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A Friend to Practice With
A River to Paddle
Understanding what momentum and tracking are.
How momentum affects your boating.
How to manage momentum for the river you are running
In this lesson, Nathan and Trevor discuss the theory of momentum and tracking. The best definition of momentum is the product of the boat’s mass and velocity, typically expressed at p = mv. While tracking on the other hand is the ease at which the boat can continue travelling one the course is set. These two concepts are deeply intertwined, but we have to clearly define and understand several of the following physics concepts:
Mass
Velocity
The first law of motion
Inertia
Mass is the overall amount of matter within a given area. In boating a good example would be an r2 team in a 14-foot boat vs an r6 team in a 14-foot boat. In both instances it is the same craft, however since there are more crew members in the r6 team it will have more mass if the velocity remains constant.
Velocity is best defined as an object’s speed combined with direction. Speed being typically measured in mile per hour or kilometers per hour has to have some sort of direction attached to it to create velocity. In boating we will typically use terms like:
Left Angle
Right Angle
Downstream
Upstream
These terms help us orient ourselves as well as establish the velocity of a boat on the water. Understanding which way your velocity is headed will help you determine how best to manage your momentum on the water by allowing you to see what action you need to take to punch a feature or slow down to avoid hitting a feature you don’t want to hit.
An object in motion stays in motion unless acted upon by an outside force.
This principal helps us to establish why a boat moves the way it moves and also helps create the basis for tracking. A closely related concept to this law is Inertia, which is an object’s ability to resist a change in movement. Mass is the most important factor in determining inertia, however overall momentum will also influence the inertia of a boat. Where this matters for boating is when a high inertia craft impacts another outside force. That force can be a rock, a wave, or even the boat’s own drag in the water which creates friction.
If the boat impacts a feature with more inertia than the boat, such as a massive wave or a house sized rock, then the boat will likely come to an immediate stop or even bounce off the feature. if this happens your paddlers will be violently tossed as their individual inertia propels them along with their momentum. It is possible the boat can bounce off of the feature, flip off of it as the boat’s momentum is deflected, or even wrap or surf the feature.
If the boat encounters a force with less inertia than the boat it will still experience a sudden decrease in velocity from the opposing momentum countering the boat’s own momentum. The most common scenario with this is a large wave or another raft. in either case the boat’s own momentum is deflected, reduced, or negated which can also lead to flips, surfs, or swims depending upon the difference in momentum.
As previously stated, tracking is the craft’s ability to maintain its course and there are several factors that influence tracking including:
River features
Hull design
Draught
Tube inflation
Weight distribution
Of the factors listed above river features are the only one that is not within your control. eddy fences and currents can drive your boat into odd places, by exerting addition drag forces on your boat. It is important to note though that river features are not actually part of how the boat tracks, however many paddlers, especially newer paddlers, can have a hard time distinguishing when the river channel is a factor vs. element of the boat influencing tracking.
Hull Design - this can influence drag by putting more elements in the water which increase friction. A great example of this is a cataraft vs a raft of the same size. the cataraft does not have a floor, thus the water can easily flow over the tubes which helps to reduce drag. conversely, a floored raft has much more surface area in contact with the water thus increasing the amount of friction with the water thus causing the raft to move more slowly due to drag.
Draught (pronounced draft)– This is the depth at which the raft penetrates the water and draught contributes to drag by causing the boat to have a greater displacement. Greater displacement has two implications on the raft one it creates more friction by forcing the boat to have to plow more water out of the way to achieve the same velocity. Second, the draught is influenced by a mass – buoyancy interaction. The greater the buoyancy compared to a similar mass, the higher the boat sits in the water creating less drag. This is best shown by the floored raft and cataraft example. The cataraft has less buoyant pockets of air thus causing it to sit lower in the water creating more displacement and thereby drag. The floored raft on the other hand is likely to be more buoyant thus creating less displacement.
Tube Inflation – The amount of air in your tubes directly affects the buoyancy of the raft which leads to less drag the greater the buoyancy as stated above. When a boat hits the water and the water temperature is lower than the air temperature, the air in the chambers will condense. This reduces buoyancy and creates drag. When one tube is lower pressure than its neighboring tube, it will have less buoyancy thereby creating additional drag. Proper raft inflation will help to alleviate this drag issue and prevent your boat from tracking poorly because one side of the boat has more friction in the water.
Weight Distribution – this can increase drag if the crew and equipment are not evenly distributed across the boat. More weight on one side will cause deeper draught and higher displacement on one side of the boat thereby causing that side to move more slowly than the other side.
Understanding how to put this knowledge to good use on the water is going to go a long way to helping you become a better boater so here are a few techniques you can use to help you be prepared:
Understand the fundamental nature of the river. A big water river requires more momentum to punch features A technical river on the other hand will require constant maneuvering to avoid feature in the river and thus requires lower momentum to safely navigate.
Changing your momentum can only be achieved in 2 ways: you can either alter your mass or Alter your velocity. Changing your mass by changing the number of paddlers or volume of equipment can help you either have more momentum to punch big water features or less momentum to weave through technical features. Changing your velocity by hoping on a faster current (lookin’ at you outside bend of the river) or switching from an r3 setup to a oar/paddle combo can help you achieve higher velocities with a similar mass. Conversely you can get on a slower current (an eddy for instance) or backpaddling, can help you make more efficient technical moves by reducing momentum.
Always remember that the river has many different features at play and as such a big water run can have creeky rapids in it just as a technical run can have a tight chute ending in a huge feature that you need to punch. When in doubt scout comes from the principal that the river is a dynamic place that requires boaters to understand when to increase momentum and when to decrease momentum. For more insight on this please review the scouting video.
Finding the right balance of momentum takes practice so we always encourage people to get out on the water and more importantly set up your gear consistently when you are starting out. Once you have a baseline for how your boat moves and reacts you will start to have a better understanding of when to apply these principals of momentum.
