12
Marine Mammal Physiology: Requisites for Ocean Living
and walrus (pelvic flippers) (Fish 1993a, 1998a). This swimming mode, also described as
thunniform or carangiform with lunate tail (Lighthill 1969; Webb 1975), is similar to propulsion by certain fast swimming fish. Lift-based swimming produces forces up to five
times greater than drag-based propulsion (Weihs 1989).
An oscillating hydrofoil (i.e., flukes, flippers) generates lift at a controlled angle of
attack (Lighthill 1969; Feldkamp 1987a; Fish 1998a,b; Vogel 2003). The angle of attack should
be small to avoid separation of the flow from the hydrofoil surface, which reduces lift and
increases hydrodynamic drag. Lift is directed perpendicular to the path traversed by the
hydrofoil, so it has a component that can be resolved as an anteriorly directed thrust force
(Weihs and Webb 1983; Fish 1993a,b; 1998a,b). Thrust is generated almost continuously
throughout a stroke cycle. Although the hydrofoil produces some drag, it is small relative
to the lift (high lift-to-drag ratio) (Vogel 2003).
Lift-based oscillation is characterized by high propulsive efficiencies. Propulsive
efficiencies for cetaceans are 0.75–0.9 (Fish and Rohr 1999), where maximum efficiencies are achieved within the range of normal cruising speeds (0.8–1.5 body lengths/s).
For pinnipeds, oscillation of the fore flippers by Zalophus and hind flippers by Pusa and
Pagophilus provide maximum efficiencies of 0.8 and 0.88, respectively (Feldkamp 1987b;
Fish et al. 1988).
Propulsive flippers and flukes have a planar geometry with a high aspect ratio, where
aspect ratio is defined as the square of the span over the planar area of the hydrofoil (Webb
1975; Feldkamp 1987b; Fish 1993a,b; 1998a,b). A high aspect ratio reduces drag while maximizing thrust. An aspect ratio works in concert with the sweep of the hydrofoil geometry.
Sweep is the rearward inclination of the leading edge. A combination of low sweep with
a high aspect ratio allows for high-efficiency swimming, whereas high sweep may compensate for the reduced lift production of low aspect ratio hydrofoils (Liu and Bose 1993).
The cross-sectional geometry is similar to symmetrical engineered foil sections with
an elongated teardrop design. This design has a rounded leading edge that increases to a
maximum thickness at approximately 24%–36% of the chord (i.e., the linear distance from
the leading to the trailing edges) (Fish 2004). From the maximum thickness, the hydrofoil
slowly tapers to the trailing edge. This configuration prevents stalling due to boundary
layer separation as the hydrofoil is oscillated.
The typical planform of cetacean flukes has a sweptback, winglike shape with tapering tips. The aspect ratio varies from 2.0 for the Amazon River dolphin (Inia geoffrensis)
to 6.1 and 6.2 for the fin whale (Balaenoptera physalus) and false killer whale (Pseudorca
crassidens), respectively (Fish and Rohr 1999). The flukes of fast-swimming cetaceans have
higher aspect ratios than slow swimmers. The sweepback in flukes ranges from 4.4° for the
killer whale (Orcinus orca) to 47.4° for the white-sided dolphin (Lagenorhynchus acutus) (Fish
and Rohr 1999). The flukes of mature male narwhals (Monodon monoceros) have a slightly
concave leading edge without a sweepback (Hay and Mansfield 1989).
Pinnipeds utilize foreflippers (Otariidae) and hind flippers (Phocidae, Odobenidae)
as propulsive hydrofoils. The foreflippers of Zalophus have a high aspect ratio of 7.9 and
are flapped in a manner reminiscent of underwater flight (Feldkamp 1987b). The foreflippers generate thrust throughout almost the entire stroke cycle. The stroke cycle is divided
into a forceful downstroke of the flippers during a power phase that terminates with a
paddling phase before the flippers are lifted in a recovery phase (Feldkamp 1987a). The
hind flippers of phocids and walrus are oscillated laterally. The two flippers are alternated
with the digits of the trailing flipper are fully abducted to generate thrust as the digits of
the leading flipper adducted (folded) (Fish et al. 1988). Aspect ratio for the hind flippers of
phocids is 3.4–4.0.
