Sunday 26 September 2021
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Newtons Laws of Physics

Newton’s Laws. Blade Orientation & The Paddler’s Box

There are two basic physics principles that power paddle sports. They are Newton’s 2nd and 3rd Laws of Motion:

2nd Law: Force equals mass times acceleration

3rd Law: For every action there is an equal and opposite reaction


Due to Newton’s second law of motion, the force exerted on an object equals the mass of the object multiplied by its resultant acceleration. Equally if you were to divide the numeric value of the force by the mass of the kayak/kayaker combination, you would get the resultant acceleration that the boat experiences. (i.e. If the kayak and kayaker total a mass of 50 kg and they exert a force against the water (and thus the water exerts against them) a force of 150 N then one would need to divide 150 N by 50 kg in order to find the acceleration, 3 m/s^2)

In other words, the lighter the kayak and the more force that is exerted, the faster it will accelerate.

Think of the power face of the paddle as the palm of your hand. Just like when you are swimming, you reach forwards, grab some water and pull backwards in order to propel yourself forwards. It couldn’t be simpler. But what is actually going on?

While the paddle is being pulled back, it is exerting a force on the water and because of Newton’s third law of motion, the water is simultaneously exerting an equal, but opposite, force on the paddle blade. Since you are pulling backwards the force exerted inversely onto the paddle blade is in the forward direction.

Or is it? ONLY if your blade is oriented correctly. Imagine a Force Arrow sticking out perpendicular from the back of the blade. That Force Arrow needs to be pointing as much as possible in the direction you want to go when you pull on it.

Think about the forward stroke. The blade (and paddle shaft) should be as vertical as possible at the point at which you apply force. If the blade is at too much of an angle your Force Arrow will be point towards the sky and you will lifting the kayak out of the water instead of propelling it forwards. Likewise if your stroke is too long and leave your blade in the water past your hip, your Force Arrow will be pointing down into the water and you will be lifting water, pulling the kayak down into the water which will slow you down.

Law: Force equals mass times acceleration

How do we achieve that vertical blade and paddle shaft? By extending the top arm out so that the BOTH arms are almost straight at the start of the power phase of the stroke. This is called “The Paddler’s Box”. We try to maintain the Paddler’s Box during every propulsion stroke. The Paddler’s Box also allows us to engage our core, back and shoulders. If you allow the Paddler’s Box to collapse, the chances are you are just using your biceps which are tiny and weak in comparison to your core, back and shoulders.

We also need to ensure that the force exerted by the water on our paddle blade is transferred to the boat. After all it is the kayak we are trying to move! If the connection between the blade and the boat is compromised, a lot of the force will be lost. Thus we need to keep a strong posture and drive with kayak forward the leg and foot using the foot brace. Coordinating the pull on the paddle blade with the drive of the foot will ensure the boat surges forwards and does not rock from side to side.

Finally let’s think about Torque: Though Newton’s Laws explain the basic concept of how we are able to propel ourselves forward, we also want to be able to turn our kayak. Torque is the reason why the kayak swings in the opposite direction of each stroke. If a stroke is done on the right side, the kayak rotates counter-clockwise, but if the stroke is done on the left, the kayak rotates clockwise. This is because that while a stroke is taking place the paddle acts as a lever arm for the vessel. Torque is the length of the lever arm times the perpendicular force exerted on the arm. Thus the further from the kayak that one places the stroke the greater torque that will be made, and the more the boat will rotate.

When paddling forwards we want the power phase of the stroke to be as close as possible to the kayak to reduce the amount of torque. But we do this WITHOUT bending the elbow and collapsing the Paddler’s Box! That is why the forward stroke is short  from the feet to the knees and then out!

When performing a turning stroke we want to increase the amount of torque by LOWERING the top hand, allowing the paddle blade to sweep in a wide arc. But it is equally important to apply maximum force to the blade by maintaining the Paddler’s Box and have a good connection with the kayak through the foot brace, back band and thigh hooks ensuring an efficient transfer of force between the blade and the boat.

Understanding these principles will allow you to become a better paddler.


Top Tips

Remember, the power face of the paddle is like the palm of your hand. If you get confused, just think what you would do if you didn’t have a paddle in your hands. How would you propel yourself in the direction you want to go? Now do it with the paddle.

You can practice paddling with your ‘Hands Only’ this will help you develop a FEEL for the water and understand more about what is going on.