Re: How I decided which new sculls to purchase

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. :-)

Upon re-reading before posting, I had some more thoughts:

Whether we see a "catapult" effect depends on if the force bending the beam is release more quickly than the rate at which the beam can unbend.

So, hypothetically, a rower might see a catapult effect if they, upon reaching maximum force on the oar (i.e. roughly mid-stroke) just stopped pulling altogether and let the oar unbend (with the idea to then extract the oar from the water at just the moment that the oar is completely unbent.

Duvall Hecht (my college coach for those not keeping track, and an old-school rowing stylist if there ever was one) taught a release that supposedly took advantage of the bend in the oar to assist with the extraction of the blade. I won't go into the details here. I think I posted about it back in the mid-90's if you want to search. The problem with this concept though, is that by the time you get to the release, even if you're still pulling with all your might with your arms&shoulders toward the finish, the force is dramatically less than what it was at mid-stroke, & so there really isn't a whole lot of energy left to be recovered from the bent oar: it's already mostly unbent.

Charles, I feel like I've rambled without directly answering your questions. Hopefully this has helped some, and maybe having read it, you can ask some more questions.

-KC
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