Marine Mammal Physiology: Requisites for Ocean Living
and walrus (pelvic flippers) (Fish 1993a, 1998a). This swimming mode, also described as
thunniform or carangiform with lunate tail (Lighthill 1969; Webb 1975), is similar to propulsion by certain fast swimming fish. Lift-based swimming produces forces up to five
times greater than drag-based propulsion (Weihs 1989).
An oscillating hydrofoil (i.e., flukes, flippers) generates lift at a controlled angle of
attack (Lighthill 1969; Feldkamp 1987a; Fish 1998a,b; Vogel 2003). The angle of attack should
be small to avoid separation of the flow from the hydrofoil surface, which reduces lift and
increases hydrodynamic drag. Lift is directed perpendicular to the path traversed by the
hydrofoil, so it has a component that can be resolved as an anteriorly directed thrust force
(Weihs and Webb 1983; Fish 1993a,b; 1998a,b). Thrust is generated almost continuously
throughout a stroke cycle. Although the hydrofoil produces some drag, it is small relative
to the lift (high lift-to-drag ratio) (Vogel 2003).
Lift-based oscillation is characterized by high propulsive efficiencies. Propulsive
efficiencies for cetaceans are 0.75–0.9 (Fish and Rohr 1999), where maximum efficiencies are achieved within the range of normal cruising speeds (0.8–1.5 body lengths/s).
For pinnipeds, oscillation of the fore flippers by Zalophus and hind flippers by Pusa and
Pagophilus provide maximum efficiencies of 0.8 and 0.88, respectively (Feldkamp 1987b;
Fish et al. 1988).
Propulsive flippers and flukes have a planar geometry with a high aspect ratio, where
aspect ratio is defined as the square of the span over the planar area of the hydrofoil (Webb
1975; Feldkamp 1987b; Fish 1993a,b; 1998a,b). A high aspect ratio reduces drag while maximizing thrust. An aspect ratio works in concert with the sweep of the hydrofoil geometry.
Sweep is the rearward inclination of the leading edge. A combination of low sweep with
a high aspect ratio allows for high-efficiency swimming, whereas high sweep may compensate for the reduced lift production of low aspect ratio hydrofoils (Liu and Bose 1993).
The cross-sectional geometry is similar to symmetrical engineered foil sections with
an elongated teardrop design. This design has a rounded leading edge that increases to a
maximum thickness at approximately 24%–36% of the chord (i.e., the linear distance from
the leading to the trailing edges) (Fish 2004). From the maximum thickness, the hydrofoil
slowly tapers to the trailing edge. This configuration prevents stalling due to boundary
layer separation as the hydrofoil is oscillated.
The typical planform of cetacean flukes has a sweptback, winglike shape with tapering tips. The aspect ratio varies from 2.0 for the Amazon River dolphin (Inia geoffrensis)
to 6.1 and 6.2 for the fin whale (Balaenoptera physalus) and false killer whale (Pseudorca
crassidens), respectively (Fish and Rohr 1999). The flukes of fast-swimming cetaceans have
higher aspect ratios than slow swimmers. The sweepback in flukes ranges from 4.4° for the
killer whale (Orcinus orca) to 47.4° for the white-sided dolphin (Lagenorhynchus acutus) (Fish
and Rohr 1999). The flukes of mature male narwhals (Monodon monoceros) have a slightly
concave leading edge without a sweepback (Hay and Mansfield 1989).
Pinnipeds utilize foreflippers (Otariidae) and hind flippers (Phocidae, Odobenidae)
as propulsive hydrofoils. The foreflippers of Zalophus have a high aspect ratio of 7.9 and
are flapped in a manner reminiscent of underwater flight (Feldkamp 1987b). The foreflippers generate thrust throughout almost the entire stroke cycle. The stroke cycle is divided
into a forceful downstroke of the flippers during a power phase that terminates with a
paddling phase before the flippers are lifted in a recovery phase (Feldkamp 1987a). The
hind flippers of phocids and walrus are oscillated laterally. The two flippers are alternated
with the digits of the trailing flipper are fully abducted to generate thrust as the digits of
the leading flipper adducted (folded) (Fish et al. 1988). Aspect ratio for the hind flippers of
phocids is 3.4–4.0.