Vessel Speed Reduction
 The San Pedro Bay Ports’ voluntary speed reduction (VSR) program requests ocean going vessels (OGVs or ships) entering or leaving the ports observe a 12-knot speed limit when they are within 20 nautical miles of Point Fermin. The objective of the VSR program is to reduce nitrogen oxide (NOx) emissions from OGVs by slowing their speeds as they approach or depart the ports. Reduced vessel speeds demand less power from the main engine, which in turn reduces NOx emissions and fuel consumption.
Ocean Going Vessels have two types of engines: main engines and auxiliary engines. Main engines are used for vessel propulsion through the ocean. Auxiliary engines spin electrical generators that provide power for a ship’s non-propulsion needs such as lights, navigation computers, bow thrusters, air conditioning, satellite communications, etc.
How Does Slowing a Ship Down Reduce Emissions?
For main engines, the energy required for vessel propulsion varies with the vessel speed; the faster the speed, the greater the energy requirements to maintain that speed. Conversely, as speed is decreased, energy requirements are also significantly reduced. Since the energy required to operate a ship’s main engine is directly related to emissions, actions that reduce energy consumption typically reduce emissions from these main engines.
Two questions are often raised about the emissions benefits of vessel speed reduction. The first is that main engines are somewhat less efficient at slower speeds. It is important to note that this reduced efficiency is more than offset by the benefits of reduced energy consumption (and therefore associated emissions reduction) that result from slower transit speeds. The second is that reducing ship speed means that the ship takes longer to move from point A to point B, and therefore spends more time transiting. Auxiliary engines do operate during transit, and therefore the auxiliary engines are operating for a longer period of time over the same distance, when the vessel is moving more slowly. It is true that the additional auxiliary engine operation for longer transit times will increase auxiliary engine energy consumption. However, the emission reduction benefits resulting from decreased main engine energy consumption significantly outweigh the increased emissions from increased auxiliary engine operation at slower speeds. This reduction in main engine energy consumption provides for reduced emissions overall of NOx, particulate matter (PM), and sulfur oxides (SOx). In addition, reduction in fossil fuel consumption results in a reduction of greenhouse gas emissions.
In 2007, the ports of Long Beach and Los Angeles estimate that the VSR program resulted in the following emission reduction benefits: |
