Huge Mis-Understanding of Oar Fluid Dynamics: Oars are Drag — Not Lift Devices

This morning I was astounded and then annoyed to read a leading manufacturer and designer of competitive oars – get the fundamental reason, the fluid dynamic reason, oars work – entirely wrong — to the detriment of the field.

Fluid dynamics can seem slightly difficult to understand, but with good explanations it can easily make sense. (The field is daunting to many due to the excessive use of math required to take courses barring entry to this priesthood. So lets see how well I can convey some math-free principles here.)

In any case, the unsigned article strongly, and wrongly, claims that oar blades operate using “lift” as though they work like aircraft wings.

No. The truth is that in competitive rowing, oar blades operate almost wholly as a drag devices, like a traditional circular parachute, employing drag only  – not at all like flying wings. (This is not true for all oars. Some oars not used for competitive rowing, such as Gondola oars, do use lift to a small degree.)

Competitive Racing Oars Don’t Employ Lift – Only Drag

The effective fluid dynamics, kinematics, of oar blades is based entirely in how they employ drag – not lift.

Yes, you could go way out on a limb and argue competitive rowing oars can be operated as lift devices with a horrific lift to drag ratio (of at best maybe 0.5-1) — but almost anyone can understand that’s just silly. No one in rowing competition operates oars at an angle of attack where any meaningful lift is generated. (Typically 3 to 7 degrees from vertical or roughly an 83 to 87 degree angle of attack.) In my opinion, no surface (wing or not)  is employing lift until its lift exceeds its drag (an L-D ratio of 1-1 or a 45 degree angle of descent).

What’s more damning is that to employ lift – the oar blade would have to be operated at a small angle of attack – which with typical competitive rowing schemes would indisputably slow the boat down.

It is my opinion, that if competitive rowing DID genuinely employ lift surfaces to propel them, the boats should go dramatically faster. Which is why the Hobie Mirage drive is so amazingly fast with tiny wing surfaces and ordinary people operating them.

What makes the article even worse is the extremely misleading science experiment which demonstrates something that has nothing to do with, is contrary to, how oars are actually used. They show water running down the back of several oars to demonstrate how a vortex generator keeps fluid flow attached.

There is so much wrong with this, it makes my stomach churn.

1. Oar blades never USE the angle of attack shown in the pictures to propel a boat. Blades in competition are at right angles to the fluid flow – not the small angles of attack depicted. If the blades operated at the depicted angles of attack, the blades would be slicing horizontally (not vertically as is normal) in the lake or river – with nearly zero drag and almost total ineffectiveness.

If the picture was a reasonable analogy, it would show the blades horizontal to the water pouring out of the sink faucet. Of course then the flow attachment principle wouldn’t work – thereby showing the whole idea is a farce.

2. While vortex generators can indeed keep flow attached on Lifting Wings, there is no evidence they help with parachute (pure drag) operations.

3. With wings, detached flow is usually bad, but in parachute operations – detached flow might provide more drag – a benefit in this case. So oars might be worse off by employing vortex generators. (Parachute drag is primarily composed of Surface Area and a Drag Coefficient. Lowering the drag coefficient would make a parachute fall faster – not good for parachutists or rowing.)

4. Effective vs Efficiency: Because the article’s author believes lift is involved s/he claims the feature is more “efficient.”

Ow. No. The only thing a rower cares about is how much drag (lack of slip) blades can provide. Lift is simply not part of the equation, so there is no “efficiency” (a ratio using lift) to be calculated.

Oddly enough, an associated article on the same website gets a lot right – and thankfully never mentions “Lift.”

There’s more to cringe at (but I’ll stop after this one) — including the classic “[Delta wing] Aircraft designed to fly at greater angles of attack . . . ”

Wrong again. Aircraft are designed to fly at a range of air speeds and carry a payload range – not an angle of attack. Those air speeds often demand a wing design which can handle the dynamics – sometimes making a delta planform a good choice. A delta planform needs high angles of attack at low speeds. But as far as I know no Delta wing aircraft designer ever wanted the craft to fly at higher angles of attack – because that causes several other problems.

I invite anyone to show an example of a aircraft explicitly “designed to fly at greater angles of attack” – other than for testing purposes.

End of rant.

Summary : Oars are Drag devices – Not Wings.

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2 Responses to Huge Mis-Understanding of Oar Fluid Dynamics: Oars are Drag — Not Lift Devices

  1. Freddy Fender says:

    Consider a kite at a 45 degree angle to the ground with a horizontal wind, with a tight sting so the kite is stationary. This also is nothing like an airplane wing. There is a force on the kite in the direction of the wind, which is drag. There is a force up, counteracting gravity, which is lift. If there was no lift the kite would fall.

    Lift is strictly a geometrical concept. There is lift on an oar blade.
    But you are right about the article you reference being full of misinformation; which is not to say that the vortex edge they describe does nothing, just that their explanation is ridiculous.

    • David says:

      Thank you for your thoughtful and helpful comment Freddy.

      You are right of course, there is some “lift” on an oar blade – even if it has a nearly perpendicular angle to the fluid flow.

      Lift continues to exist at any angle of attack – far beyond stall, up until lift ceases, becomes zero, when the angle of attack reaches 90 degrees.

      As you recall my article conforms to that with — “kinematics, of oar blades is based entirely in how they employ drag – not lift.” This is also effectively correct, since oar blades are at or very near a 90 degree angle of attack. Thus there is not any useful lift providing any propulsive benefit to the oars and the boat.

      A Kite is a “different kettle of fish” which I once believed operates like a wing. It took some experiments with controllable “two-fisted” kites for me to realize that a traditional kite operates as a generally (as opposed to precisely) controlled drag device. Unlike an oar, a kite intentionally uses its lift (in this case to rise). In the case of a steerable kite, it can be turned and controlled quite precisely.

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