Oar shaft stiffness - engineering

[Rebecca Caroe]

Just found this on the Rowing in Motion site
Carl.... your views?
http://www.rowinginmotion.com/oar-shaft-stiffness/

[carl]

Interesting to see points which, for so long, I've been putting before
RSR at last being taken up there as if they were original, including:
1. energy storage in bending of shaft
2. undesirability of extracting a still-bent shaft

However, stiffness has only a secondary influence of gearing, & only to
the extent that you are fast or slow to apply the load.

What was not mentioned, is never mentioned, yet I've rasised it on RSR
over many years (so now expect it to crop up shortly as a supposedly
original thought, i.e. without acknowledgement, in some quasi-scientific
article :) ) - is that the blade's alignment WRT the shaft is very
dependent on shaft flexure & hence on loading & stiffness. The angular
deflexion of the blade from its unloaded alignment WRT the shaft axis is
proportional to load, inversely proportional to shaft stiffness _&_
proportional to the cube of shaft length. This deflexion can be very
pronounced, greatly altering the geometry of the oarblade's interaction
with the water with consequences which are _never_ considered, let alone
addressed.

Thus the blade of very strong rower with fast catch loading may
experience a markedly different flow geometry to that of a weaker,
slower-catching rower. To some extent (but take care before rushing
into interpretations!) this may be like taking a longer catch. And it
will have interesting effects throughout the stroke.

Cheers -
Carl

--
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[madmar...@gmail.com]

On Tuesday, 21 July 2015 18:59:11 UTC+8, carl wrote:

>
> Thus the blade of very strong rower with fast catch loading may
> experience a markedly different flow geometry to that of a weaker,
> slower-catching rower. To some extent (but take care before rushing
> into interpretations!) this may be like taking a longer catch. And it
> will have interesting effects throughout the stroke.
>

At least one oar manufacturer does suggest different shaft types for different rowers - basically stiffer shafts for bigger rowers in faster boats and 'softer' shafts for smaller and less powerful rowers.

[thomas....@googlemail.com]

Croker recommend shafts for different erg scores

http://www.crokeroars.com/#!sculling-shafts/cbaj

SlidingSeat also has a post on bending shafts which repeats Carls point that one of the main issues with a bendy shaft is changing the angle of the spoon in the water

http://www.slidingseat.net/howtorow/howtorow.html#oarbend

[John Greenly]

Carl, this is one of the important complications in my analysis of the Sinkovic video, because the deflection does affect the blade angle significantly as it is loaded after the catch. I'm comparing results to see how much difference it really makes whether or not I include this effect. It appears to be most significant at large oar angles (near the catch), and tends to reduce efficiency there.

Cheers,
John

[John Greenly]

Rebecca, thanks for posting this! These data from Sliasas and Tullis are very interesting in several ways. What happens near the finish really catches my eye, I've been measuring this in video I'm analyzing.

The comment made in the text there is good, but I think there is more to say. If the shaft is still bent, that is, the load is still held on, as the oar approaches the finish, then it can possibly help with clean blade extraction. As the hands reach the body they have to reverse from moving bow-ward to moving sternward, of course. As they come up to the reversal point, if the oars are still flexed they will then un-flex, giving some extra sternward motion of the blade. If timed right, this will both couple the stored energy in the bent shaft to the water just before extraction, and also give a little extra effective length at the finish to get the blades out cleanly.

Good rowers do take advantage of the flex this way. I think it's natural and probably unconscious, just by their coordination of the feel of load in the hands with the extraction. In the Sinkovic video I am working with, the top of the blade emerges as the load (flex) has been rapidly decreased to about 25% of its peak value, and at that instant the sternward motion of the blade is roughly half due to handle motion and half due to shaft unflexing. So he is indeed neatly coordinating the unflexing (unloading) with the extraction.

The feel will be that the load holds on for a little longer, as the hands slow down in the moment before reversal, than it would without flex. I think you can actually feel this if you pay close attention. I remember years ago rowing a very heavy fixed-seat boat with very long, flexible oars, and you had the feel that you could stop your hands at the finish and wait for the oars to unflex before lifting them out neatly- a great, springy feeling! I think that this is why my finish and extraction are much cleaner- the blades pop out neatly- when I have the feel of holding the load on right to the end, rather than slacking off sometime earlier.

many thanks,
John

[carl]

These brief moments, as the shaft straightens with the blade still
immersed, with the sense of continuing load but the hands not moving,
have long been the unappreciated basis of what, on this side of the
pond, some coaches like to term "send".

