bentshaft physics

I use a bentshaft often on flatter sections of the rivers I paddle. I can feel the difference in ease and efficiency. I know much of the ease comes from being so much lighter than my WW paddle. I have wondered about the geometry of the whole stroke. If I am right, they are more efficient because you can maintain a more verticle position past your hip during a forward stroke? My question is, why wouldn’t this be canceled out by a shorter distance forward from your body on the placement to maintain vertical? If you use the same body/arm motion, wouldn’t the distance the paddle is vertical stay the same? Should one compensate by altering your body position by reaching farther forward for placement? or, do I have this all wrong. thanks.

great question, and a new one here

– Last Updated: Dec-01-06 9:51 AM EST –

... I think.

I really don't know the answer. But, like you, I think about it.

My main question about trade offs comes from angle of the blade at the entry and catch. While the bent angle is more efficient in the power phase, it would have to be less efficient than a straight in the catch.

Here is a nice illustration of the power phase from Wenonah's site: http://www.wenonah.com/wenonah/stock/a_CPadBent_b.jpg

But, it seems the bent angle is going to have to result in less power during the initial catch, and perhaps during the beginning of the power phase.

I suspect the answer is that there really isn't much power in the catch and early power phase of a straight shaft paddle either. I guess almost all the power comes from the aptly named "power" phase. Now, if this is true, why is getting as much forward as possible before entry so important? I dunno? More questions than answers here as far as the physics are concerned.

But, one thing is for sure, bents are faster. I use them to try and make the "damn kayaker" I paddle with at least act like she is exerting herself just a little.

body position?
I’m sure no expert but I think I read somewhere that by moving the stroke back relative to your torso you are in a stronger position muscle wise.

I’m not sure how much performance I’ve gained from that vs. the higher stroke rate from the super light paddle. But I do know that I’m faster and my tennis elbow doesn’t bother me near as much.

Now all I’ve got to do is remember not to go for the cross forwards with the bent! Instant pretzle man!

Tommy

My thoughts
A bent shaft is almost always vertical until it comes out of the water beside your hip.

A straight shaft usually enters at a slight angle forward, and then doesn’t reach vertical right away, and then immediatly after reaching it starts to angle backwards.

Cheers,

jackL

Leverage
Note: The following is how I understand it . . . feel free to correct me as necessary.

I think you hit the nail on the head when you mentioned that the power phase is where the power is. The catch is far from your body, with your arms acting as levers on you torso. Near your body, your abdominals and ab. obliques can crunch through, giving you tremendous power (or, in the case of flatwater paddlers, great endurance). This requires a higher stroke rate and shorter strokes, but in the end the results are worth it.

Kneeling changes things a bit, allowing more rotation earlier, so a straight or lesser bent is not as much of a disadvantage.

Despite believing bent-shafts are efficient, I can’t quite get used to them.

bentshaft efficiency
As all others have stated in slightly different terms; bentshaft,combined w/ shorter stroke & faster turnover, allows blade to remain in the power phase for higher percentage of fwd stroke. As Wenonah pix shows downward (& upward) motions of blade don’t get you where you wanna go

vertical catch?
Descriptions of bent shaft paddles giving a more vertical stroke have puzzled me, but I had been picturing similar arm positions reaching out for the catch. With arms in the same position, it seems a bent shaft paddle would be even less vertical on the catch. Now I’m guessing you don’t reach out as far with the bent shaft so the catch is closer to your body to make it more vertical? It is hard for me to picture keeping a bent shaft vertical and still reaching out enough to have much of a stroke at all before reaching the hips.

– Last Updated: Dec-01-06 1:53 PM EST –

there is no way that a bent has a closer-to-vertical orientation than a straight throughout the entire length of the stroke. Just to keep it simple, conjur up in your mind two paddles, both 53", one bent and one straight.

They are good, but they ain't magic.

for me it’s in the catch
I’m no physicist, but my own experience has been that the bent shaft advantage is in the catch–dramatically increasing the power in that phase of the stroke. By emphasizing a strong vertical entry of the shaft of the paddle, the blade enters the water bent forward, acting like a wedge as the tip of the paddle moves down. To me, it is that wedging action during the catch that creates the advantage of a bent shaft.

I might get that wrong

– Last Updated: Dec-01-06 9:42 PM EST –

but that sounds to me as if you're using the blade the wrong way around?
As said before:
the advantage is during the powerphase of the stroke. The 'as far forward as possible' rule doesn't really apply to bentshafts. It's rather 'as far forward as nesseccary'. And the stroke ends once your hipp reaches the shaft. That, together with a catch that is closer to your body than with normal shafts, leads automatically to a higher stroke rate.....

Other subtle differences

– Last Updated: Dec-01-06 10:12 PM EST –

the things you mentioned. So true. And the idea of increased rate with shorter stroke length make sense to me too. Thanks to all for your ideas. larry

From the "damn kayaker"
I dunno if this link on bentshaft paddles and how they work (sent to me by a canoeist) will be of any assistance. I humbly submit it for your perusal.

However, I have noticed that Clarion keeps up pretty easily with me when using the bent shaft paddle on the lake. (He’s no slouch of a paddler anyway.)

The day he’s faster than me, probably in one of those racing canoes he’s bought, I’ll need to buy a newer, faster, lighter boat.

