bentshaft physics

lucky
that I’m not the only one wondering about this ‘attak angle’ description…

Whut are ya’ll talkin’ about?
Lift angle, vortices? I just bought a bent shaft to appear cool before bring out the kayak paddle.

Think down, not forward

– Last Updated: Dec-03-06 10:41 PM EST –

Read the article, re-read what I wrote. Think down as I wrote it. I read it several times over 3 months, pondered it, re-read, and spent several hours writing the note on it. It is not real obvious how it works.
A demonstration of the lift force: In a stationary boat with a straight paddle: plunge the blade straight down into the water. Any forward force generated? Probably not, it should be zero. Angle the entire straight paddle 5-10 degrees forward and push straight down, not forward or angled, feeling for the forces developed. The blade will go forward. Is it forward lift, or just the angle? It should be both an effect of lift and angle. The old paddlewheel boats on the Mississippi got some lift, but nothing like modern boat propellers or wings get.
When the bent shaft is forced down, that 14° angle going downward is what produces lift, in the forward direction. Somewhere around 80-90% of the propulsion comes from moving the water backward, while with the bent shaft, roughly 10-20% comes from lift of the paddle driven downward and the lift force acting 90° to the descending blade, in the forward direction. Pretty slick.

No misunderstanding here

– Last Updated: Dec-04-06 9:54 AM EST –

That's what I thought you meant. I only tried to say that the 14-degree angle you speak of only works well because of the natural way of holding a paddle. Offhand, I'm not sure that plunging the blade with the shaft being perfectly vertical is natural, meaning that if 14 degrees of blade offset really DOES translate into 14 degrees of approach angle when plunging, it's a coincidence that has no more "meaning" than if the angle of the pitch controller on an airplane's propeller were accidentally the same as that of the propeller blade. All the attachment mechanics are irrelevant in both cases.

Totally off the subject addressed here, I'll have to look at that rowing info. How this plunging effect (outward "plunging" when rowing) can counteract the fact that a large componant of the thrust is in the wrong direction to start with, seems a little hard to swallow unless they demonstrate it with actual measurements of force. I'll try to look at it later today. In my own experience, the thrust of the oars is not too good at the beginning and end of the stroke compared to at the middle, when the oars are straight out and the force is directed parallel to the line of travel, but that's just my experience.

Here’s a Link
that may shed a little light on the path of the oar tip:

http://www.concept2.com/us/products/oars/testing/default.asp



Pete in Atlanta

Thanks

– Last Updated: Dec-04-06 11:16 AM EST –

I see right away that this is based on a curved blade, and that things could be significantly different with a straight blade, though the principle should hold for nearly as much of the stroke with a straight blade. Also, this article says what I think SHOULD be said, that theory is no substitute for actually measuring the forces being described. Anyway, I can see what canunut is saying about getting power on the "plunge" phase of the paddle stroke. I just didn't want to confuse mechanical "holding" angles with actual "orientation relative to path through the water" angles.

Interesting stuff, and I don't question what they say is happening. My doubts have to do not with whether those forces are there, but their magnitude, and that's just based on my own experience with various steering maneuevers which take advatage of using various portions of the total available stroke when rowing. These observations suggest that whatever lift is occurring isn't great enough to counteract the force which is due simply to the path and orientation of the blade in the water, which makes me think the main advantage of the bent-shaft paddle is in putting the blade perpendicular to the path of the strongest part of the stroke. Admittedly these observations are based on flat oar blades, not curved ones, but then, the blade of a bent-shaft canoe paddle is flat too.

For what it's worth, I don't like curved blades for rowing because for general-purpose stuff, I need a blade that works well in both directions, especially when really good turning ability is needed.

and still
we don’t row with bentshaft paddles, do we?

I believe, the mechanics in canoeing are totally different and not comparable. But I bet, nobody has done a true sientific test with a canoe paddle yet either. Should prove more difficult than with oars as well since there are a few more variables involved.

Yup
I was daydreaming last night of a way to install a strain gauge on the gunwales of a row boat which would measure only the forward componant of the rowing thrust. It would be very easy to do (not for me, but it would be for people with the proper equipment at their disposal), and then you could just test different blade designs and plot the forward force occuring at all parts of the stroke. This would be a lot harder to do with a canoe, but I think it could be done.

agree,
but with oars, you have at least one fixed point of leverage and with competition row boats, usually the oar’s length doesn’t differ much. With a canoe paddle everything is variable except the blade surface itself. You would have to do a lot of testing with the same paddler in the same canoe using the same blade but different bentshaft angles (down to zero), shaft length, hand positions, seat positions/heights,…to get a valuable sientific study on the forces that work on the blade. And you better hope, the paddler’s form and mood is consistent throughout your testing. There is a word for that in english: rigmarole



OMHO

exactly
I think you are right on the money, canunut. You clearly stated what I was trying to say, that the vertical planting of paddle (the catch) pulls the canoe forward, even before the power phase of the stroke begins. Therein lies the difference, in my opinion, between the performance of straight vs. bent shafts.

vertical position of the catch?

– Last Updated: Dec-04-06 4:34 PM EST –

That I don't follow. How can a bent have a more vertical catch than a straight? That's why you see some newbees using them backwards, to achieve a more vertical catch than you get with a straight. It seems counter intuitive to them to start with a less vertical catch than they are used to with a straight.

Now, if you tell me the more vertical catch is because bents are shorter, I can maybe buy that. But, that would make them more vertical in the catch because they are short, not because they are bent.

I'm ready to be shown the error of my ways.

not convinced
I’m a big fan of bentshafts for everything < class 2 but surely no physicist.

When a straight shaft paddle is planted @ start of fwd stroke, it too is forced downward. If this lift theory of propulsion & the resultant force vectors produce so much fwd motion, wouldn’t the same hold true for a straight shaft ? Realize the angle of blade would be somewhat different than w/ a bentshaft but that angle changes as the paddle (either kind) is moved thru water. I still think the big difference in efficiency is in power phase of stroke (but then there are many tecnnical issues my old brain can’t seem to grasp)

Proposal for a practical experiment
I have an idea to test whether a bentshaft is more efficient than a straight in the catch phase of the stroke.



See how fast you can go with each using only the plant and catch and no power phase. In order to control the experiment, place something across the gunnels (clamp it maybe) so that you can not cross that point with the shaft. The piece of wood, pvc or whatever, would prevent your stroke from entering the power phase. Anyone with GPS could use it to measure the results.



I think it would be an interesting experiment and I plan to do it when I get the chance and maybe a GPS.

comments
Without any data on lift from the bent shaft paddles, my initial estimate is probably way high. I think around 5% is more likely. I don’t think anyone has measured canoe paddle lift or other data, compared to that of oars in rowing with the interest of the university rowing and olympic rowing groups.

It’s an interesting excercise to carry both bent and straight paddles for a simple trial- paddle 10 strokes with one, then swap for 10. Quite a dramatic difference, but doesn’t prove much. No question that the bent shaft is biomechanically better- shorter strokes, faster, more direct power applied. Both straight and bent paddles are planted before power is applied. The ~14° bend has to put the blade in the proper position for lift to be developed, and with more downward pushing, even more lift is gained.

The reference in a previous post about a compromise paddle at 7° bend being easier to steer with and still get some benefit- I’ve never had a problem steering with a 14° bent shaft (except when trying to go fast and not wanting to rudder or j-stroke to correct).

I don’t think we will prove much in this forum unless someone else has scientifically tested paddles.

vertical shaft
for me, vertical means the shaft of the paddle is vertical, thus the blade is planed forward, and pulls the canoe forward as the paddle is planted downward.