Kármánsche Wirbelstraße and vertical stiffness

As Rory Cowan points out, some blades when pulled through the water show an up-and-down shuddering which is very unpleasant to row with. It is a phenomenon first studied by Von Karman and called after him the "Karman Vortex Street".

Basically certain shapes in a moving stream of fluid, shed vortices first from one side and then from the other, which causes the shuddering. If they are flexibly mounted the effect gets worse, so the vertical stiffness of our shafts does matter, and I think torsional stiffness can also be a factor.

The only way to find out if we have a vortex shedding problem is to try, but I do know one cure, which is to mount a longitudinal spine down the centreline of the blade, on the front, back or both faces. I think it works by shedding the vortices from the centreline rather than the edges.

Yes, our blades suffer a little from this.  (820 x 150 mm needle)
It is only noticeable when paddling light though and when under full power is not really a problem.
I did anticipate this phenomenon before construction and tried to leave part of the loom protruding along the power face of the blade to improve this.
Eastern have done a better job with their blades and have a central spine for perhaps 40% of the blade length. (My guess from looking at photos.) In conversation with them I know that their blades are stable.

I'm sure there are other clubs with similar stable oars though.

I also considered building in slight dihedral ie curving the power face part of the cross section rather than flat. I rejected this as I thought it would be less efficient.
As i said earlier, I am not unhappy with the blades and am not going to change them this winter. Perhaps the effect is more prevalent at slow speeds?

Vertical Stiffness - yes important and for this reason our looms start off at 60 mm which gives quite a stiff oar in the vertical plane.
I would be concerned with using a 14 foot oar which is less deep than this.

Ian

P.S.
I have an idea where we make a blade with a single sheet of ply and use a central spine to hold the curve. (Not necessarily for whole of the length) This is then faired in with epoxy and micro balloons to create an easily made spine. Cut a curve in the end of the loom and stick blade in place. Fair forward face into end of loom.

Would improve the flutter characteristics me thinks and fairly simple to make. Could still be too complex for public consumption though?

Blade flutter is only a problem if your rowers are too strong! Heart flutter is more of a problem than blade flutter with the age group I row with!

Ha Ha! Rowing keeps you young though...

In fact our blades do have slight curvature to the power face so it is slightly convex - which maybe helps in a dihedral manner (though not as much as Ulla's exisiting long thin blades, for example, which have pronounced dihedral.)

Would be slightly concerned about Topher's new blade which I have just realised is very anhedral and hence potentially unstable.
You've probably clocked this already though, Topher?
Probably adding a spline (which I'm sure you were going to do anyway) would help.
Ian

I agree it has anhedral in the aircraft wing sense but vortex shedding is a bird of a different feather to aircraft roll stability. The vee is just an approximation to the vertical curve in section that a spoon has. (admittedly in wooden spoons there is a spine in the middle). Anyway we will soon know.

I find this to be a fascinating revelation, as it's not something I've experienced in oars before (though I have come across the effect in the context of a yacht foil that had the maximum chord located too far aft ... but that's a story to be reserved for rudder design forums elsewhere!). I'd like to suggest that angle of entry will also have an impact on vortex shedding (i.e. the combined angle of the pins and the boat's trim when under weigh) as this subtly changes the pressure differentials across the blade.

Hiya,
yup it is interesting!
I did quite a bit of research on the subject before designing our blades and found more info on the subject in kayaking web pages and forums than rowing pages.
(Sea kayaking is my other obsession on the sea  - hence my interest.)

The use of a dihedral x-section is the oft quoted solution to the problem (Hence my use of the term) and the central spine described above is  a form of the same thing. Also hence my referral to Anhedral for Topher's spooned design.
Though yes it works in a different manner.
From memory I think thin walled spooned designs are less stable than thick convex/dihedral sections.
I think NB oars have probably always been stable as I think your original longer thinner design had a central spine and your current Macons do also I think?
Similarly Eastern's blades have a central spine for nearly half the length and are stable in the water.

In your yacht rudder - I assume the hinge line was at the max thickness? Hence flutter if too far back?

Anyway, we digress!!

There's a bit of central spine where the loom attaches to the plywood blade, though this doesn't extend very far. Our blades are made from 9mm ply (if I remember correctly) so they don't really require much support.

(In response to the digression - it actually wasn't the rudder at all. A friend of a friend had built his own yacht and had trouble controlling it at speeds above about 5 knots; I diagnosed that it was probably because of vortex-shedding on the keel, which was pretty thick and with its maximum chord about 50% aft of the leading edge.)

oh right ta!  

At Catch at the beginning of the Drive , there is flow over the blade and this creates vortices. The one on the leading face tends to be retained whereas the one at the back of the blade is shed. Just as with an aircraft wing this gives Lift (in the horizontal plane).This is an important part of the Drive . Curved blades help to retain the vortice and Lift for longer.
When the oar is perpendicular to the boat in the middle of the Drive , the blade is effectively stationary in the water and all vortices are shed. Towards the Finish there is again flow on the blade and vortice formation but they tend to be swept off the blade. This is an inefficient part of the stroke and excessive "lean-back" looks good but is not effective.
The practical outcome from this is that the Catch should clean and powerful and the finish should not be prolonged.
More details can be found at: http://eodg.atm.ox.ac.uk/user/dudhia/rowing/physics/rowing.pdf