New class on waterfall drops! - Page 2 - Mountain Buzz
 



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Old 11-05-2011   #11
 
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While your at it you should see if freewheels would be safer than boofing.

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Old 11-05-2011   #12
 
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Question for the good professor

Gannon,
Here's a question: It seems like having speed coming off a drop decreases the impact on landing. This seems even more true in skiing but still seems to hold in kayaking. However, I can't think of any logical reason why that would be true. Gravity is gravity and surface tension doesn't change (assuming an aeration constant) so the impact should be the same. What say you?
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Old 11-05-2011   #13
 
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oh,i thought you were gonna talk about rating good sized but low skill and/or low consequence vertical drops , class plop [n' drop] aka glorified seal launches at stupid low flows. I like the rating one guy gave easy creeks with wood issues and poor eddy options; class tree. The class you're teaching sounds interesting.
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Old 11-05-2011   #14
 
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Correct on gravity is gravity. A couple of ways you can look at it...and there may be no correct answer.

Don't be offended by my simplified comparison example as I have no idea how much "science" you have under you or anybody reading this.

Give me some assumptions we can make to simplify (unless one of these simplifications is the answer!). Ignore air resistance. You seal launch from a 10 m cliff with a 2m/s vs 5m/s Vh. You land completely flat in calm water both times (which will take 1.428 s regardless of Vh). You feel like the 5m/s has an easier landing right?

This, above, is probably the greatest simplification:

Seems the easiest answer is that the fluid reaction force (FRF) in the AP direction (which will be directed posterior) for the 5m/s condition will be greater. The fact that your upperbody is "independent" of the boat/lower body segment means as a result of that AP FRF you will incur flexion of the trunk.

Think about wearing only the lap belt in a car then rear-ending a car at 2m/s and 5m/s. You're upperbody is going to be thrown forward more quickly (towards the dashboard) during the 5m/s collision as your lower body essentially stays in the seat.

When you land from a waterfall drop with speed you will have some resultant velocity that increases only as Vh increases as Vv prior to impact will stay the same due to the constant 10m drop. That resultant velocity vector will have a FRF directed along the same line but opposite (Newton's 3rd) which will also increase only as Vh increases (Fv stays the same regardless).

****The long technical answer****

Now what do you know to do when you are going to (unintentionally) land flat from a high drop...If you sit strait up the compression forces are directed upwards through the spinal column (compression mostly absorbed by the IV disks and a small % by the Vertebra) and shock in attenuated to the head in a vertical fashion. It is the high head acceleration (change from a head Vv of 14 m/s to 0m/s) that results in the initial headache/neck pain you sometimes feel right after a drop. BUT we know to tuck! So the Fv is absorbed this time by bending forces, compression on the Anterior side/tension on the Posterior side of the IV disks. This force is applied to the spinal column which is reflexively involuntarily countered by the back extensors due to feedback from muscle spindles. This results in a lesser head acceleration partially due to it moving through an arc (increases time so decreases force Imp/mom relationship) and not having the spinal column directly below it "forcing it to stop" in such a short distance.

So, what do you think? Sounds like a thesis or dissertation I'd act as an outside committee member on if you're a grad student! Strap a few 3D accelerometers on and seal launch!

It sure would have been nice to have this as a class discussion when I was an undergrad (though I never heard of kayaking then).
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Old 11-05-2011   #15
 
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Oyeah, stop giving me ideas to include more lectures/different material...now I have more work to do! Most of my examples are the major sports examples.
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Old 11-06-2011   #16
 
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Film and post? I'm curious now!
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Old 11-07-2011   #17
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Sorry I was late, but way back there spencer said something not quite true. Hull design does NOT affect impulse during the landing. The impulse is the one thing completely independent of hull design, since it's the net change in momentum. Hull design and boof affect the amount of time over which the collision takes place, which can decrease the size of the forces involved.

And Gannon, I think that Tyler Bradt and Rush Sturges experimented with 3d accelerometers while they were traveling in africa several years ago. The results were inconsistent, but I think they just had one in the boat, not several attached to different locations on the boat and paddler.
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Old 11-07-2011   #18
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">

Shit, would one of you nerds fix my link?

http://youtu.be/gZEdDMQZaCU
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Old 11-07-2011   #19
 
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water fall class and biomechanics of the boof

great thread!

Gannon_W - While on the topic, I would love to hear your thoughts regarding the "biomechanics of the boof". Feel free to start a new thread on this.

The dogma is that a boof off of a waterfall is accomplished by 1) the boof stroke and 2) pulling the knee upward (i.e. hip flexion).

However, I have seen videos of text book boofs using hand paddles, i.e. applying minimal force to the water. Therefore, I wonder whether the most important parameter is - not the boof stroke per se - but hip flexion? Is the primary purpose of the boof stroke to aid in aid hip flexion? Or is the primary purpose of the boof stroke to generate horizontal acceleration? And if both are important, which is more important? To put it another way, is the boof stroke analogous to a pole plant when skiing steep terrain?

A related question is why does leaning forward during and after coming off the boof flake enhance the boof? It seems that there is a fundamental relationship between leaning forward and hip flexion. And it seems that this relationship is also operating when one executes a bow stall in a playboat, i.e. to counteract toppling over forward one leans toward the bow.

I would love to hear your thoughts on this.

Many thanks!
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Old 11-07-2011   #20
 
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Quote:
Originally Posted by leif View Post
Sorry I was late, but way back there spencer said something not quite true. Hull design does NOT affect impulse during the landing. The impulse is the one thing completely independent of hull design, since it's the net change in momentum. Hull design and boof affect the amount of time over which the collision takes place, which can decrease the size of the forces involved.

And Gannon, I think that Tyler Bradt and Rush Sturges experimented with 3d accelerometers while they were traveling in africa several years ago. The results were inconsistent, but I think they just had one in the boat, not several attached to different locations on the boat and paddler.
I see what you're talking about... kind of. Either way not going to question you here. But remember, there is a pattern to everything in the world, like branches and shit.
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