## Monday, May 19, 2014

### Dragon Boat Physics 101: Stroke Efficiency

To get a better understanding of how to get more efficient in your stroke, you need to understand what exactly happens when you take a stroke. Obviously, it moves the boat forward, but have you ever taken the time to consider what physics are at play here? Understanding the fundamentals can greatly help you to understand what you are doing right and wrong. Today's lesson is about improving your stroke efficiency... with science!

The basics

Paddling (of any kind) relies on Newton's third law of motion, which states that for every action there is an equal and opposite reaction.  This means that the force you apply on your paddle results in an opposite directional force for the boat.
 Picture courtesy of HowStuffWorks
The above graphic from HowStuffWorks explains canoes, but the same concept applies here.  The angle of the blade as we apply force affects in what direction the directional force is applied.  This graphic is using it to explain the purpose of a bent shaft paddle, but since dragon boating only uses a straight shaft paddle, you can see that as the paddle goes negative, the directional force moves from forward to down-and-forward.  This downward force will actually slow the boat down (because the downward force on the boat increases the surface area of the boat and the water, increasing water resistance, but that's another lesson for another day).  So ideally, you want to exit your stroke before you start applying a downward force on the boat.

 Time lapse of 3 phases of the stroke
Now here's where things might sound a little crazy. With the basics explained above, you should now realize that any force applied to the paddle should completely go towards moving the boat forwards. So you apply force, and the boat moves forward - not water moving backwards. That means that during your stroke, you are actually touching the exact same spot of water the whole time. Don't believe me? Check out some old race footage on YouTube. This effect is most evident when passing by a stationary object like a buoy. In the time lapse photo above, you see that the paddle's point of entry is the exact same as the point of exit as the boat moves forward.
 Animation of stroke. Courtesy of Wikimedia Commons.
The above animation (unfortunately which shows a canoe stroke with an extremely negative exit) shows the effect of the paddle (and thus you and the boat) moving forward as you take the stroke. So the actual area that the paddle comes in contact with is a single point in the water. When we talk about water compression, we literally mean it because the force we apply is compressing a small section of water, making it act as a solid that you are "pushing" against. As we apply pressure (force) to the water, it will continue to push back on our paddle... to a certain point. When we break the maximum tension of water-to-paddle (a measurement of force per area), we actually start moving the water backwards instead of pushing ourselves forward. The technical term for this is "slippage".

Slippage

As stated just above, slippage is the phenomenon where the force applied on our paddle is no longer completely moving us forward, but also moving the water backwards. This can be visibly seen as waves coming off of the paddle. I know that some paddlers use this as a cue that they're putting in a lot of force (which they definitely are), but unfortunately they are sacrificing on efficiency.
There are times when slippage is hard (if not impossible) to avoid; case in point, the first strokes of the start. Since the boat is at a stand-still, it takes a lot of force to get it up and moving. This amount of force can normally be in excess of what the water tension will allow and your paddle will slip, so there's no way to apply the appropriate amount of force without sacrificing a little bit of it towards slippage. But since we're talking about only a few strokes in an entire race, I'm completely fine with that. What I'm not a fan of, however, is slippage during the race (especially longer races, where we can't spare wasted energy).

How to prevent it

You can avoid slipping by making sure that your paddle is in contact with the largest surface of water possible when applying pressure. This means you have to bury the whole paddle blade, especially if you're applying a lot of pressure. If you only have half your paddle in the water, then you're limited at half the amount of force possible before slippage occurs.

#2) Apply force gradually and throughout the whole stroke
Shooting all your force into the water in a single shot will increase your chances of slippage. Instead think about balancing the pressure during the entire stroke; beginning with the least amount of pressure at the catch and maximum amount at the power phase (when the paddle is perpendicular with the water). This also ensures maximum forward directional force. N.B. I'm not encouraging "soft catches" here, I'm saying that the pressure at the power phase should be greater than that at your catch.

A good stroke starts with a good entry, and making sure you have a clean entry can be the first step at minimizing slippage. A missed catch can be identified by its signature "ker-plunk" sound. The reason you hear that is because the paddle is actually trapping an air pocket underwater. This air pocket contributes to slippage because the paddle moves through air much easier than water. Eliminate the "ker-plunk" to eliminate the slippage, and your teammates will probably thank you for less splashing too.

#4) Eliminate the bicep flex at the exit
In most of the scenarios that I've witnessed slippage, it occurs most heavily at the end of the stroke. Paddlers will have the urge to finish their stroke with a bicep pull just before the exit, which allows them to apply a quick burst of force into the water. While it might seem like a great idea, this sudden burst usually results in slippage, and worst, the slippage is usually in the form of scooping water backwards and onto the lap of the paddler behind you. Furthermore, the angle of the blade when executing a bicep pull is driving the boat downwards (!). Consider instead of ending your stroke with tricep extension to "press" off the exit, rather than to "pull" into a negative exit.

#5) Don't hold back pressure, just be smarter about where to use it
What I definitely don't want to happen here is that paddlers become overly self-conscious about slippage and will hold back on their stroke. I want you to just be smarter by acknowledging when it happens. If you're paddling at 80% pressure during the piece, and on power series (100% pressure) you are slipping a lot (and can't correct it with #1-4), then consider making it a more incremental increase and maintain more pressure during the piece. E.g. Increase to 85% pressure during the piece, and decrease to 92-95% on the power series. The overall energy output may be the same, but the total forward directional force will increase.

1. Brilliant. Our coach Cath Little has been explaining this for years. Good to see it in print and to have the graphics.

2. Number 1: Water is not compressible. Air is compressible.
Number 2: If the paddler is not "shoveling" or lifting the blade up at the exit, the force is still towards the helm with less surface area due to the negative angle. For efficiency the blad should be edged out to the side, slipping forward out of the water.
Number 4: The captured water at the back of the blade is dynamic, not in a fixed position. It slides off the blade because the paddle is constantly in a pendulum from positive to negative, though we try to maintain vertical/perpendicular/horizontal as much as possible and still maintain a long and deep stroke. The water is constantly replaced by new "captured" water. Eddies are produced off the corner tips of the blade during the stroke.

3. The above is pretty much correct, but a few thoughts. Winter's did some tests indicating that paddleblades are most efficient between +/- 10 degrees to the direction of travel. The rowing community looking at similar data claims the efficiency fall off is +/- 20 degrees. Kinda depends of where we draw the line, lets compromise at +/- fifteen degrees to square to the stroke.

Both straight and bents have their +/- windows. The straight blade's most effective travel is forward of the paddler's body, starting at an extended catch and ending when the paddler's knee comes abeam the blade. The reach increasing torso rotation to catch suggest straight blade paddler's should kneel to increase forward reach.

Bending a paddle moves the sweet spot, which is actually an arc, aft, closer to the paddler's body. The reduced need to reach forward lessens torso rotation and allows a shorter paddle both increasing cadence and therefore speed. Lessened reach also allows the paddler to sit, again shortening the paddle and further increasing cadence. Works for marathon racers.

Unfortunately, the Dragon boat game requires sitting and straight blade paddles. What to do? For starters, emphasize torso rotation to extend the catch forward. Secondly, stop the stroke abruptly mid thigh to eliminate the inefficient aft portion of the straight blade's stroke and increase cadence. And, of course, have fun, damn few of us get paid to paddle.