Re: How I decided which new sculls to purchase

KC wrote:

Charles Carroll wrote:
Kieran -

I am addressing this to you, but I am also addressing to Carl and Paul
and Mike and anyone else who may be interested.

When it comes to choosing sculls I have often wondered about shaft
stiffness. While every oar maker offers sculls of different shaft
stiffness, no one (except Croker USA) seems to offer advice on how to
choose shaft stiffness. Time and again one hears and reads that shaft
stiffness is a matter of personal preference.

But is it?

Recently I know that both Carl and Paul have written about how a flexed
shaft stores energy. And I believe, if I have understood them, that they
think this stored supply of energy is significant.

So how does it signify? How much should a shaft bend? And during which
part of the stroke should this stored energy become available?

I am not sure, but I wonder if this isn't one of the reasons I am
enjoying the Croker sculls so much. They are a little less stiff than my
old Apex-Rex sculls. I can actually feel them bend as I take the catch.

This leads me wonder if I had the power at the catch to bend the
Apex-Rex sculls properly? It doesn't seem so. Leastwise during the drive
I never experienced a stored supply of energy snapping into place. So
then, were the Apex-Rex sculls too stiff for me?

Given how slow I scull, all this may seem a little ridiculous. But I
find it interesting. While I may not be strong enough to use all of the
fluid, I still like to know about all of it.

Cordially,

Charles

Charles,

Admittedly, I have not done as much thinking about the affects of oar
stiffness on the rowing stroke as I have other aspects of the stroke, so
this is sort of off-the-cuff:

Imagine a bendable plank, fixed at one end and free at the other end.
Similar to a diving board (aka a "springboard" in the diving world).

If you add weight to the end of the plank, it will bend. The more
weight you add, the more it will bend. The bending of the plank is, in
engineering/physics terms, called "stored energy". If you remove the
weight the plank returns to it's original state/position. In other
words, all the energy stored by bending the plank is released when the
weight is released. If you put 10 50lb weights on the end of the plank
and it deflected some distance (say "d") each time you added a weight,
then the plank will come back that same distance "d" each time you take
away one of those weights. So when the last weight is removed, the
plank is right back where it started. In this case, the stored energy
hasn't really helped you much (a little maybe, but probably not
noticeable.)

Now imagine a catapult. The same principle applies, except this time
the force that bends the beam comes from people pulling back on the
catapult with much more force than the weight (rock?) sitting in the
catapult "cup". In this way, you've stored more energy in the bent beam
than the rock weighs, so when you let go of the beam, the energy is
released, and the rock is "thrown". Here the energy you stored in the
catapult helped you a great deal. Note that if you had gradually let
the rope out, inch-by-inch, all the energy would still have been
released, but the rock would not have been thrown, instead it would drop
to the ground below the cup, thus again, like the springboard case, the
stored energy didn't help you much. In fact, you could argue that
bending the beam was a "waste" of energy on the part of the people who
did it, for nothing was really gained.

Note the similarities above: any flexible beam will bend a distance
proportional to the force applied. The more force applied, the more it
bends. KEY CONCEPT: the "stored energy" is not "released" until some or
all of the force applied to bend the beam is removed. This is easy to
visualize: If you left all the weights sitting on the end of the spring
board, the board would just sit there bent for as long as the weights
were there. It would never bend more, and it would never "recover"
(move to a state of less bent) until or unless more weight was added or
some removed. Likewise, the catapult will stay "cocked" and ready to
fire as long as the rope holds the beam bent. Until the rope is cut,
the beam just sits there with energy stored in its flexed material.

Now to rowing:
Even the stiffest oars will bend (if slightly) when rowed by even the
weakest rowers. The amount of bending that occurs during the stroke
will be maximum when the force applied is maximum. *GENERALLY* most
people row with a style/technique that yields maximum force somewhere
near the middle of the stroke (when the oar is ~90 degrees to the hull).
After this point, the force applied to the oar decreases steadily
until the release.

The energy stored in the bending of the oar will be recovered - there is
no disputing this fact. The question is, will it be recovered in the
way that the energy in the springboard is recovered as you gradually,
one-at-a-time remove weights from the end of the board, or will it be
recovered in the way that the energy in the catapult is recovered when
the rope is cut, and the stored energy is much greater than the
resistive force?

In the diving board case, the stored energy doesn't help you much. In
the catapult case, it helps you a lot. I tend to think that the way in
which energy is recovered from the oar is more like the diving board
case than the catapult case: the force applied is gradually reduced in
the latter part of the stroke, so the energy is gradually recovered.
However, as I said I haven't spent a lot of time thinking about this, so
maybe others have something more to add. I'll have to contemplate it
more before I can make any strong statements about what I think is
really going on during the stroke. I'm not sure if/when I'll get to
that point though. :-)


-KC

You could also use a catapult analogy the other way around, though:
any 'catapult' effect is energy wasted on sending water backwards at high
speed (e.g. taking the oar out of the water while it is still under load,
causing it to flick water backwards), while 'taking weights off the diving
board one by one' gives the maximum transmission of energy to moving the
boat forwards at a slower speed.
I don't know whether this is a better analogy, but the implied 'optimal
force curve' (steady taper into the finish) is probably more conventional.

.