In rowing, the mass of the crew is much more than the mass of the shell. Therefore, any movements by the crew affects the momentum of the boat. Because of this, much research has been done to minimize the detrimental momentum changes and maximize the positive momentum changes caused by body movement.
Dragon boating has the same situation, with some fundamental differences.
- A dragon boat crew has up to 20 paddlers, a drummer, a sweep, paddles and the boat itself.
- The crew faces the way they wish to travel.
- The paddle blade and our body movement are in the same direction.
A fully loaded dragon boat is not just a 2000kg solid body - it contains two separate components:
- Crew with paddles, representing 70-80% of the total mass; and
- Boat (drummer and sweep), representing 20-30% of the total mass
During a stroke the individual components of the dragon boat move relative to each other. Sometimes the crew is pivoting backwards while the boat is moving forward. Other times the crew is pivoting forwards while the boat is moving forward.
Unlike rowing, pivoting backwards and forwards is not moving your whole body. In dragon boating, only some of the total mass of the crew is moving in a way to affect the momentum.
How much crew mass is moving backwards and forwards? Let’s find out how much each part of the body weighs and which parts are moving.
Mean Segment Weights http://www.exrx.net/Kinesiology/Segments.html
I will assume that the pelvis and legs don’t move (Note: elite padders do drive their hip back but most recreational paddlers do not) and therefore only 68% of the crews mass is moving and the other 32% of the crews mass is considered as part of the boat, sweep and drummer. I will also assume the paddling crew weighs 1600kg in total and the sweep weighs 80kg and the drummer weighs 70kg.
Therefore the part of the mass of the crew that is moving is 55% of the total mass of boat and crew.
While the momentum of the individual components can change relative to each other, the momentum of the whole system cannot change unless external forces are applied (via the water, usually) (Newton's 1st Law).
So let’s remove the paddling and drag from the system for now and just leave a boat coasting along.
Momentum = mass x velocity
If a crew with upper body mass, mc, is in a boat, mass mb, that is moving at velocity vt, the total momentum of the system is:
Momentum = mcvt + mbvt
Remember mb is the mass of the boat, the drummer, the sweep, the paddles and all the parts of the paddlers that do not move, and mc is the mass of the moving part of the paddler’s bodies.
So far so good.
The crew will now begin to pivot their torsos forward and backwards a small amount to see the effect on the boat. Not paddling, just rocking forwards and backwards.
Now we need to modify the formula to include the velocity, vc, added by the crew’s movement.
|(1.1)||mc vt + mbvt = mc(vt+vc) + mb (vt+vb)|
|(1.2)||-mc vc = mb vb|
In English this means the change in momentum produced by the paddlers moving is equalised by an opposite change in momentum of the boat.
Since, in this case, the mass of the crew is equal to 1.2 times the mass of the boat
|(1.3)||-(1.2mb) vc = mb vb
-1.2 vc = vb
Again, in English this means in order to maintain momentum, the change in velocity produced by the paddlers moving is equalised by an opposite change in velocity of the boat multiplied by a factor of 1.2.
So what effect does these momentum changes have on the speed of the boat?
- A stroke rate of around 60spm. 1 stroke per second.
- 0.6 seconds on the drive
- 0.4 seconds on the recovery
- The body pivots forward with the chest moving a distance of 0.25m.
That means the pivot forward is +0.25m in 0.4 seconds, which is +0.63m/s
Correspondingly, the pivot backwards is -0.25m in 0.6 seconds, which is -0.42m/s.
Torso Moving Forwards - the Return
Using these formulae, let’s see what happens to the velocity of the boat when the crew pivots forward.
From an upright position if the crew starts to pivot forward, the boat must move backwards at a different relative velocity to conserve momentum.
Using (1.3). If the crew move at vc = +0.63 m/s (i.e. forwards), the boat’s velocity will change by vb = -0.75 m/s (i.e. slow down).
Pivot forwards and you reduce the speed of the boat.
Torso Moving Backwards - the Drive
If the crew then starts to sit up (pivot backwards), the boat must move forwards at a different relative velocity to conserve momentum:
Using (1.3). If the crew move at vc = -0.42 m/s (i.e. backwards), the boat’s velocity will change by vb = +0.50 m/s (i.e. speed up).
Pivot backwards and you add to the speed of the boat.
What does this mean?
For crews traveling at an average speed of 4 m/s. This body movement creates a velocity swing of 1.25 m/s which equates to a swing in velocity of 31%. Significant.
Note: The crew pivots forward when their paddles are out of the water. This results in a shift in boat momentum backwards at the same time that the drag on the boat hull is at its maximum. Together they wash off a lot of boat speed. This is not so good.
The crew pivots back to upright(-ish) during the drive phase, which creates momentum in the same direction as boat travel. This is good.
This also means the difference between the boat’s average maximum (4.5 m/s) and average minimum (3.25 m/s) speeds during a full stroke is caused by the crew’s body movement - which is not desirable.
The overall speed of the boat is the average of the boat’s maximum and minimum speeds.
Looking at graph below, you can see that during a stroke, we are in the water longer than in the air, therefore the return phase of the stroke is compressed into a shorter time and hence produces are larger momentum change.
So the questions begs:
Is this pivoting movement of the body good, bad or neutral to boat performance?
Why should we pivot?
- It allows us to engage an extra swag of muscles that we wouldn’t if we didn’t pivot.
- It allows us to have a longer stroke length at the front.
- At least during the drive, we are adding to the speed of the boat.
Why shouldn’t we pivot?
- The difference between the boat’s fastest velocity and slowest velocity within a stroke can vary by 31% not including drag and other factors.
- At least for the return phase, we are slowing the speed of the boat.
So there are positives and negatives to pivoting.
The real question is:
Do we gain more than we lose by pivoting?
The rowing fraternity move their whole bodies and therefore, this effect is magnified even more. However, they use different techniques and speed profiles of body movement to try and minimize the negatives. However, that sport has had years of research to determine these movements.
In the absence of similar data for dragon boating, we need to determine the answer through trial and error while we wait for more studies to be done on the subject.
Questions that come from this post:
- Should we not pivot at all – or very little?
- Can we pivot in such a way that we too can minimize the negative effects on the boat whilst keeping the positives?
- Maybe the twisting of the torso whilst pivoting forward minimises the slowdown momentum change as the inside part of the torso will not be moving as fast forward as the outside part of the torso.
- Could adjusting the speed of return, and the rate of twist and untwist, to help in minimising the backwards momentum?
- Of course if the leg is used to drive the hip back, then more of the body mass is moving and the momentum change will be greater. But we DO gain the use of our leg muscles, and extend the length of the stroke. Again, are the gains worth the losses?
I am hoping to carry out some on water trails to answer some of these. But if anyone has some answers, I would be most interested to hear from them.