Graphics Reference
In-Depth Information
(a)
(b)
FIG 4.82
Swimming with hydrofoils
and oars. a: Demonstrates lift on the
upstroke. b: Demonstrates lift created
on the downstroke. c: Creates lift on
the downstroke and the upstroke. The
red arrows denote the direction of lift.
d: Illustrates propulsion by a rowing
action. The green arrows denote drag.
(c)
(d)
much shallower than in the former examples. During the downbeats, the
wing creates both an upward lift and forward thrust; during the upbeats, the
wing creates downward lift and forward thrust. To achieve a steady horizontal
movement, the upward and downward lift must cancel each other out,
leaving the forward thrust to propel the bird through the water.
Unlike with the use of limbs or fins as oars, whereby they are required to
be furled or bent on the forward recovery stroke to reduce drag, the use of
hydrofoils depends only on the adjustment of the angle of attack of the limb
or in. The shallower angle of the recovery stroke in the hydrofoil action has
the same effect of reducing drag.
The net result of movement with oars or hydrofoils with regard to acceleration
is the same: On the power stroke the animal achieves greater acceleration,
and on the recovery stroke the effect of drag reduces acceleration. For
very small animals this may result in deceleration. Those animals that
use a hydrofoil action by paired fins or limbs to create lift on the up- and
downstroke achieve a more consistent acceleration throughout the sequence.
Hydrofoils are not just used in pairs, as in turtles and penguins. Whales
and dolphins use single hydrofoils. Nor is it necessary for hydrofoils to be
limited to a horizontal orientation; tuna and other fish use their caudal fins
as vertically oriented hydrofoils in much the same way as a dolphin uses its
tail flukes. There is no significant difference between the way a whale uses
