# Rowing: blade travel extension through flow induced vibration

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, Nov 12, 2023.

1. Joined: Jan 2008
Posts: 122
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Location: nyc,ny

### lunaticSenior Member

A slow dinghy with oarlocks fixed to gunwales has a different blade motion geometry than a fast racing shell with outriggers, FIG 1, 2. FIG 1 Phase 1, 2, 4 shows blade travel beyond the slippage of Phase 3 common to both systems. Flow induced vibration could extend blade travel of the dinghy by blade design, with no additional hardware or compromise of utilitarian function, FIG 2.

Can a rower's muscular input be better distributed to the water by the increased travel per stroke of a vibrating blade? FIG 11 is a possible model. Vibrations are 3D flow of Karman vortex street FIG 11 A, C, E creating 2D flow of vertical travel FIG 11 B, D. There is much research of energy harvesting from flow induced vibrations, why not energy in, flow out, for propulsion?

http://people.eng.unimelb.edu.au/imarusic/proceedings/11/Jackson.pdf may have an example of extended blade travel with a stretched out U shaped vortex and the intriguing quote "for least effort the aim should be to generate diffuse vortices travelling parallel to the hull as frequently as possible". There is a pronounced ease in rowing with the 7G oars, could be diffuse vortices?

Blade section seems critical for flow induced vibration. FIG 7 B, C, F, G vibrated, FIG 7 D, E, H did not, with difference being much less performance in startup and braking for the latter. Vibrating blades are best for sudden maneuvers in rough conditions and symmetrical sections are essential for backrowing, turning and braking and, with these narrow blades, strength. Good vibrations: high frequency / low amplitude or low frequency / high amplitude? One very annoying characteristic of the latter is the blade bottoming out in shallow water. A flatter section would swing the force vector further forward and reduce amplitude FIG 11. Trade off should be reduction in vertical travel but actual rowing showed no difference.

Hard to know parameters for optimal vibration, section and area, or why non-vibrating blades do so well when the boat is in motion. Perhaps vortices' concentration of low pressure on blade edges leaves interior area less effective FIG 12, especially with no spanwise flow nor growing conical form typical of forward moving sweptback slender foilsâ€™ LEV. Short period of drive stroke and shedding of vortices might also limit vortical growth on 7A making it comparable to FIG 7 H2. Failure at startup and braking could be high load separation on small FIG 7 H2 blade or changes in flow and Reynolds numbers affecting drag FIG 13? Time for a premature prototype: a bi-bladed oar FIG 12A extending edge, reducing area. It has a pleasing asymmetry with oar 7A, edgier with some humor, an elongated tennis racket without the ventilating strings.

Some advantages of these thin blades: less vertical travel for recovery stroke clearance and less impact of oar-wave contact, less windage, less weight including less water clinging to blade on exit, less disturbance and ventilation on entry and exit, better balance, less prone to damage, easier to build, carry and store. Downside: shallow water ground contact, a real problem in rough beach launching at low tide, minor annoyance of additional stop to retain oarlock since blade is smaller than the loom, though a through pin would retain oarlock and maintain alignment, inadvertent feathering has large negative results, and is this system applicable only to slow boats? Many speed runs (SOG on NavX, not great for the slow and variable) at 2.5 - 2.9 knots gave 7A a .1 - .2 knot advantage over 7G oars, gained with considerable forearm fatigue, but the noticeable sound of heightened water resistance indicates a hull with less drag could be even more favorable to the 7A oars. Must be more negatives, but none obvious after a year of rowing.

Hard subject for words or diagrams, I have no numbers only the qualitative results of sustained use. I have reached my limits, need some feedback.

PHOTOS

PHOTO 1

Left to right 7H2, 7G, 7A sections are in FIG 7

7H2 split welded thin wall 1-3/8" fence pipe waterproofed with duct tape

7G whittled down from 1-5/8" dowel

7A well worn traditional ash oar as a base comparison

PHOTO 2

Rich in variety or "any stick will do"

Greenland section G-G, F-F, E-E similar to oar-blade G section

PHOTO 3

Obvious solution yet to be tested or tasted

PHOTOS 4, 5

Improvised non-vibrating crosscountry ski paddle, slow startup but did just fine against bladed one once boat was in motion

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Last edited: Nov 12, 2023
DogCavalry and Will Gilmore like this.
2. Joined: Jan 2008
Posts: 122
Likes: 7, Points: 18, Legacy Rep: 28
Location: nyc,ny

### lunaticSenior Member

Boat rows straight with unmatched oars.
Clip 1 and 2: 7A starboard oar matched with 7H2 port oar, boat turns to starboard due to poor braking of non-vibrating 7H2 blade.
Clip 3 and 4: 7A starboard oar matched with vibrating 7G port oar, boat goes straight on braking.

DogCavalry likes this.
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