Need analysis of rowing experiments reducing blade area while maintaining performance

The amount of research on competitive rowing is overwhelming but little is applicable to utilitarian dinghy rowing. High aspect blade motion is horizontally rotational, out and in from boat, and vertical only down to blade immersion with much ventilation (Fig. 1); competitive rowing seems to move little water quickly with great effort by trained athletes in specialized craft. For a dinghy, high aspect blade has less rotational motion, speed, ventilation, but more vertical travel (Fig. 2, 3), and is user-friendly.

To judge performance chose routine use over several days in various conditions switching, port to starboard, mismatched prototype oar with traditional 5” blade (Fig. 4B, 7A) as base comparison. Using boat rotation from line of travel had too many variables in open water, and the instinct to row straight, with its unconscious corrections, is very strong.

Experimental prototypes, based on dubious theory, did surprisingly well, empirically leading to less area through chord reduction and the higher aspect thin blade. Thin oar (Fig. 4C, 7BCFG) matched base comparison oar (Fig. 4B, 7A) in general performance, even in first stroke and braking which others could not, and produced my first experience of flutter. Steel was quick and easy for prototypes but reached its limit with the pipe oar (Fig. 5); switched to wood for stiffness and more sectional variation (Fig. 6).

Further research found the Greenland paddle, a precedent for maintaining performance with reduced area, but much discussion was of avoiding flutter, rather than theory. And Petros on this forum https://www.boatdesign.net/threads/what-is-the-propulsive-efficiency-of-oars.47468/ , page 4: his downward slice at an AOA (Fig. 8) seems a far better model for dinghy rowing than the racing research. My simplistic diagram (Fig. 3) looks possible but does not account for the persistence of flutter, high at startup and braking, less at sustainable speeds. Petros attributes this to lack of technique and Karman vortex street, clearly evident in braking; but no change of AOA or technique eliminated flutter in the thin blade, maybe a difference between oars and paddles.

Although initially annoying, the extra travel per stroke and/or the turbulence of flutter seem beneficial. Again Petros’ “The forward facing surface is of course the low pressure side of the foil”, seems good condition for LEV with high lift and no back side drag penalty; but sharp edged flat side of Fig 7D had no flutter, less performance, more ventilation. With a simple 180 degree flip of the oar (Fig. 7C), there is flutter, better performance, less ventilation.

I’ve been rowing with matched thin blades (Fig. 7G) through the fall and into the winter in various conditions with good results but with no insight or basic diagram as to what’s going on. Time for a more objective rower -- a reluctant friend expecting to be rowing in circles with mismatched oars (Fig. 7A, 7F), said upon return, “Now if I lose an oar, I know any old stick will do.” Need help in finding a better stick!

 

Isn't the point to move as little water as possible while moving the boat as much as possible?

"Need analysis of rowing experiments reducing blade area while maintaining performance."
Why? What is your objective?

 

Why not do your own practical research with an electric model rowing device, and a collection of paddle types, in your bath or pool. If you're only testing paddles, it won't matter if the row machine isn't 'anatomically correct' in its motions, because it's all relative to the conditions being identical for different paddles. Even a single oar position giving flow speed of fluid in a channel of water, or volume 'paddled' (pumped) over time, could inform you of any shape benefits. Whittle up as many model oars as you like, with standard or radical shapes. Try a hockey stick or cricket bat. You may find the next big thing, or the best marine store oar/paddle shape. Keep in mind that people have already been paddling for thousands of years, so probably everything has already been tried, right up until outboard motors. Leg oars aren't much different, sculling is a bit different, ancient Greek Tireme oars look modern, Pacific Island traditional oars are a bit different. Maybe modern materials may make a difference, eg. light weight hollow carbon fibre oars, or flexible sections distorting to benefit in different ways throughout the stroke. I'm sure some college or uni would have already done materials science study on rowing gear, since it is a prestige sport. Try searching in google scholar for research. Reduce the blade area with a long propeller shaft, electric motor, solar panel, battery, and a beverage of your choice, while towing a shiny lure. Unscientific but sometimes worthwhile.

