Force Alignment and Moment Balance
There is a really cool universal law out there that states: “For every force, there must be an equal and opposite force.” When you are docking, if your boat pulls on a line attached to the dock, the line pulls back on your boat. If the forces are aligned, then nothing really happens.
When the forces are not aligned, the forces work on their own volition to try to align themselves through a phenomenon called torque—also known as “moment.” (Not a moment in time, but rather a force applied at a distance.) The greater the force or distance, the greater the moment. Think of it as a turning force. It is this concept of moment that you must fully understand. So the second part of the universal equation above is that “For equilibrium (no spinning) to occur, for every moment there must be an equal and opposite moment.” Don’t worry, it is all easy and the exercises will allow you to gain an excellent feel for the dynamics (forces and moments).
The image shows the forces aligning. To make things easier, the universe decided that multiple forces applied at the same place are additive. And forces applied at distances can be represented by a turning force in between them.
In the above image, the thrust force and the rudder force combine into one force, F3. To test out this phenomenon, take your phone and place it on a desk. Hold the top right corner down with a finger then push the bottom middle of the phone in the direction of F3. You’ll see the phone turn. It’s kinda obvious when you do this but, needless to say, this is how you can apply this process to your boat to get you into and out of some tight places.
The image below shows what happens to a boat when tied to the dock at a sternside cleat and propeller thrust is engaged. All the forces align and moments balance to move the boat to an equilibrium position.
The boat on the left is not in equilibrium and will thus move. The boat on the right has stopped moving. It is in equilibrium even though forces are being applied from the thrust, the dockline, and the rudder. All the forces and turning moments balance. Notice the rudder is slightly turned. The rudder is applying an additional balancing force to stabilize the boat.
