Some thoughts on water flowing over a paddle

[blueplanetsurf]

New post on the ZenWaterman blog:

Quickblade's Jim Terrell recently came out with an excellent video breaking down the SUP stroke with high tech video analysis.   If you have not seen it, watch it in the window below (double click for full screen view).
The video analysis clearly shows that good paddlers plant the paddle and move past it.  It shows Rob Rojas in slow motion pulling the blade out in front of the spot he planted it.  Once the blade is planted, the water is compressed against the face of the blade and there is very little "slippage" or backward movement.  This made me think more about how a paddle blade moves through the water.  I'm not a scientist or paddle designer but just want to share some of my thoughts.
When people talk about paddle blade design they usually explain how the shape and design of the blade moves through the water, visualizing how the blade face gets pulled backward through the water.
In reality, there is very little movement of the blade once it is planted.  The water is compressed against the face of the blade and if the force is applied at the right time in the right dosage, there is very little slippage or movement of the blade backwards, it is effectively planted in the water, not moving through the water.
So, most of the movement of the blade through the water occurs when the blade is sliced into the water during the catch and when it is pulled out of the water during the release.  In both cases, the paddle moves sideways, edge first with water rushing past both sides of the blade.
When you think of it this way, most of the water flowing over the paddle is not flowing over or past the face of the blade but moving sideways, during the catch and release.  When designing a paddle the concern should be to make the sideways movement during catch and release as smooth and efficient as possible as this is the way the paddle travels through the water the most: sideways.
Instead of looking at the face of the blade, look at the edge and side profile of the paddle as that is the direction the blade moves through the water mostly.  It seems to me that a thin, flat blade should be most efficient slicing through the water sideways, while paddles with big spines, concaves, curves or other features designed to "catch" more water will only disrupt a clean sideways entry and exit.
It also means that it does not matter wether you put a sticker on the face or back of the blade, the sticker edge might cause a tiny bit more friction on either side during catch and release, but should not make a difference during the power phase.
So what's up with those "magic" golf ball dimples on the face of the Quickblade elite racer blades?
Some people seem to think they are designed to "hold" more water.

According to the Quickblade website FAQ section:
Q -
What is the difference between the Elite racer and the Magic?

A -The Magic paddle is identical to the Elite racer with the difference of the blade surface.

Q -What is the special feature on the QB Magic paddle?

A -The QB Magic paddle has dimples on the backside of the blade similar to a golf ball. This has proven to be most effective for very strong powerful paddles that can drive it with force.

I'm kind of puzzled by this.  The dimples are on the face of the blade, not the backside (I guess it depends how you look at it).  I think they mean to say it works well for powerful paddlers. I would think that the dimples would be most effective at reducing friction when the paddle is moving through the water sideways, much like dimples in a golf ball make it travel farther with less friction(I think that's why they have dimples anyways, correct me if I'm wrong).  Do they work?  I can't tell the difference and neither can anyone I talked to that tried both but it's one of those things that if you believe in it, it works.  It's called the placebo effect.  At $10 more than the non-dimpled elite race paddle the dimples are cheaper than a holographic power band bracelet and might actually do something.
Just don't put a fat paddle edge guard on a Magic blade, that would just be silly.

If the purpose of the dimples really is to reduce friction during catch and release, the dimples really should be on both sides of the blade, not just on the face of the blade, as the water rushes past both sides equally.

[adios pantalones]

I don't know about paddles, but my understanding is, the dimples on a golf ball are to create lift.

In Yachting circles, experiments with irregular surfaces (think sharkskin) have been to relieve drag, not "hold" water.

Yes, I agree about your slicing theory, but my experience is that my dihedral face paddle indeed flutters less under power than the smooth face.

My .02

[rpmooreii]

The dimples on a golf ball do not create lift; rather they transform the flow from laminar to turbulent so as to reduce fluid (air in this case) separation.  Turbulent flow actually creates higher skin friction drag but the delayed separation around the aft half of the ball greatly reduces pressure drag.  In fact if one could completely eliminate separation there would be ZERO pressure drag and the ball would fly almost forever (skin friction drag on a ball is exponentially less than the pressure drag.  In sum, dimples are used on ball like surface to reduce the high level of pressure drag caused by the separation of flow.

...Roger

Roger Moore
MS, Aero/Astro Engr (Stanford, 1986)

[PonoBill]

Interesting notions. I find the paddle analysis very useful, it's sparked a lot of thinking for me. I'm not so sure that the paddle coming out of the water ahead of the insertion point means the blade doesn't move, I think it might have a lot more to do with recovery and the speed of the board. Obviously it couldn't happen if the board was still, and it couldn't happen if the paddle was slipping grossly.

