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  #21  
Old 01-25-2011
gerz gerz is offline
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Quote:
Originally Posted by Lawrence View Post

As to the propellor analogy, I think the point is that by entering steeply but then flattening the arm as it extends forward, the trajectory of the arm is a curve, the convex side of which faces the pool bottom.
It seems to me the arm works like the wing of an aeroplane or a sail and thus generating some additional forward propulsion.

Last edited by gerz : 01-25-2011 at 10:41 PM. Reason: misunderstanding because of poor English
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  #22  
Old 01-25-2011
Lawrence Lawrence is offline
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I'm not sure I buy the wing analysis. As I understand it, a wing generates lift because the fluid medium travels at different speeds over its upper and lower surfaces. It isn't clear to me how that would apply to a spearing arm.

I find the propeller analysis is easier to understand, although perhaps we don't need to mention propellers. When the spearing arm enters the water steeply and then flattens out, it moves along a curve. At the steepest section, the arm is at about 45 degrees to the water surface and the high shoulder is beginning to fall into the water. As it does so, body weight presses on the diagonal leading arm, so that the arm in turn presses along its diagonal length against the water. You can resolve the force of the arm on the water into perpendicular components: one straight down, providing lift (as the water presses back), and the other pointing backwards, providing propulsion (as the water presses back).

If you re-enter at a shallower angle, you weaken the component that provides propulsion. Similarly, if you re-enter at a steep angle but don't flatten out, you lose propulsion because you displace less water as you reach to full extension.

Last edited by Lawrence : 01-25-2011 at 09:35 PM.
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  #23  
Old 01-25-2011
gerz gerz is offline
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Quote:
Originally Posted by Lawrence View Post
I'm not sure I buy the wing analysis. As I understand it, a wing generates lift because the fluid medium travels at different speeds over its upper and lower surfaces. It isn't clear to me how that would apply to a spearing arm.
The lift occurs because the arm is slightly curved (fingers and palm pointing downwards) and by mail slot entering a stream of water is applied to the arm like a stream of air to a wing of an aircraft.
Since the arm during mail slot entering is not horizontal but pointing downwards ca. 45°, the "lift" has a considerable forward component.

Last edited by gerz : 01-25-2011 at 10:38 PM.
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  #24  
Old 01-25-2011
Richardsk Richardsk is offline
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Hi all

First of all, let me state that I am no physicist and no expert on any of the following
Aerodynamics
Hydrodynamics
Propeller theory
Marine screw theory

However, I have built a lot of model airplanes and carved many balsa propellers and made many wings for these models. Almost any shape of propeller will move an airplane forward and even with full-sized aircraft I understand that designing propellers is something of a dark art. The principle of the propeller is indeed the same principle as the principle of the wing. The propeller blade is in effect a very small narrow wing rotating at great speed and creating lift and drag, with the lift being mainly in a forward direction. A propeller is not very like a human arm but the fundamental principle is the same. As the arm moves through the water it will create some lift and probably a lot more drag. The consensus nowadays seems to be that drag predominates as the motive force, because neither the human hand nor arm is ideally shaped to create lift. However, the consensus is not always necessarily right and the arm and hand may create more lift than people think.

By experimenting with the angle of entry we may be able to find a method of producing maximum lift and minimum drag during the reach phase under water. If possible we want the lift to be in a forward direction, which means, I think, that the arm must be at an angle to the surface of the water. What that angle is I don't know, though. Maybe it's slightly different for everybody. I'm guessing that for me it's quite deep and clearly for Terry it's deeper than for Shinji. For elite swimmers it seems to be quite shallow, although camera angles can be very deceptive and refraction plays tricks with our judgment of objects under water.

It would be interesting if some clever physicist who was also a swimmer could devise an experiment to settle this question.
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  #25  
Old 01-26-2011
boken boken is offline
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I'm going to have to agree. I have video coming as soon as YouTube finishes loading it.

My fish will forgive my trespass into their aquarium.
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  #26  
Old 01-26-2011
boken boken is offline
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Default Fun With Spatulas

I took a silicone spatula from my kitchen and did an aquarium experiment this evening.

http://www.youtube.com/watch?v=kXkcgmarpNE

Sorry how dark the video is, but I think you can see what's going on. If it were better lit you'd just see how much more algae was in there. Anyways...

The spatula is wider on one end and roughly flat in cross-section. It has a bit of metal that runs most of the length on the inside of the handle that gives it a bit of weight and rigidity.

Each time I put it in the water I had one end butted up against the glass wall to the left and a little bit of space between the side of the spatula and the wall in front of the camera. I selected several angles and let the spatula drop.

On the 'angled' drops, the spatula shot forwards by varying degrees. On the flat drops, the spatula floundered until one end got angled, then it proceeded to drop on that angle. I tried a few drops each with the wide end first or the narrow end first. Wide end first was the least consistent to me. On at least one drop, the wide end drug along and let the narrow end drop further. Once the narrow end was down it followed the angle carved by the narrow end. Perhaps a more subtle lesson here is to not have a 'wide end' on the front of your arm when you spear!

