Regardless of the angle the oar makes to the forward direction during a rowing stroke, there is negligible loss of energy between the oar and the gate. However, it does not follow from this that there is no decrease in oar efficiency at the extreme oar angles necessary for a very long catch. The loss of energy from the oar occurs, not at the gate, but at the blade. The water which is moved by the blade, against the water reaction force on the blade, absorbs mechanical energy from the oar. Calculations, experimental data and “on-water” measurements of rowing show that the energy efficiency of the oar is generally lowest for oar angles less than 55 degrees to the boat-forward direction (i.e. in the early part of the stroke)

Macrossan, M.N.

University of Queensland eSpace

Does the pinching force dissipate the rower’s energy?
University of Queensland, Mechanical Engineering Report No. 2008/10
M. N. Macrossan
Centre for Hypersonics, School of Engineering, University of Queensland
January 19, 2009
Abstract
Regardless of the angle the oar makes to the forward direction during a rowing stroke,
there is negligible loss of energy between the oar and the gate. However, it does not follow
from this that there is no decrease in oar efficiency at the extreme oar angles necessary
for a very long catch. The loss of energy from the oar occurs, not at the gate, but at
the blade. The water which is moved by the blade absorbs mechanical energy from the
oar. Calculations, experimental data and on-water measurements of rowing show that the
energy efficiency of the oar is generally lowest for oar angles less than 55 degrees to the
boat-forward direction (i.e. in the early part of the stroke).
Introduction
The blade force is the (vector) sum of the forces exerted by the water on the submerged
rowing blade as it moves through the water. This force can be resolved into a forward
(propulsive) component, and a sideways component (pointing in towards the boat). Fig-
ure 1 shows schematic views of an oar at the beginning of the stroke and later in the stroke.
In each case the blade force is shown acting in a direction approximately perpendicular to
the oar-shaft axis. Thus the sideways force just after ‘the catch’ is much greater than the
forward force, whereas the sideways force is near zero when the oar angle ψ approaches
90◦.
The term ‘pinching force’ is often used for the sideways component of the force exerted
by the oar on the boat (at the gate/pivot). For an ideally rigid boat and rigging, the gate
does not move sideways as this force is applied. Thus, the gate force transmits energy
from the oar to the boat without loss and this is true regardless of the angle of the oar.
Hence one might conclude that there is no extra waste of energy when the oar is near
the catch [4]. Nevertheless, some rowers might ask themselves ‘Isn’t the blade moving
water more sideways than backwards near the catch, and isn’t this sideways movement
a waste of energy?’ This ‘intuitive theory’ is approximately correct1. Apart from energy
converted to heat by friction, or by bending and twisting of the hull or oars, the only
losses of mechanical energy from the system (i.e. everything which must be moved from
the start to the finish in a race) are due to external forces acting on the system. If we
1It is not only when water is moved sideways that energy is dissipated. Energy is also dissipated when water
is moved backwards.
1
ﬀ
½
»
¼
ﬀ
½
»
¼- -boat forward boat forward
ψ1 ψ2»»»
»»»
»»»
»»»
C
C
CW&%
'$
-Bf
?
Bs
FORCE COMPONENTS
ENLARGED VIEW
¤
¤
¤
¤
¤
¤
¤
¤
¤
¤
¤¤
XXXz&%
'$ -Bf
?
Bs
FORCE COMPONENTS
ENLARGED VIEW
Figure 1: Schematic top views of boat and one oar for two oar angles, ψ1 and ψ2 (measured from
the forward direction). The components of the water reaction force on the blade are shown (in
enlarged views) as Bf (forward force) and Bs (sideways force). The resultant force is approximately
perpendicular to the oar shaft.
ignore the slight up and down motion, the only relevant external forces are those acting
in the horizontal plane; these are exerted by the water and air through which the system
moves. Ignoring the air forces, the efficiency of the oar depends on how much energy is
dissipated by the force of the water on the oar (an external force).
Affeld et al. [1] calculated that the efficiency of the oar was lowest almost immediately
after the catch. The efficiency increased as the oar became more square to the boat.
Similar results were given in [6], and these are shown in Figure 2. The figure shows
the oar efficiency for two different assumptions for the direction of the blade force: (a)
perpendicular to the oar shaft axis and (b) with a small deviation from that direction as
suggested by experiment. In both cases the oar is relatively inefficient for oar angles less
than 55◦ to the forward direction and least efficient at an angle of about 45◦.
Dissipation at the blade
The gate force transfers energy between the oar and the rigger/hull (and does so effi-
ciently), but the energy transferred is not all the mechanical energy put into the oar by
the rower; it is only the remaining part, after accounting for the energy dissipated at the