[carl]

I would expect such a serious student of this subject as yourself, John,
to see this effect, especially with these 2 strong guys. but I would
myself be cautious about supposing that this angular deflexion under
load was necessarily adverse in its consequences at this or that point
of the stroke. After all, who's to say that blades are optimally
aligned WRT their shafts in the first place?

We're dealing with such evidently sub-optimal kit that I'd prefer to
stand back & consider every aspect of a well-loaded stroke sequence
before letting my own prior assumptions cloud my interpretations. Which
is not to say you are wrong, but just that I hope that preconceptions
will not cloud your view.

IMHO the typical oar is likely to be very far from the perfect implement
of the job, the focus on blade profiles, "vortex edges", spines (or not)
& lips at the top (or not) already showing no obvious performance
superiority of any 1 of these options over any other. Meanwhile we've
seen no experimentation with the inclination of blade chord to the shaft
axis, no discussion of blade surface curvature, or of blade tip & root
geometries, while retaining the presumption that area should be maximised.

I would suggest that we'd do well to suppose that the oar has evolved
but slightly from that original flattened stick, & therefore to exercise
some caution before presuming that their blades can be taken as
representing meaningfully advanced design.

[carl]

Re "Sliding Seat": Magnus is always quick on the uptake.

Re Croker: recommendation is one thing, but an erg score tells so very
little about a rower's technique, whereas blade deflexion around the
catch may be as much affected by technique as by erg score.

I also worry about such statements as:
"These 100% carbon shafts exhibit crisper catches and allow higher
rating than our elite level S2 Soft models." That doesn't sound too
scientific to me, since the catch is created by the rower's technique &
speed of action, not by the blade.

[John Greenly]

On Tuesday, July 21, 2015 at 2:24:02 PM UTC-4, carl wrote:

> I would expect such a serious student of this subject as yourself, John,
> to see this effect, especially with these 2 strong guys. but I would
> myself be cautious about supposing that this angular deflexion under
> load was necessarily adverse in its consequences at this or that point
> of the stroke. After all, who's to say that blades are optimally
> aligned WRT their shafts in the first place?

Hi Carl,

no supposing, just trying to follow the data to correct conclusions- that's why I'm still working on it (I'm not delaying to enhance the suspense, I just don't have much time these days unfortunately). The question of how best to orient the blade with respect to the shaft is a really good one that I certainly don't know the answer to.

cheers,
John

[John Greenly]

On Tuesday, July 21, 2015 at 2:00:47 PM UTC-4, carl wrote:

> These brief moments, as the shaft straightens with the blade still
> immersed, with the sense of continuing load but the hands not moving,
> have long been the unappreciated basis of what, on this side of the
> pond, some coaches like to term "send".

"Send", what a nice word! Now I remember a Maine fisherman using that word in that way when he taught me to row his dory. He showed me the spring in the oars as he rowed, but I was only 10 yrs old I think, and not strong enough to bend the oars! Thanks for bringing back a fine memory to me!!

"The spring in the oars sends the boat along"... or something like that.

John

[Alexander Lindsay]

I'm not sure how relevant this is.

Many years ago, in the 1950s, my club had a set of oars made to the specification of a previous Captain. (In those days, with wooden oars made by hand, you could tell the maker what you wanted. Happy days!) He had specified that they were to be as rigid as possible, in the belief, I was told, that that would lead to quicker catches. They had mahogany backs, and weighed a ton, but of course all oars were heavy then. The result was a set of oars that everyone hated; it was almost impossible to get a clean finish.

Ever since, I have been convinced of the need to have flexible oars which maintain a bit of pressure as the hands stop.

Alexander Lindsay

[Jim Dwyer]

"The Oar represents a first class lever with the gate being the fulcrum (see
[1] for discussion). “Bend” describes the deflection of the oar under load
applied to the blade with a “softer” shaft having more deflection at similar
load than a “stiff” shaft. Various ways exist to measure shaft stiffness,
for example this is the method used by Concept2 to describe their oar shafts
(see Fig. 1)."