 

Point was trying to show the geometric difference between two distinct rowing systems. My objective was to satisfy my curiosity, after kayaking next to a fast swimming deer, that there might be other blade possibilities.

 

Way too many suggestions for my limited capacities, but a tennis racket came to hand, lousy oar but good ventilator, "sometimes(not)worthwhile" but looked great in action.

 

If you are interested in very narrow blades, you should check out the Irish curragh, which is rowed with oars often described as "bladeless".

 

Thank you and the Irish, so any stick, even a deer's leg, might do, but why? Foil seems to be high aspect operating at low Reynolds numbers, with beneficial flutter, any clues here?
Enjoying the ease and visuals of rowing with silly sticks, still looking for better ones.

 

The blade is a foil, so the blade actually slips less due to lift generated as the blade moves away from the boat. This gives the appearance of moving little water when viewed perpendicularly from a fixed point on shore. More water actually moves over the blade as the stroke rate increases. Once the face of the blade starts to move toward the boat, the efficiency of the blade drops. In a kayak using a wing blade, I always keep the blade moving away from the hull to maintain lift. Once the blade starts to move toward the boat, the blade can dive quickly. When I used a flat bladed paddle, I didn't have to worry about the blade diving.

 

Little experience with kayaks but there seems to be much more nuanced control than with oars. Fig 2 dinghy diagram shows little horizontal travel due to vertical sweep down, seems a very different geometry needing a different blade? Ever use a Greenland paddle? Suprised by its arctic origins; with a small area, it still has the force to right a kayak in critical conditions. Rough cold water seems to breed small, or even, no blades?

 

I can anticipate practical reasons for this.
1.) Less likelyhood of the blade snaggging the water on the return stroke. This would be especially true in rough conditions.
2.) less wind drag on return stroke.
3.) Less likelyhood of splashing frigid water into the boat.

My understanding is that, with the bladeless oars, there is not an even speed stroke. The stroke gets sped up during the middle third of its arc, where it will do the most to propel the boat foreward. Since its drag (propulsive force) is the square of its velocity through the water, I can imagine that this technique would be effective.

 

The middle third of arc is also the deepest avoiding ventilation of high speed entry. Also, this the area of flow reversal over the blade. and what might be the consequences of such an extreme transition and is this a possible source of flutter? Flutter and tumble in fluids – Physics World https://physicsworld.com/a/flutter-and-tumble-in-fluids/

 

Thanks for this article. My rowing experience is very limited, but I'm found of fluid dynamics, and I've done some experiments with swimming palms, and also like very much bare feet swimming, as the natural sensation of the vortices shedding along the skin is a very nice one. I've carefully read the diagrams you provided, and, from a fluid dynamicist point of view, I would say that there is two phenomena that influence the rowing performance and its fit to one's use :

- Static force generation : The static force generated by the blade is the combination of the lift and drag generated by the profile. As Seasquirt said, this static force can be experimented quite simply, by putting the blade at angle into a flow vein. This static force is proportionnal to the surface of the blade, and to the aerodynamic coefficient of the blade profile. For a symetrical profile ,without inversion, to generate lift, typical values of the relative cord thickness are 12% to 18%. At constant delivered power, integrated along the beat cycle - and subsequently the lenght of the blade path, high surface or high thickness ratio will tend to lower the beat frequency and increase the effort.
With the ability of a circular profile to produce drag, it can be explained why bladeless oar can be used for rowing. Also, squares or short rectangle profile should work as well, as long as the beat stroke is not too fast. On the contrary, streamlined sections or curved sections would require higher beat frequency, and a better ability of the rower to manage the incidence angle during the whole cycle, as the static force is mainly generated by the ability of the profile to produce lift, except in Phase III. A circular profile would be the more tolerant, and the most easy to build. It may explain partially why the Irish and the Celts in general have use it so often, to navigate in open seas.


