Rowing: does every oar need a blade?

[rower]

TL;DR Summary

Why would a large reduction in blade area have so little negative effect on the performance of a dinghy oar?

Blades in Photo 6 and Fig 7 can be mismatched as pairs and the boat rowed straight (video 1) indicating some parity with variations at startup, braking, and with/without flow induced vibration (FIV). Fig 2 and 16 show likely blade travel (based on repeat observations). On entry FIV blade is a bluff body in apparent flow of horizontal slippage, minus boat motion, producing FIV for vertical travel where a more streamlined positioned blade, with high aspect ratio and small angle of attack, could generate a favorable lift phase with a good longitudinal component. At each peak of amplitude the FIV blade returns to being a bluff body and repeats the cycle with more cycles in startup, braking and strong headwinds.

Despite the nice construct of Fig 17, seems little evidence of a lift phase in actual rowing as blade exits downstream of entry ( "V. the rowing stroke" from The Physics of Rowing by Chris Pulman ) eodg.atm.ox.ac.uk/user/dudhia/rowing/physics/rowing.pdf Fig 18, with vortices' low pressure at the edges might explain less need for blade area, but outline blade has double the edge length to no noticeable effect.

Extended blade travel of FIV might counter lost blade area in the momentum transfer of a drag driven system. Although non FIV 7H2 (Fig 7) has equal performance to 7A when boat is in motion with less apparent flow, it has poor performance in higher flow of startup and braking where FIV does improve performance as in 7G. Or, could this effect be changes in Reynolds numbers, Fig 14? Seems rowing can be a mental exercise.

FIV is of practical interest since my rowing conditions can be extreme, requiring quick, forceful maneuvers where non FIV thin blades are poor performers, but FIV blades are problematic in grounding out in shallow water. Would reduced amplitude and higher frequency still retain effectiveness? and how to get there?

With 7G section applied to the paddle of a kayak, a lighter craft with less drag, startup and turning by braking were much slower with no vibration, maybe due to the asymmetry of paddling verses rowing, but once boat was in steady forward motion and blade vibrating, it could match performance of pictured paddles, Photo 8 and video 2.

7G blades have been in use for 2 years replacing 7A, they are easy to build, store, carry, use. They generate amusing conversations which have yet to reveal the physics involved, hope this posting will.
Note: am posting for first time and am unable to upload videos…

 

[Baluncore]

Why would a large reduction in blade area have so little negative effect on the performance of a dinghy oar?

A kayak is propelled with a double ended paddle. The kayaker can adjust the position of their hands on the paddle shaft, and so change the leverage to the large blade area. When starting, they will hold the shaft near the blades, once at hull speed their hands will be closer together. They are, in effect, changing gear, by impedance matching their anatomy to the blade area in the water. Much of the drive from a kayak paddle, comes from moving the paddle sideways in the water, like one blade of a propeller. The blade is generating lift, which both propels the kayak and increases their stability. Variation of the paddle blade angle provides another way to adjust or match the paddler's anatomy to the water, like a variable pitch propeller.

A dinghy is rowed with an oar, that passes through a fulcrum, the rowlock. There is little opportunity to adjust the hand position, to match the anatomy to the blade area in the water. The key adjustment available, is to change the area of blade and oar shaft that is submerged. Much of the work done by an oar is being done by shaft drag through the water, the shaft including the midline of the blade. The blade outline, does increase the area of contact, but it is mainly being used to control, by shaft rotation, the entry, depth, and extraction of the shaft from the water. At the end of the stroke, water is spilled from the blade as it lifts through the surface. That effect is lost when the oar blade area is reduced. The rectangular hole in the oar does not greatly alter the drag, since the oarsman will plunge the oar deeper into the water, to increase the shaft drag.

 

[sophiecentaur]

A kayak is propelled with a double ended paddle.

The paddle is two types of lever at the same time. When the right paddle is in the water, the left arm and the right arm both move so each is a kind of fulcrum for the other one. It achieves a (very intuitive) form of infinite ratio gear. Changing to the left paddle reverses the functions. There are so many different applications for a basic kayak that you could expect a wide range of shapes and sizes of paddles.