I stopped reading after the first few sentences.
The oar is a second class lever! If the author doesn't know that then why
bother continuing to read?


Jim


"Rebecca Caroe" wrote in message
news:cec3dbce-f8e2-4b72...@googlegroups.com...

[Jim Dwyer]

When choosing the correct shaft stiffness you must also consider how stiff
the boat/rigger combination is as well. A very stiff boat/rigger and shaft
combination will feel harsh and leave you feeling beat up and sore at the
end of the row. It may lead to injury as well! Many masters rowers in
Ontario are choosing boats that are not as stiff. They are more forgiving
and comfortable to set up and row.

Jim

wrote in message
news:4126de36-b4e9-4b1f...@googlegroups.com...

[Kit Davies]

To be fair he recognises this discrepancy in his footnote at the bottom.
If Einstein is correct and all notions of time and place are relative,
he has a point. Personally I'm with you, that for practical purposes, it
is 2nd-class.

Kit

[James HS]

I always think that the opposite of send is when the rower looks surprised because they have not allowed that stored energy to release, pull their blade out 'square' (as instructed by their coach) and then the blade 'jumps' clear of the water leaving them to have to correct before they begin their (subsequently) delayed recovery.

When I ask why they are releasing so early I am normally told that they are worried about getting stuck ...... so I ask them to allow (if they are in a 2X and one sitting the boat) their blades to float free - and at the end they beautifully stop and feather themselves out of the water .... all by themselves :)

James

[John Greenly]

On Tuesday, July 21, 2015 at 5:27:56 PM UTC-4, Alexander Lindsay wrote:
> I'm not sure how relevant this is.
>
> Many years ago, in the 1950s, my club had a set of oars made to the specification of a previous Captain. (In those days, with wooden oars made by hand, you could tell the maker what you wanted. Happy days!) He had specified that they were to be as rigid as possible, in the belief, I was told, that that would lead to quicker catches. They had mahogany backs, and weighed a ton, but of course all oars were heavy then. The result was a set of oars that everyone hated; it was almost impossible to get a clean finish.
>
> Ever since, I have been convinced of the need to have flexible oars which maintain a bit of pressure as the hands stop.
>
> Alexander Lindsay

That's great! It seems to illustrate just what we're talking about, thanks for posting!

--John

[Lucy]

James, that's how my coach taught me to dare have a second attempt - the first time I thought about it too much and got the blades stuck, but once I let them do the work for me everything "clicked".

[martin...@gmail.com]

Some of these comments take me back to when I started.

Like people have said - using the wooden blades could be interesting - the age, design and manufacturer all added to the bendiness of the loom. Not sure how much wqs by design or just age.

I seem to recall being told to "sit" and hold the finish an let the blade spring through to its finish then extract. it did mean adapting to different sets of blades as they were never the same. But for totally rigid blades we had a set of aluminium shafted ones - never did get used...............

I also seem to think that we were shown videos of international crews using "softer shafts" to get a whip effect? i.e. they loaded the blade at the catch causing it to bend and then allowed it to release the energy as though it were a spring. this is all in the dim and distant past so could be wrong of course

M

[marko....@gmail.com]

On Tuesday, 21 July 2015 19:00:47 UTC+1, carl wrote:
> These brief moments, as the shaft straightens with the blade still
> immersed, with the sense of continuing load but the hands not moving,
> have long been the unappreciated basis of what, on this side of the
> pond, some coaches like to term "send".
>
> Cheers -
> Carl

I wonder if this is why a good finish happens so easily after a good catch. Whether with the blades loaded early the shaft bend is well established and is maintained through the stroke.

[carl]

Marko -

A good catch sets up a good stroke - because it's then so much easier to
follow through to a good finish. A less good catch mucks up every
subsequent element of the stroke, including the finish.

While it is important to get an early load/bend on the shaft, the catch
is some way from the finish so I doubt, other than for the reason
already stated, that a worse catch should affect the amount of bend on
the shaft towards the finish. It's just that there is so much more
command & confidence in any action which starts well - a good finish is
dead simple whereas a clumsy stroke is complex all the way.