The term "Propeller" might be hard to wrap a brain around as many have stated, but I can see how an angled spearing arm is using the water as a "ramp" to "sled" or "ski" down. I believe a propeller is like a infinite version of a ramp so I think using it as an analogy is still technically fine. But since my arm's not rotating like a propeller, I think the simpler, 'ramp-like' descriptions are better words for our purposes here.
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  #27  
Old 01-26-2011
haschu33 haschu33 is offline
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boken - great! I love it. That skiing, or 'ramp' analogy is lovely.
A bit like the sharks seeing the Titanic going down...
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  #28  
Old 01-26-2011
gerz gerz is offline
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Quote:
Originally Posted by Richardsk View Post
Hi all

First of all, let me state that I am no physicist and no expert on any of the following
Aerodynamics
Hydrodynamics
Propeller theory
Marine screw theory

However, I have built a lot of model airplanes and carved many balsa propellers and made many wings for these models. Almost any shape of propeller will move an airplane forward and even with full-sized aircraft I understand that designing propellers is something of a dark art. The principle of the propeller is indeed the same principle as the principle of the wing. The propeller blade is in effect a very small narrow wing rotating at great speed and creating lift and drag, with the lift being mainly in a forward direction. A propeller is not very like a human arm but the fundamental principle is the same. As the arm moves through the water it will create some lift and probably a lot more drag. The consensus nowadays seems to be that drag predominates as the motive force, because neither the human hand nor arm is ideally shaped to create lift. However, the consensus is not always necessarily right and the arm and hand may create more lift than people think.

By experimenting with the angle of entry we may be able to find a method of producing maximum lift and minimum drag during the reach phase under water. If possible we want the lift to be in a forward direction, which means, I think, that the arm must be at an angle to the surface of the water. What that angle is I don't know, though. Maybe it's slightly different for everybody. I'm guessing that for me it's quite deep and clearly for Terry it's deeper than for Shinji. For elite swimmers it seems to be quite shallow, although camera angles can be very deceptive and refraction plays tricks with our judgment of objects under water.

It would be interesting if some clever physicist who was also a swimmer could devise an experiment to settle this question.
Richard, very well said!
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  #29  
Old 01-26-2011
Lawrence Lawrence is offline
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To make sense of this I think we need to distinguish (a) vertical lift from (b) forward-directed propulsion.

Item (a) gets you nowhere fast although it may help you avoid sinking.

Item (b) is what is in point on this thread, if we're trying to explain why a steeper entry angle provides a seemingly effortless 'squirt' of propulsion.

The only way to be pushed forwards is to push backwards on the water. That's just Newton's third law of motion. So we need to understand why a steeper entry angle results in a greater push backwards against the water.

I find the airfoil analogy confusing because it's comparing apples and oranges. A wing doesn't change position in relation to the air while the plane is flying. The lead arm does change its position in relation to the water while one swims. It is also far from clear, to me at least, that the water travels at different speeds over the upper and lower surfaces of the arm. Contrary to what has been said above, the arm isn't shaped like an airfoil (bulbous on top, flat beneath), even if you allow for a drooping hand. If you need proof, try measuring the distance from fingertips to elbow (or shoulder), first on the upper side then on the lower. It's basically the same. So I think we should ignore airfoils.

The propeller image is better because the propeller's angle of attack on the water changes as the propeller moves relative to the water. The central insight, I think, is in Terry's use of the word 'squirt'. What happens is exactly that. The spearing arm collapses on to the water beneath as it flattens out during forward reaching, pushing water backwards as it does so. It also pushes some water downwards, but the backwards component is increased with a steeper entry angle, due to the parallelogram of forces analysis I mentioned earlier.
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  #30  
Old 01-26-2011
haschu33 haschu33 is offline
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Still don't get it...

Quote:
Originally Posted by Lawrence View Post
...
The only way to be pushed forwards is to push backwards on the water. That's just Newton's third law of motion....
Yes, complete agreement.

Quote:
Originally Posted by Lawrence View Post
...So we need to understand why a steeper entry angle results in a greater push backwards against the water.
Quote:
Originally Posted by Lawrence View Post
...
The spearing arm collapses on to the water beneath as it flattens out during forward reaching, pushing water backwards as it does so. It also pushes some water downwards, but the backwards component is increased with a steeper entry angle, due to the parallelogram of forces analysis I mentioned earlier.
The spearing arm always travels forward, and forward only. It travels with a faster speed than our body thus creating a high drag.
It never travels backwards, and it never pushes water backwards, it pushes some water down at the most.

If the arm was stationary and you push it down at an angle, it would move water backwards (you feel pressure at the lower side of your arm).
See the wonderful video from boken. The spatula glides because it has a flat shape. As it is very lightweight it pushes a little water backwards and 'glides' down on the water resistance. If you push it down at the same angle it will push water backwards.
But our arm is not stationary. It moves forward all the time. If we are lucky and find a perfect angle we manage not to feel any pressure on the front side of the arm, meaning not to push water up and forward. By the time the hand hits it's target the shoulder will have passed the original 'mail slot' entry point of the hand: there is a constant forward movement.

It is literally impossible that an object (our arm) that you push through the water moves water backwards. The only point where you can have water molecules moving backwards is when vortices get created, and they probably will get created. But at least the same amount of water molecules in the vortex move forward, so there will be no net backward movement.

Which doesn't mean that the correct angle will not give us a benefit. But the benefit is (neglecting the contribution to our overall movement cycle) that it creates the least drag possible.

Last edited by haschu33 : 01-26-2011 at 11:02 AM.
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