The blade outline, does increase the area of contact, but it is mainly being used to control, by shaft rotation, the entry, depth, and extraction of the shaft from the water

I see where you're going here but there has been a lot of development in rowing blades for racing so that rather implies that the blade supplies a significant amount of torque. I found (Google) many racing blade images and (I'm 60 years behind) was surprised by the shapes of modern oars. They seem to be very asymmetrical. Doesn't that require a constant controlling torque to keep the blade vertical; it can't be locked because it has to feather on the return stroke.

In sculling (everyday getting places as opposed to rowing races) it is common to move the oars in and out over a fair range in the rowlock (leather sleeve) and even overlap the hands for maneouvering.

Matching up all the rowers in a racing eight must be difficult, with different arm and leg lengths and strength.

 

[Baluncore]

When the right paddle is in the water, the left arm and the right arm both move so each is a kind of fulcrum for the other one.

With a kayak, most of the power comes from the body, twisting and leaning, reaching forwards then leaning back. The arms, holding the paddle shaft, are then more like fixed links, with one wrist controlling the paddle shaft twist angle.

In an emergency, a kayak can be propelled by the hands without a paddle, the cadence is much higher, and the stroke shorter. That is similar to a surfboard rider, paddling out with their hands, or catching a wave. The area of the hands and wrists is much less than a kayak paddle blade, but the speed reached is still limited by hull speed. It is also easier, with practice, to hand roll a capsized kayak back upright, with one sweep of the arm and hand. You would not throw your paddle away to do a roll, but the paddle tends to get in the way, is slower, and requires more coordination.

When rowing a dinghy, the thwart, (bench seat), is fixed, there is little secure bracing, and the force is limited, by the weight of the oarsman, sitting on the thwart. The oars, used to manoeuvre a dinghy, do not need big blades to handle the forces generated mostly by the arms.

In a rowing race, most power comes from the legs and the trunk, with the seat being a slide, on rollers. Outriggers are used on racing shells, to fix the position of the oar's fulcrum.

 

[rower]

Could there be increased lift with more sideways travel of vibration? Found less performance when thin oar blades are feathered. Now row with no rotation and blades fixed perpendicular to water surface. Blade still fallows a good trajectory and is small enough for little need of feathering on return stroke.

 

[Baluncore]

Could there be increased lift with more sideways travel of vibration?

Probably not. Vibration or fluttering will spill water faster, and reduce the "grip" of the oar on the water. Maybe the oar surface is ahead of the shaft, so is unstable, spilling water on alternate sides? Maybe the blade is too flat?
I would want to know the source of the vibration or flutter. Cavitation occurs more easily near the surface where there is only atmospheric pressure, less at depth, where hydrostatic pressure is greater. Keep the blade away from being part submerged at the surface, where the conditions are less predictable.

Found less performance when thin oar blades are feathered. Now row with no rotation and blades fixed perpendicular to water surface. Blade still follows a good trajectory and is small enough for little need of feathering on return stroke.

That may be true under still conditions, but with a strong headwind, you will tire quickly, and find yourself being blown downwind. Notice that the blade moves ahead through the air, at more than twice the speed of the vessel, with more than four times the drag from wind resistance.
The feel of the feathered blade skipping, just before it enters the water surface, will give you a good sense of blade angle, without looking. You can therefore apply power immediately it enters, as you are confident that the blade will bite immediately.
Move the blade forwards, feathered, almost skipping on only the tops of some ripples. That will minimise windage, as the blade will be moving upwind in the thin boundary layer over the water. Learning to feather the blade, will quickly become instinctive, you can stop thinking about it, and you will find it easier in the long run.

A kayak blade, as it moves forwards, is high above the water surface, so it is subjected to air resistance at double the vessel velocity, plus the full wind velocity. The drag encountered is proportional to the square of the combined air velocity.
Kayak paddles are double ended, with a close to 90° angle between the blades on the shaft. The blade-end, that is moving forwards through the air, is feathered automatically. Only a kayak beginner, would accept being given a toy paddle, without the feather twist, by someone who did not know better.

 

[rower]

Maybe the oar surface is ahead of the shaft, so is unstable, spilling water on alternate sides? Maybe the blade

Blade is in line and centered along shaft. Blade sections in Fig 7 have descriptions of actions. Vibrations could be Karman street vortex. After entry blade runs deep and even with vibration never disturbs the surface until exit. With width of blade and diameter of shaft being equal, there is less need for feathering on recovering stroke but I referred to submerged feathering for blade travel. The kayak paddle was for comparison since this work was developed on my rowing dinghy which might have unique characteristics.