Oar Shaft Stiffness

Oar Bending has been discussed in various venues before, but the descriptions often remain diffuse and imprecise. Without claiming this would be an exhaustive description of the mechanics, I want to reiterate the basic mechanics and discuss what effect oar shaft stiffness may have on rowing performance.

The Oar represents a first class lever with the gate being the fulcrum (see [1] for discussion). “Bend” describes the deflection of the oar under load applied to the blade with a “softer” shaft having more deflection at similar load than a “stiff” shaft. Various ways exist to measure shaft stiffness, for example this is the method used by Concept2 to describe their oar shafts (see Fig. 1).

Let’s for a second assume we have a theoretical shaft of infinite stiffness, i.e. a shaft that has no deflection under any load. In this case the ratio of blade and handle force (called “gearing ratio”) would be exactly that of inboard to outboard. Since outboard is in practice is always longer than inboard, the force at the blade is smaller than the force at the handle. A really accurate description though would need to consider that the point of force application on the handle is not the tip of the handle but rather somewhere near the center of the hands (see Fig. 2).

Furthermore, it moves throughout the stroke, especially in sweep rowing where the force applied by the inner and outer hand are not balanced. Similar thoughts need to be applied to the blade. See this paper by Dr. Kleshnev for more information. Leaving the issue of the actual gearing ratio aside, the transfer of mechanical energy is unaffected by a lever of perfect stiffness. All the work performed on the handle is transferred to the blade, even though the force on the handle is larger than on the blade. This is because the mechanic work on the handle $$W=F*s$$ is performed with a higer force on a shorter distance while the work on the blade has lower force but longer distance.

Shafts in the real world of course do not have infinite stiffness and deflect under load. What effect does this have? We need to look at this from two different perspectives: The energy balance and the force balance.

Let’s look at energy balance first. A bent oar stores mechanical energy, very much like a spring. It is however not a perfect storage for mechanical energy but exhibits elastic hysteresis due to internal friction in the shaft (Fig. 3). Some of the energy put into the shaft will thus dissipate as heat and cannot be recovered for propulsion.

This phenomenon is one of the reasons for the often claimed “inefficiency” of softer shafts.

Second, we need to look at the change in forces introduced by shaft flexibility. When the blades enter the water, the shaft is fully relaxed. The subsequent force application on the handle by the rower now however has a different temporal loading profile. When the shaft flexes, the handle force profile will ramp up slower because the oar handle needs to travel a further distance until the shaft can produce a counter force for handle force at the same magnitude as if the shaft were stiffer. In Figure 5 below we can see the result of simulating how shaft stiffness affects handle force and blade force (called “blade propulsive force” in the figure), assuming an identical rotation profile of the gate between both simulation runs.

We can observe multiple interesting results in this figure. First, we can see the effect of gearing at work: blade forces are much smaller than handle forces. Second, a more rigid oar requires a much more front-loaded handle force profile if the rower was to maintain a similar gate rotation profile. Third, handle force becomes negative for the simulated rigid oar towards the end of the drive. This is of course an unrealistic result and a consequence of the simulation constraint (identical gate rotation profile). Nonetheless, it is a consequence of the blade velocity falling below shell velocity towards the end of the drive.

This hints at a realistic problem though: as the shell accelerates throughout the drive and handle force decreases naturally after the leg drive is finished because the weaker trunk and arms muscles are at work, the crew must maintain a positive difference between blade and shell velocity in order not to stop the boat. A shaft that is still flexed at the end of the drive phase helps maintaining this positive difference as it ensures positive blade force is applied. Problematic however is a shaft that is still flexed during extraction of the blades. Not only will it make extraction of the blades more difficult, but also the potential elastic energy stored in the shaft can not be efficiently recovered for propulsion of the rower+boat system.

As the rower applies force at the relaxed handle at the catch, the shaft flexes and will store mechanical energy (called elastic potential energy). This portion of mechanical effort of the rower is thus not available for propulsion at this point in time. The total amount of mechanic energy stored in the shaft has been estimated to be between 2.5% (stiff) and 3.3% (soft) of total work per stroke.

What effect does storing mechanical energy in the shaft have for the propulsion of the rower boat system? There are a lot of factors at play here so that it is difficult to make a reasonable judgement about the advantages or disadvantages of using a softer or stiffer shaft. Let’s recap:

Energy stored in the shaft during the beginning of the drive phase does not accelerate the boat, which would be advantageous to maximize fluid drag efficiency. Theoretically, using a stiffer shaft should result in a more front-loaded boat acceleration profile which would be easy to measure with an acceleration measurement system like Rowing in Motion

Energy release of the shaft is subject to hysteresis (probably of marginal effect), energy may thus be lost

A flexible shaft helps maintain positive blade force towards the end of the drive phase

The change in handler force profile due to a more flexible shaft may be more suited to the physiological capabilities of rowers (working angles of muscles, isometric vs. isokinetic force, muscle contraction speeds)

Flexion of the shaft has a considerable influence on the blade force profile, which in combination with the variation of blade propulsive efficiency at different angles may lead to a higher or lower total blade efficiency for the stroke

Softer shafts may be lighter (decreasing oar inertia and the work required to accelerate the oar) and of smaller diameter (decreasing wind resistance)

The interplay of all these factors is complex.

A recent thesis by Brock Laschowski from the university of Western Ontario, Canada (supervised by Dr. Volker Nolte) has set out to empirically test the effect of varying oar shaft stiffness. The full text of the thesis is unfortunately still under embargo, but the available abstract indicates that no statistically significant differences could be found.

I hope this article gives a reasonable overview of the different arguments to consider when choosing oar shaft stiffness. Curious whether changing oars and inboard/outboard setting has any effect on your stroke? Give Rowing in Motion a try to measure it and see for yourself.

Try Rowing in Motion

[1]: here has been some discussion about this statement in the comments. My current thinking is that from the perspective of the rower, the oar represents a first class lever with the gate as a fulcrum. From the perspective of the water through which the boat is propelled, the blade’s tip is the fulcrum (although this fulcrum moves quite a bit!) and the oar is a second class lever. Happy to discuss this in the comments!)