Drag coefficient of basic shapes


Swimming cattle across from Aran to the S.S. Dun Aengus


- Dynamic force generation : The dynamic force generation is caused by the flutter, ie, the coupling between the structural response of the whole oar, with the generated perturbation of the flow. The more static force are produced by the blade, the more perturbation is induced to the flow. This perturbation should be kicked out of the blade by elastic response, for its influence to give benefits to the rower. This kick out effect is obtained with a good balance of the oar stiffeness with the flow perturbance. Energy is stored in the material during the cycle, and this energy is released at the right moment, to produce this kick out effect.
When using an oar with a circular blade, ie "bladeless", its occurence will mainly depends on the lenghts of the handle. Too short and you won't be able to kick the flow out. The stiffness distribution along the handle could be used, at constant oar lenght, to create this effect. Too stiff and you will have to add a certain movement to the beat cycle, in order to make the kick out yourself, increasing the effort and the technique required. When using an oart with a highly profiled blade, full carbon built, this effect is enhanced by the flow circulation produced by the lift. So, in general, flutter would arise more often with profiled blades, as long as the oar and its blade are flexible enough.

As for the lazy rower that I am, not very fit with a high power delivery potential, I wish I could make some test with a circular oar, made in aluminimum or glass fiber, with a variable diameter. In that case, I will seek to obtain this flutter, while maintaining a low beat frequency.

Hope this will make sense to you. In a second time, I will describe how these two phenomena influence the results you have presented in the figure 7. Cheers

 

Thanks for your response, sorry for the delay.

Figure 3 shows idealized rotating blade maintaining angle of incidence to flow vein for lift but in practice a wide range of angles made no difference in performance. Phase 3 is where blade 7G becomes a bluff body producing the flutter of a Karman vortex street; a circular profile did not, with loss of performance. This was especially evident in start up and braking when blade is moving as a bluff body relative to the water. The additional blade travel of flutter (greater perturbation of flow) may explain performance despite reduced blade area.

Elastic response is unlikely as my oars are short, 6 1/2 feet and 7 G blade, best performer and winter tested, is quite stiff. Flutter and low beat frequency are my objectives too, but I think you will be disappointed by a circular section.

Need to correct mistakes in FIG 7 and will post diagrams of blade travel with flutter as I understand it.

Photo looks more like towing cattle! Deer are fast swimmers and can keep a kayaker working hard, as I found when I pursued one.

 

The above diagrams and text describes the process of reducing blade area and embracing flutter to good performance effect. Vortex induced vibration, Fig 9 (Data-driven prediction of vortex-induced vibration response of marine risers subjected to three-dimensional current https://www.arxiv-vanity.com/papers/1906.11177/) seems a reasonable model of the mechanics of my rowing, though a round section is a poor choice for rowing. The flattened diamond section of Fig 7G blade rotates alternating lift force further downstream Fig 9A. Optimum shape would maximize low pressure forward facing surface while still maintaining flutter, blade Fig 7C flutters, 7D does not. The simplified blade path diagram Fig 10 illustrates the action and corresponds to actual rowing. Flutter rate seems to respond to load without conscious effort, high flutter rates in startup, braking and stiff headwinds, low rate with continuous strokes and lower rate downwind.

After a winter of rowing in various conditions the7G oars still seem on a par with 7A oars but have less weight and imbalance, handle well in rough water and survived beach launches. They have become my oars of choice, though the predominance of large area blades on oars and paddles could make one skeptical of this work. I have yet to give away my traditional oars, still looking for a better pair of sticks.

The version of Fig. 7 below this text is the corrected version.

Disclaimer, this work was done using a 7' X 31/2' dinghy with 61/2'oars and may not apply to other rowing craft.