of the aircraft 

into the wind. An H-tail empennage arrangement has been used earlier in the conceptual design 

phase for the said purpose (Haque et al., 2015b). The analytical results have shown earlier that 

this configuration is marginally stable in the yaw, and any further increase in the size of a twin 

tail will make the weight of the tail heavy. Similar to a shark fish, use of multiple dorsal fins was 

earlier proposed for HB aircraft. However, such fins should be placed away from the center of 

gravity, Fig. 2(a), and should not be located at the attachment points of the bulkheads, Fig. 2(b).  

The body of marine animals is quite flexible, but a flying machine should be rigid enough to bear 

the flight loads. Such a structural rigidity can be reinforced by the use of bulkheads and additional 

frames in the lateral direction. Now let’s discuss the anatomy and the biological science of dorsal 

fin in the swimming animals. 

 

 
(a) Side view                                                                     (b) Isometric View 

Fig-2. Extruded view of  different components of  a hybrid buoyant aircraft model 

 

3.1. Biological Science 

A lot of  research has been carried out earlier on the role of  dorsal fin towards stability and 

turning (Harris, 1937; Harris, 1938; Harris, 1953; Webb, 1975; Webb, 1977; Webb and Keyes, 

1982; Webb, 1988; Webb, 2002). For example, Drucker and Lauder (Webb and Keyes, 1982) 

carried out a thorough study about the understanding of  the design and function of  dorsal fin for 

different types of  fishes. They found that the dorsal fin produced a pair of  counter-rotating 

vortices during turning. During steady swimming, these vortices interact with the caudal fins that 



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are located downstream for wake energy augmentation and also increase the thrust. Such fins are 

located close to the center of  the mass and produce about one-third of  the required lateral force 

for turning (Standen, 2008). It was observed by the scientists mentioned earlier that irrespective 

of  the fineness ratio of  the body, the location of  the paired pectoral fins (a lift producing 

component, marked with red colour) is always ahead of  the CG. Interestingly, it can be observed 

from Fig. 1 (Standen, 2008) that Percoid has small fineness ratio and also it has a big dorsal fin as 

compared with Salmonid. But as the fineness ratio increases, the location of  dorsal fin moves 

downstream for Sturgeon to fulfill the requirement of  the long moment arm. Among all, Shark 

has the highest fineness ratio and has multiple dorsal fins, with the first fin of  big size as 

compared to the secondary dorsal fin, located downstream of  the center of  the mass. 

 
Fig-3. An evolutionary transformation: paired fin position throughout fish evolution 

Source: Standen, E.M., 2008. Pelvic fin locomotor function in fishes: Three-dimensional kinematics in rainbow trout oncorhynchus mykiss. 
Journal of  Experimental Biology, 211(18): 2931–2942. 

 

Maia and Wilga (2013) has recently conducted experiments to evaluate the functionality of  

dorsal fin in bamboo shark for the steady condition. Their experimental data indicated a 

continuous oscillation in the lateral position. They also estimated the thrust produced by such an 

oscillation motion and found a loss in lateral stability due to the different orientation of  dorsal fin. 

Moreover, deflection in the tip of  the dorsal fin was found. Though, the major contribution 

towards longitudinal stability is due to the tail, but such a deflection can be related to longitudinal 

stability as well. Deflection of  the complete dorsal fin is also observed in other aquatic animals, 

having more than one dorsal fin. Harris (Harris, 1937; Harris, 1938; Harris, 1953) found that the 

first dorsal fin of  spiny dogfish behaves like a stabilizer, and it moves independently. But the 

second dorsal fin moves with the body and also acts as a thruster for forward motion. Based on his 

experiments, he concludes that “vertical lift force at the posterior end produces a negative pitching 

moment that neutralizes the positive moment of  the trailing pectoral fins.” 



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3.2. Application 

For all wireless communication systems, one important factor is the design of  the antennas at 

low frequencies with desired compact size and enhanced performance characteristics such as 

matched impedance bandwidth, better efficiency, and good gain. In an aircraft communication 

system, antennas are used for various operations such as for direction finding (DF), VHF 

communications, distance measuring system (DME), global positioning system (GPS), microwave 

landing system (MLS) and radar altimeter (RadAlt), etc., covering the frequency range from 

30MHz up to 4GHz and more (Macnamara, 2010). It has been studied in the past that wrong 

placement of  antennas and any additional weight and size on the aircraft increases the 

aerodynamic drag that negatively affects the flying ability of  an aircraft, for example, an increase 

in the fuel consumption and puts a limitation on the speed of  aircraft (Jahoda, 2006). They 

introduce the noise and increase the parasitic drag. By making the antenna low profile, it comes 

very close to the metal body of  an aircraft which degrades its performance (Josephson, 1957; 

Sievenpiper, 1999). Such antennas can be housed inside the dorsal fin as well. 

 

4. CONCLUSION 

Fineness ratio, the location of maximum width and buoyant independent drag of a California 

sea lion are the three quantities, which are common with hybrid buoyant aircraft. Hence, the 

known knowledge of hydrodynamic characteristics of swimming animals is adjoined with the 

aerospace application so that aerospace when dealt in line with biology, can provide inspiration 

for the design of HB aircraft. In aircraft, antennas are used for communication as well as for 

various navigation systems of aircraft, which can be shrouded by a dorsal fin. However with a 

slight yaw, dorsal fin causes a lot of drag, and it can produce a restoring yawing moment but not 

that enough such as a rudder produces. 

 

Funding: The support of the Ministry of Science, Technology and Innovation (MOSTI), Malaysia, under the 
grant 06-01-08-SF0189 and of the Ministry of Education, Malaysia under the grant FRGS 13-020-0261 are 
gratefully acknowledged. 
 

Competing Interests: The authors declare that they have no competing interests. 
 

Contributors/Acknowledgement: Authors are thankful to the management of Zoo Negara, Kuala Lumpur 
for providing assistance in taking measurements of the California sea lion. 

 

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