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Advice, Suggestions and General Help New Topic Printer Friendly Version

  Depth of paddle's center of pressure
  Posted by: Glenn_MacGrady on Jun-10-14 2:06 PM (EST)
   Category: Canoeing Technique 

It is better, worse or irrelevant for a paddle to be deeper in the water for forward strokes and turning strokes?

"Deeper" could just mean burying the paddle down further than the throat, but let's be a little more technical.

Let's assume the "center of pressure" (COP) is at the geometrical center of a paddle blade. Thus for a 26" long animal tail paddle, the COP is ~13" deep; whereas for a squat 16" racing blade, the COP is ~8" deep.

Which of these two COPs is better for forward and turning strokes? You can assume the total areas of the animal tail and racing paddles are the same if it matters.

If you conclude a deeper COP is worse for efficiency and leverage, what functional reason is there to use a long and deep paddle? There must be something. These paddles are ubiquitous. So, maybe they are better for efficiency or leverage or something.


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Messages in this Topic

 

  All Depends
  Posted by: clydehedlund on Jun-10-14 3:07 PM (EST)
On how much fulcrum you need or want to use?
 
 
  that's what she said
  Posted by: slushpaddler on Jun-10-14 3:15 PM (EST)
 
 
  LOL Clyde ... I can see that !
  Posted by: onnopaddle on Jun-11-14 4:59 AM (EST)

signed,

single blade grasshopper
 
 
  Although I've messed with the center of
  Posted by: ezwater on Jun-11-14 1:35 PM (EST)
pressure concept myself. I confess I'm unable to answer your question.

I started using a 66" Clement with a 26" or so blade. We were paddling tandem then, and that paddle seemed OK for slow tandem maneuvers.

Over time, I've gravitated to ~61" paddles with short 20 inch slalom blades. A lot of my paddle work is relatively shallow, though the heart of the forward stroke goes fairly deep.
 
 
  paddle depth
  Posted by: rblturtle on Jun-11-14 1:36 PM (EST)
I had a bow paddler, a fellow scout leader, that used a real long paddle and dug deep. He claimed that the deeper the paddle blade is in the water, the less "slippage". I don't know about that, but he could sure move the boat! We need a submarine propeller engineer the chime in.
Turtle
 
 
  Pressure
  Posted by: carldelo on Jun-11-14 3:13 PM (EST)
-- Last Updated: Jun-11-14 3:20 PM EST --

I don't work on submarines, but I am a mechanical engineer and teach fluid mechanics regularly.

In simple terms, static pressure on either side of the paddle cancels, so the net hydrostatic pressure force is zero and the depth is irrelevant.

The dynamic pressure force depends on paddle shape and area, fluid density and relative velocity squared. Water is essentially incompressible, so has constant density. The remaining variables are independent of the local fluid pressure, so again, the depth of the center of pressure is irrelevant.

The benefit of a long paddle is not that it buries the center of pressure deeper, but that the lever arm is greater, so the relative velocity of the blade through the water is increased. Force varies as velocity squared, so the effect can be large. The bow paddler no doubt perceived the increased paddle force as reduced 'slippage', which is nonsense, I'm afraid.

In the atmosphere, depth is relevant, as density is a strong function of vertical location. This is why airplanes, helicopters and balloons are altitude-limited.

 
 
  I'll throw in my two cents,
  Posted by: tdaniel on Jun-11-14 4:20 PM (EST)
none of which is physics based but allows for some generalities regardless of craft or even paddle type- canoe or kayak.

for slow steady paddling a deeper stroke is used

for "bursts" of speed shallower strokes are used

for paddling straight a deeper stroke is used

When turning the strokes become more shallow.

Here's the why- slow steady paddling results in a longer stroke because the paddle stroke is extending past your body- best achieved with a deeper more vertical stroke. Many stern paddlers prefer a longer paddle so that the correction stage of the stroke (like the j) extends toward the end of the boat where it is most effective.

bursts of speed are best achieved by increasing your stroke rate, by shortening the stroke in both depth and distance one can paddle more quickly over a short distance

when the paddle is more vertical and parallel to the paddler, less correction is needed- the stroke is thus deeper

turning strokes- such as sweeps are most effective near the ends of the boat- since the stroke goes outward a shallow stroke is used in order to reach this position.

