Why do ships have a bulbous bow? Many ships have a bulbous bow to create a wave that helps reduce drag, which increases fuel efficiency.
Any object that is moving, unless it is in a vacuum, is subject to forces that act upon it. These can be friction with the ground or surface, and wind resistance. The moving object has kinetic energy, which makes the object move. When that energy has been dissipated, the object will no longer move. If you are on your bicycle and you are on a flat surface, you can give a hard pedal and lift your feet because that energy will carry you for a while. However, after a short distance, the energy will be swallowed up by friction with the road and wind resistance. You can make yourself go further in this situation by having narrow tires and well oiled wheels to lower friction and by leaning down to keep yourself out of the wind. Ships are obviously subject to the same forces. A ship has to contend with friction, wind resistance, and water resistance.
Water resistance is a little different from wind resistance because water is much denser and harder to push out of the way than air. When a plane is moving through the air, the wind moves out of the way more easily than water moves out of the way of a ship. The ship has to push through the water, and then there is friction all along the sides of the ship as the water comes into contact with the hull. The ship needs more power from its engines to fight against the water. The water that the ship pushes aside heads out from either side in a long wave called a wake. The energy from the ship is transferred to this wake, making the waves larger and travel further. This is where the ship’s wasted energy goes. A bulbous bow works to prevent this.
The bulbous bow is a protrusion on the front of many ships. The base of the hull continues out past where it would naturally end and culminates in a bulb that is usually as far forward as the front edge of the upper part of the ship. It kind of looks like a nose on the front of the ship. The bulb is sometimes curved up, known as a “ram” bulbous bow, and sometimes just straight. It is at the point of the ship where it will be below the waterline.
When the ship plows through the water, it is hitting stationary water, and its energy forces the water up and to the sides. It goes on either side of the bow, so it is called a bow wave. This brings the water higher, so it hits more of the bow and takes more of the ship’s energy. The idea behind the bulbous bow is to reduce this bow wave. We learned before, when we were looking at noise cancelling headphones, that two waves can interact with each other. If you have two waves where the peaks and the troughs are in the same place, they will build on each other and make a much higher wave. However, if you have two waves where the peaks of one wave are where the troughs of the other wave are, they basically cancel each other out and reduce the size of the wave drastically. A bulbous bow does this. It pushes the front of the ship out far enough so that the waves coming over the bulbous bow are exactly at the right place for the troughs and peaks to be between the troughs and peaks of the ship’s regular bow wave, cancelling the waves out. This is called destructive interference. The ship has to use extra energy because it is now producing two bow waves, but because the two waves cancel each other out, the pressure along the hull is greatly reduced, and overall the ship can travel more easily through the sea and needs less power. A bulbous bow can improve a ship’s efficiency by up to 15%, which is a huge saving in fuel.
Bulbous bows only have an effect if a ship is large enough or if it is travelling fast enough. Oil tankers and cargo ships that are huge but slow need them. Passenger ships that are smaller but faster need them. If ships are smaller than 4,000 tons or travel slower than 12 knots, then a bulbous bow can actually reduce their efficiency because they need more power to make the second wave than they get from the reduced pressure along the hull. And this is what I learned today.
Sources
https://www.marineinsight.com/naval-architecture/why-do-ships-have-bulbous-bow
https://en.wikipedia.org/wiki/Bulbous_bow
Photo by abdo alshreef: https://www.pexels.com/photo/large-cargo-ship-docked-at-seaside-port-aerial-view-33218942/