Our friend Arman asks us to clarify a term that was used in the BoatSafe.com contest. The situation was that a large passenger ferry was overtaking a pleasure yacht and, because they were in a narrow channel, the ferry passed very close to the passenger vessel. Because of the “hydrodynamic effect” and the suction of her propellers, the ferry pulled the passenger vessel into her side causing damage.
Hydrodynamic effect can be broken down into its word elements which may help explain what it means. Hydro means water, dynamic means energy or physical force in motion and effect means anything brought about by a cause or a result. So…if we put this all together, hydrodynamic effect means the result of water in motion.
On vessels of any size, but especially on large vessels, as the boat moves through the water the friction of the water on the bow and on the hull causes the water to create turbulence. This turbulence, as the force of the water moves toward the stern (back) of the boat, tends to pull floating objects into the side of the boat. In addition, as the large propellers (which are also toward the stern of the boat) turn, they, too, pull water down and toward the center line of the boat. So, if a large boat passes too close to you in a small boat, you could be caught in this hydrodynamic effect and pulled into the side of the larger boat. Not a good idea.
To demonstrate the effect of hydrodynamics next time you are out in the family boat, ask the responsible adult in charge if you can try an experiment. With the boat moving forward at a slow speed, drop a small ball that floats just off the stern of the boat. Actually, just let it roll down the transom until it hits the water. You will notice that the boat does not run away and leave the ball behind but actually pulls the ball along behind it as if it were attached. This is hydrodynamic effect. Don’t forget to stop the boat before retrieving the ball with your fish net. If you imagine that your small boat were the ball and you go too close to a large ship, you too would be pulled along and into the ship.
Be careful out there. There are strange and mysterious forces at work. But, you can avoid this one by staying far away from large boats.