My two cents- depth is a function of verticality
 
 
  that was good.
  Posted by: slushpaddler on Jun-11-14 4:38 PM (EST)
 
 
  That brings up a question though.
  Posted by: Guideboatguy on Jun-11-14 5:43 PM (EST)
-- Last Updated: Jun-11-14 5:44 PM EST --

Why do the propellers of outboard motors undergo more stress at the bottom of the rotation circle than at the top? This is, after all, the reason the propeller imparts such a strong turning force on the motor itself, and counteracting that force is the reason for having an angled tab for steering trim. Small outboards, like my 20-horse, have no trim tab, and if the pivoting friction is set too loose the "torque steer" is really very strong, and if one were to let go of the tiller the motor would be pushed by that unequal force into a fully sharp turn in less than two seconds (it would happen a whole lot quicker if the pivoting friction were eliminated entirely).

I always figured that the water that is "flung" backward and at an outward angle from the spinning prop blades encountered less resistance when the outward angle was toward or along the surface than when the outward component of that motion was toward or within the "more confined" water below. That "confinement" is not the same as pressure, but it's my idea of why the blade encounters noticeably more resistance at the bottom of the rotation circle than at the top. There must be something to this, even if I'm not using the right terminology.

I don't believe this phenomenon would be present if the prop simply "screwed itself through the water" without slippage. I believe it has something to do with water being set into motion by the blades, and it's easier to accelerate that water in some directions (upward and horizontally as the blade approaches and passes the top of its circle) than others (downward and horizontally as the blade approaches and passes the bottom of the circle).

 
 
  This is a good question:
  Posted by: pagayeur on Jun-11-14 4:07 PM (EST)
Over the years I've seen many folks who paddle Canadian style (tandem canoes paddled from the center station, at a standing heel), using long otter-tail paddles (regular shaft, long blades). I always wondered why? Turns out the sq. inches of area on a long otter-tail blade is about the same as some of my conventional paddles. But they paddle mostly on lakes and I paddle mostly on rivers where a long paddle can be a detriment, so it's moot question for me. But each time I head for a lake, I wonder if I'm missing something. sounds like our Mech. Eng. poster says no.

Pag
 
 
  My opinion
  Posted by: Glenn_MacGrady on Jun-12-14 1:55 AM (EST)
-- Last Updated: Jun-12-14 1:57 AM EST --

For a blade of any given area, it's best if the blade's center of pressure (COP) be moved through the water in the shallowest arc possible.

The COP is simply the point where the sum of all the pressure fields act on the blade, the point through which the vector force of the blade is focused. I agree this is dependent on the shape of the blade, but I proposed a reasonable approximation assumption, for the sake of discussion, that the COP is half way down the blade.

Force may vary as velocity squared, but we must ask how much velocity a paddler can impart to a really long paddle and, more importantly, in what direction the COP force is pointed during a stroke.

A stroke describes an arc. At the beginning the stroke arc, part of the vector force through the COP is directed downward, lifting the bow of the canoe vertically upward rather than moving the canoe forward. At the end of the stroke arc, part of the vector force is directed upward, pushing the bow downward.

I propose that that the longer the paddle blade, the deeper the COP will arc through the water. The deeper the COP arc, the more vertical bow lifting and plunging there will during the beginning and ending of the stroke. These lifting and plunging vectors through the COP waste force and are inefficient. Moreover, the longer the paddle, the more strength will be required to move it at the same velocity through the water.

To state this all a different way, the vector force through the COP would be most efficient if the stroke didn't arc downward at all, but could somehow just be pulled though the water at a constant depth parallel to the surface.

One can also think of the paddle as a lever -- the class of which is hotly disputed -- which will be more difficult to pull through the water the further the shaft hand is from the blade COP.

Imagine a paddle 30 feet long. It would be extremely hard to pull through the water with any velocity, and there would be many feet of travel at the beginning and end of the stroke that are simply bobbing the canoe up and down vertically rather than contributing forward vector force.

Long skinny paddles have other attributes, such as the ability to be pitched, sliced and fiddled underneath a heeled hull Canadian style. But I don't think those paddlers are going to win any races using that style. Racers use short, squat paddles for a reason -- one of which is to minimize the depth arc of the blade's center of pressure.

JMO, which is always subject to revision based on further understanding or empirical evidence.