blade. The power dissipation (rate of energy loss) by the blade can be calculated as the
product of the blade force and the component of blade velocity in the direction opposite
to the force [7]. It follows that, if the blade force vector happened to be exactly perpen-
dicular to the blade velocity vector, so there were no component of velocity opposite to
the force, the dissipation would be zero.2
2This can only happen if there were no water friction force. The water friction force though small [6],
produces some dissipation. Moreover, the blade force is distributed over the blade surface, and the various
points on the blade have different velocities relative to the still water. All these force-velocity vectors at every
point on the blade surface would have to be mutually perpendicular, to get zero dissipation.
2
Figure 2: Oar efficiency η at various oar angles ψ, calculated from on-water measured data, with two
different assumptions of blade force direction. Force assumed (a) perpendicular to blade-chord (lower
curve) or (b) in direction shown by scale-model tests (higher curve: ‘possible hydrodynamic effect’).
Adapted from [6].
Whether the blade force is in the direction required to reduce the dissipation to zero
(or close to it) is a question of fluid mechanical calculation or experiment. The theory
(see [5], §10.2) for a thin hydrofoil moving through water predicts that the force is indeed
perpendicular to the velocity vector so that the dissipation is zero.3 Some have suggested4
that the rowing blade might act ‘like a hydrofoil’ near the catch and, by implication, thus
be highly efficient. However, the available experimental evidence for scale-model rowing
blades [2, 3, 6] shows that the water force on the blade is not in the ideal ‘hydrofoil-
direction’ but in a direction almost perpendicular to the oar shaft. Nevertheless, for some
oar angles there is a slight shift of the direction of the force towards the hydrofoil-direction,
such as to reduce the dissipation.5 There are many reasons for caution when extrapolating
this scale-model data to the full-size oar in real rowing (see [6], appendix A) but even if
we did get the same shift in force direction for real rowing, the curve in Figure 2 labelled
‘possible hydrodynamic effect’ shows that the oar would still be least efficient near the
3The theory assumes that the water friction force is very small compared to the water pressure force, but
this is not a severe limitation of its applicability. The most important assumption is that the hydrofoil is far
removed from the air-water interface, so that the gravity force on the water may be ignored. This condition is
violated for the case of the rowing blade. It seems to me that this is the most plausible reason why the resultant
force is not in the ideal ‘hydrofoil-direction’.
4Some examples are quoted in [7].
5For one particular motion of the blade relative to the water (known as ‘zero angle of attack’), the scale-
model measurements show a force very nearly perpendicular to the blade velocity (and of course, the oar shaft).
The force however is negligible and this motion of the blade can only exist for a negligible instant at the start
of the stroke, if at all [6, 7].
3
catch.
In conclusion we can say that the blade force dissipates a larger fraction of the rower’s
applied power in the early parts of the stroke than it does later in the stroke (i.e. the
efficiency is lowest close to the catch). Part of the dissipation is due to the sideways
component of the blade force. Whether there is much the rower can do to improve the
efficiency while maintaining a race wining speed is, of course, another question, which is
discussed to some extent in Ref. [7].
Acknowledgement
I thank my brother, Neal Macrossan, for many helpful discussions of rowing.
References
[1] K. Affeld, K. Schichl, and A. Ziemann. Assessment of rowing efficiency. Int. J. Sports
Med., 14:S39–S41, 1993.
[2] N. Caplan and T. N. Gardner. A fluid dynamic investigation of the Big Blade and
Macon oar blade designs in rowing propulsion. J. Sports Science, 25:643–650, 2007.
[3] N. Caplan and T. N. Gardner. Optimization of oar blade design for improved perfor-
mance in rowing. J. Sports Science, 25:1471–1478, 2007.
[4] V. I. Kleshnev. Why is a long catch not a waste of energy? Rowing Biomechnics
Newsletter, 6, June 2006.
[5] J. Lighthill. An informal introduction to theoretical fluid mechanics. Oxford University
Press, Oxford, New York, Tronto, 1986.
[6] M. N. Macrossan. The direction of the water reaction force on a rowing
blade and its effect on efficiency. Mechanical Engineering Report 2008/03,
espace.library.uq.edu.au/view/UQ:134269, University of Queensland, Australia 4072,
2008.
[7] M. N. Macrossan and N. W. Macrossan. Energy efficiency of the row-
ing oar from catch to square-off. Mechanical Engineering Report 2008/05,
espace.library.uq.edu.au/view/UQ:134879, University of Queensland, Australia 4072,
2008.
4


Does the pinching force dissipate the rower's energy?

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Does the pinching force dissipate the rower's energy?

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