But here's my thinking.  A board traveling at 7MPH is going 10 feet per second. To keep the math simple, a cadence of 60 strokes per minute means the paddler is moving forward 10 feet over the span of a stroke. Let's assume half the duration of the stroke is in the air (full recovery). That means the board moves five feet forward while the blade is in the water. Call it 33 percent power, 33 percent coasting, and 33 percent withdrawal. Over the span of the withdrawal segment the blade is being pulled forward in the water. If you look at the shaft angle it's pretty clear that it's at least that long--the power segment may be shorter. If the blade didn't slip at all it should come out of the water more than a foot ahead of where it went in.

[pdxmike]

Robert--really interesting, as are everyone else's comments. 

One thing--at the Seattle Kalama clinic, someone asked Dave specifically whether the paddle should enter the water from the side or from above.  Dave said from above.  So, the paddle's bottom edge is entering first, and it is a downward movement, not a sideways one.  That anyway is what he believes he's doing with his stroke.  Also, when the paddle is withdrawn from the water, he has compared it to pulling a sword out of a scabbard, which is an upward rather than a sideways movement.  However, he also said he starts feathering the blade sideways almost immediately, so it could be that the blade is exiting the water sideways and upwards at the same time.

I'm not sure exactly how much of the blade movement in and out of the water is sideways or upward/downward, but in any case your main point still seems true--that when the paddle is actually moving through the water, the movement isn't the flat side pushing, but the edges (whether bottom or side) slicing through, so that the spines, concaves, etc. need to be designed to minimize resistance.

So if strength and rigidity were not issues, the ideal blade might be as thin and sharp as possible when viewed from the end or sides.  However, since they are issues, you would want to avoid say, any abrupt changes from a flat thin blade to a fat round shaft--maybe with analysis something like using wind tunnels to analyze cars or bicycles. 

[blueplanetsurf]

Thanks for the great feedback.  
I surfed on a 8'2" SUP today and realized that I should have prefaced my thoughts with: on an unlimited Raceboard moving at cruising speed, there seems to be very little slippage.  On a short surfing SUP or when accelerating form a standstill or when going into the wind, there is plenty of backwards paddle slippage.
  
PonoBill,  I can't quite follow your calculations but if you look at the slow motion footage carefully you will see that there is very little slippage even during the apex of the power phase when the shaft is fully loaded and bent.  The blade seems to merely rotate at the waterline.  I agree that the forward momentum carries the blade ahead of the entry point during the recovery.

Pantalones, I agree that a dihedral is helpful in reducing flutter, especially in lower speed situations as mentioned above where there is more "slippage".  Since most of the movement during entry and exit is straight down and pulled straight up as pointed out by pdxmike, a mild dihedral will probably not significantly affect the "sideways" water flow as it is mostly in the direction of the dihedral or spine.  

Roger Moore, since you have a scientific background maybe you can explain better how the dimples function.  I assumed that in water, laminar flow has the least friction and that turbulent flow has more friction.  If the turbulent flow reduces pressure drag, could it work on the bottom of a raceboard?

[pdxmike]

Anyone geeky enough to be reading this thread this far might enjoy skimming through Ernest Maglischo's Swimming Fastest, a very technical book about the science of competitive swimming (link is to 100 pages of the book on google books):

http://books.google.com/books?id=cSSW4RhZOiwC&pg=PA39&lpg=PA39&dq=ernest+maglischo+diagrams&source=bl&ots=qkjy7tTV44&sig=TJ745R0wHcDIRDhM4zrS6gB61Fc&hl=en&ei=fAG9TeT8O5TGsAPz_qXZBQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCgQ6AEwAg#v=onepage&q&f=false

Some sections seem very related to the paddling discussion, such as the scientific background on propulsion (around p. 16), the importance of the catch (p. 71) and some of the diagrams of how the hands and arms move through the water, and the velocities at various points in the stroke (p. 97-98).  I imagine at some point people will be doing their masters' and doctoral theses on the science of paddling--not that the typical great swimmer or paddler is really thinking about any of this (although some are). 

[ehrawn]

Here is a good illustration of what's happening with a golf ball.
http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml


"Since the laminar boundary layer around the smooth sphere separates so rapidly, it creates a very large wake over the entire rear face. This large wake maximizes the region of low pressure and, therefore, results in the maximum difference in pressure between the front and rear faces."

In comparison, lets look a flat plate. First, perpendicular to the flow:
http://en.wikipedia.org/wiki/Drag_coefficient


"fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface." Negative pressure builds at the back because of the eddies that form along the edges which adds to the effective drag. The flow around the plate will be turbulent even at very low velocities.