 
 
  I think that is pretty much correct
  Posted by: pblanc on Jun-12-14 9:45 AM (EST)
-- Last Updated: Jun-13-14 12:17 PM EST --

Photographic analysis of the forward stroke of competitive paddlers has demonstrated that the paddle rotates in the vertical plane about an axis at, or very close to the water surface. The grip and shaft above the water moves forward relative to the blade as the blade arcs backward relative to the shaft and grip.

Ideally, the paddle would remain vertical and fixed in the water as the boat is pulled up to it. Skilled paddlers can approximate keeping the rotational axis of the paddle relatively stationary with respect to the river bed, but the biomechanics of the human body do not allow the paddle to remain vertical.

Although the arcing path of the blade is not maximally efficient, it does provide forward propulsion, just as the blade on a paddle wheel does. But making the blades on a paddle wheel longer so that they bite deeper in the water, but less wide so as to maintain the same blade surface area, would not improve efficiency.

Obviously, for a given blade surface area there is a limit as to how wide the blade can be before the hull starts to interfere with placement. Slalom racers seem to have gravitated to blade widths of around 8 inches, give or take 1/4" or so.

The only mechanical advantage I see to long, narrow blades is the ability to make a deep solo C stroke or to get a longer lever arm for pries off the gunwale, or wide sweep strokes.

 
 
  Going From Positive to Negative Angle
  Posted by: clydehedlund on Jun-12-14 3:44 PM (EST)
The blade only indicates the amount of forward travel of the canoe. Propulsion is achieved when the blade is "locked" in position. Racers actually remove the blade ahead of where it entered the water.
 
 
  Narrow
  Posted by: carldelo on Jun-12-14 4:17 PM (EST)
I agree with this analysis, but would add a point - long narrow blades mean that paddling effort ramps up and down more gradually during the stroke, and is probably easier on the joints.

The GP is the classic example, and indicates another benefit, a narrow blade is likely easier to cant and put into partial wing-mode, where hydrodynamic lift (or thrust, I guess) forces are generated due to flow over the paddle section.
 
 
  Cab forward paddling with a short slalom
  Posted by: ezwater on Jun-12-14 4:40 PM (EST)
blade does involve a relatively sharp catch, which would be stressful if one were not used to it. The catch should not be savage, or slamming, but should be sharp and definite. If it is done that way, then the bow will come back during the paddle recovery, and a J stroke will normally not be necessary.
 
 
  Yes, EP vs GP kayak paddles . . .
  Posted by: Glenn_MacGrady on Jun-12-14 7:45 PM (EST)
. . . would present a similar issue.

My subjective sense with long, narrow canoe and GP kayak paddles is that they feel "softer" to pull through the water. But they have to be really narrow. My beaver tail canoe paddles, which I would not classify as really narrow, don't feel any softer to me than my ZRE racing paddles.
 
 
  $.01....
  Posted by: bigspencer on Jun-13-14 12:13 PM (EST)
-- Last Updated: Jun-13-14 12:31 PM EST --

[For a blade of any given area, it's best if the blade's center of pressure (COP) be moved through the water in the shallowest arc possible.]

Correct...but the COP shouldn't be static point, if the pull is initiated quickly after the plant ..where you eliminate as much dead eddy formed with the plant(when already moving) the COP moves from the top of the blade near the throat towards the tip(bottom edge) but only as far as the powerstroke maintains a straight path(of course).
The difference between long and short/wide blades moved through a stroke is in powerstroke distance.....the more area facing on the blade that's receiving resistance = the more resistance = power. That's why you don't see many short blades at the Summer Olympic sprint events.

 
 
  Absolutely!
  Posted by: clydehedlund on Jun-13-14 5:44 PM (EST)
And straight shaft too!
 
 
  Not that I've been paying attention,
  Posted by: ezwater on Jun-14-14 10:03 AM (EST)
but my way of looking at it is that one is trying to move the boat along or around the surface of the water, so the center of effort of the paddle should be near the plane of the water surface.
 
 
  simple mechanics
  Posted by: harry0244 on Jun-15-14 9:34 AM (EST)
The farther the center of thrust is from your hands, the more force (pushing on the top, pulling in the middle) you must exert to achieve the same force at the paddle face. As long as the paddle is long enough that you are not wasting energy stirring the surface shorter means easier paddling.
 

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