I have read many explanations about the reason why a bent-shaft paddle works better then a straight shaft paddle. So far I have not been really satisfied with any of them, because most of them contain assumptions about how paddling works, that – I think – are not really right. My own crude explanation – so far – is that a bent-shaft paddle makes the forward stroke a little more effective because your are pulling yourself less downward than when paddling with a straight shaft.

Technique?
Lots of the explanations I’m hearing seem to assume lousy straight shaft technique vs good bent.

I say there is no reason you can not maintain the same strong catch, vertical blade and short quick stroke with a straight shaft that you can with a bent. No reason you can’t sit and switch with a straight or J with a bent with a little practice.

With the right shaft lengths I’m not sure I see much difference between sitting and kneeling.

The only real difference I can see is that the stroke is further forward with the straight and back, closer in to the torso with the bent.

Tommy

thanks for posting that DK

– Last Updated: Dec-03-06 6:10 AM EST –

Interesting read. I'm not sure it unlocks the secrets to why a bent is more efficient but it does mention something not brought up here: with the bent there is less pushing forward with the top hand than with a straight.

Oh, and just let me know when you want to buy that faster, sleeker and lighter kayak (he,he,he).

Bent shaft performance
It is not just the power face of the paddle in line with the boat direction that propels the canoe forward, and it is more than the improved biomechanical movement from the bent shaft paddle that makes the boat go faster.

It is not all from the momentum transfer of shoving water backwards to shove the boat forwards. The blade is producing significant forward lift forces in the stroke when it is planted and being driven back and downward. When the blade is completely vertical to the water, the lift forces decline unless it is being thrust downward. What the good paddlers don’t mention much is that they are pushing downward on the paddle during most of the stroke, causing water to slide past the bottom edge of the paddle from the power face to the back face, thereby generating more lift in the forward direction during the stroke.

I spent years with a bent shaft working on planting the paddle and ensuring the paddle was moving straight back and not up and down, inadvertently reducing the forward lifting forces. Once I tried the downward push on the planted shaft during the entire stroke, the speed increased. It does bob some more, but that lift force and energy applied is going into a faster boat.

Before you take aim and fire potshots, consider the following points:

Flat wings fly. Snoopy’s doghouse will fly, although it has high drag. Aeronautical engineers (and I hope most pilots) call it the “angle of attack”, where the relative wind is meeting the wing and splits to go up over or under. You don’t need a curved traditional airfoil for lift to occur. In our paddle case, the paddle is going partially downward and the lift occurs across the bottom leading edge.

Angle of attack for normal flight varies from 0 to 17 degrees when stalling usually occurs. Google “Angle of attack” for more info. Ever wonder why Gene Jensen chose 14 degrees for his bent shaft paddle? I’ve tried a carbon 9 degree paddle and it has a different power phase due to different lift characteristics. After a few strokes, you adapt to the new lift effects and timing with a modified stroke.

Kayak wing paddles are designed for maximum forward lift forces, and when they quit flying (stall), you may capsize from the sudden force decrease. Normal kayak paddles fly some. Rowing oars fly. Google and read the 7 pages of “The Physics of Rowing” by Chris Pulman, Cambridge.

Summarizing his paper, when the oar is just planted and you are scrunched up and initially applying power, most of the applied force is from lift forces. When the oar is midpoint, it is mainly shoving water backward, and near the end of the stroke, lift dominates again. The lift comes from vortices that stay with the oar versus being shed (read his paper on this). He notes that the oar blade movement through the water varies- during the lift phases it moves forward in the water, during the mid-stroke it moves toward the stern as it moves water backward, and then moves forward from the lift at the stroke end, with a net movement of several inches in the direction the boat is moving. Lift moves a large volume of water slowly with low drag, while momentum transfer moves a small volume with high velocity and drag.

Other conclusions from rowing applicable to canoeing: Fast paddle plants and fast paddle removal reduces the time the leading edge vortexes add drag before they are shed.

So for good speed and efficient paddling, plant the blade fast, push downward some during the entire stroke, and remove quickly, while maintaining good form. And get out there and paddle.

Which is exactly what…
…several people here have already said, and what that Wenonah site that has been referenced says (among a couple other things).

There’s more
In order to maintain that more vertical blade orientation with a straight shaft, your upper arm needs to be more bent. You can’t apply nearly as much power to the upper handle of the paddle with a strongly bent upper arm as with your arm nearly straight. In fact, if I’m not mistaken, it’s good technique to finish the stroke with your upper arm straight, finishing the pull with a motion that’s nearly all-torso. In any case, you get a lot of power that way within minimal strain on the smaller arm muscles and maximum use of your big muscles, and that’s good for long-term efficiency and endurance. Do this exact motion with a straight shaft and you won’t be able to keep the blade anywhere near as verticle during that most-powerful phase of the stroke.

One question

– Last Updated: Dec-03-06 5:03 PM EST –

Okay, I don't have time to read the articles now, but I will. In the meantime, I think I know what you are saying about catching vorteces to increase "lift", which is a bit of a deceptive term because because we tend to think of lift as a force that opposes gravity. "Thrust" might be a better term, as is normally used when defining propulsive forces, even though the actual mechanics are the same.