But parallel to the flow:
http://www.roymech.co.uk/Related/Fluids/Fluids_Drag.html


You'll notice that there is no advantage to creating turbulent flow earlier on plate, like there was with the sphere. You won't get less wake or any other advantage because pressure drag is negligible to skin friction.

So lets talk numbers. Drag Coefficient depends on the Reynold's Number which is a function of velocity, but, common estimates for the drag coefficient of sphere is about 0.47. A flat plate perpendicular to the flow is about 1.17. A flat plate parallel in laminar flow is about 0.001, and in turbulent flow about 0.005. http://www.engineeringtoolbox.com/drag-coefficient-d_627.html

Drag force depends on the drag coefficient and a "frontal area" which we can just assume is the same for all cases: for the parallel plate, it's actually the side area, if you get what I'm saying. As such, the drag force for the parallel plate is 2-3 OOM less than the other two scenarios. So then you get into the question about laminar vs turbulent. Crunching some numbers:

http://en.wikipedia.org/wiki/Reynolds_number

L is the length of the blade. Assuming I can get the paddle in the water in about 0.5 sec, in sea water, your looking at a Re of about 1.1 * 10^9. Re > 10^6 is so we are way turbulent.

Essentially, what I'm trying to argue is that I don't think there is an advantage to making the flow turbulent on catch and release. I'm guessing that a quick catch will get you turbulent even without the dimples. Even if there was and effect caused by the dimples, it would be so minuscule compared to rest of the stroke, that it wouldn't be noticeable to the paddler. Look at the mechanics of a stroke. As soon as the blade enters the water, the perpendicular flow is going to be all that the paddler feels. At the release, there's going to be so much cavitation at the back of the paddle, as soon as you stop applying forward pressure, the built up pressure on the face and suction on the back will pull the blade into the void…not really a void but a low pressure volume. Again, it will already be turbulent flow, so dimples aren't going to do anything.

Of course, the focus of my masters was mechanics. I haven't studied fluids since my undergrad, so, Robert, how'd i do?

[blueplanetsurf]

ehrawn and pdx mike- awesome geeky reading material, I love this kind of stuff.   
Unfortunately, the formatting got screwed up which makes it almost impossible to read now.  Admin, is there a way to fix this?
Ehran, can I put your post on my blog?   
I just checked my local library and they have "Swimming Fastest"  I'll have to add it to my reading list.  pdxmike, have you read "Total Immersion" ?
I got a lot out of that book for both swimming and paddling.

[colas]

Interesting. On a SUP/surfboard, the place where you want the water to follow the round shape around is on the rails where you want them to hold on the face (except at the aft where you want a quick release).

So, dimpled rail protectors on the rails makes sense: they may actually add holding power to the rails!

[Tom]

The big difference here is that the golf ball is spinning rapidly. The dimples push air in the direction of the spinning and this creates a air foil that creates lift. The paddle, nor the bottom of the board, spins.

[pdxmike]

  pdxmike, have you read "Total Immersion" ?
I got a lot out of that book for both swimming and paddling.

I have the book and also a couple of the videos, but haven't looked at them for a long time.  PonoBill has posted about them also.  However, I think he overemphasizes rotation (as did most people a few years ago).   If you want to really see some esoteric science-of-swimming stuff, google Milt Nelms, who has been called the Horse Whisperer of Swimming.  You can look at stuff he's presented for a long time without even realizing he's talking about swimming.  Or Bill Boomer, who emphasizes reducing resistance through the water.  For technique, try Karlyn Pipes-Nielson and Sheila Taormina.  Both emphasize technique which is much like paddling--good catch, pull straight back, clean release and relaxed recovery. 

It all seems off-topic, but not when you realize that a key to swimming is managing how your body flows through the water.  In paddling, other than being able to weight your board one way or another, your board is a given, and you can't do a lot to change how it flows through the water.  In swimming, your body is the board, but you can constantly manipulate its shape for better or worse. 

Same with paddles versus your hands and arms.  In swimming, you can constantly reshape your "paddles" (hands, arms, feet, legs) as they move through each stroke.  But the principles--water flow, catch, when to apply power, etc.--have lots of similarities.  My swimming has improved from hearing about paddling technique. 

[ehrawn]

Something else to note with drag: it's really hard to model. The only way to actuall obtain the correct value is to determine it from experimental results. What I posted earlier has a lot of assumptions built in, but I'm pretty confident that dimples on a paddle blade won't do anying other than increase your manufacturing costs. Until QB shows me some testing, I'm going remain skeptical.

BPS: feel free to repost both of my comments on your blog.

[rpmooreii]

Having done many wind tunnel tests and designed a variety of flight vehicles in my prior career as and aero engr and lecturer in aero engr, I too am highly skeptical of dimples on a paddle having any effect other than psychological.