Speed management

Figure 1 source: DNV GL

Speed management includes different aspects of adjusting and planning for optimal vessel speed and engine load.

A vessel’s fuel consumption for propulsion is a result of energy needed to push the vessel through the water at the given vessel speed through water. This relationship, between fuel consumption versus vessel speed, is typically an exponential one. As a rule of thumb assuming that engine power follows the cube of speed, a displacement ship with 10% speed reduction reduces the power need (resistance) and coherent fuel consumption by 27%. However, to assess the total fuel saving on a voyage basis one has to take into account the added time it takes to sail a given distance due to lower speed, yielding a total fuel saving of approx. 19%. For a selected open hatch cargo vessel at 56 000 DWT presented in Figure 1, a 13% speed reduction saved almost 40% of the daily fuel consumption.

Applicability and assumptions

Speed management is applicable for all vessels and at all ages.

Speed planning

Figure 2 source: DNV GL

With basis in the exponential relationship between fuel consumption and power (speed), a vessel sailing with variable speed will usually, for same distance and duration, consume more fuel compared to sailing with constant speed, ref. real case from AIS analysis Figure 2. This is especially true for all diesel-mechanic propelled vessels, including most diesel-electric; however, there are examples of cases where diesel-electric vessels have engine sizes, numbers and specific fuel consumption characteristics that yields sailing with variable speed more favourable.

Optimum speeds with regards to fuel efficiency can however often be challenging due to scheduling requirements from the charterer and other influences. The consequence of strictly enforcing a speed, whether it is variable or constant, can also contradict the weather as vessels may need to “fight the waves” in harsh weather and unnecessarily reduce the power when current and wind are favourable.

Improved planning, better use of vessel specific knowledge, weather forecasts and communication between charterer, port and vessel can improve the speed profile during a voyage and consequently reduce the fuel consumption.

Slow steaming/ECO speed

By simply reducing the speed by 10%, the fuel consumption can be reduced by almost 20%. Slow steaming or ECO speed is the practice of significantly reducing the sailing speed to reduce fuel consumption not only for parts of a voyage, but for a period of voyages, a group of vessels or for a whole fleet. Speed reduction is a strategic measure as opposed to day-to-day speed adjustment depending on expected time of arrival (ETA), weather, currents etc. Large reductions can be made by sailing slower as the fuel consumption curve is exponential subject to speed. Slow steaming is relevant for all vessel types and has highest reduction potential for vessels in long transits. As a constant measure over longer periods slow steaming can inflict increased wear and tear on the engines as very low loads are experienced. Adjustments and due planning and care has to be made related to auxiliary blower operation, heat loads, lubrication and cylinder oil, etc. Significantly lower loads, i.e. at 20% to 50% MCR, can on the other hand also open up for savings potential on the auxiliary systems designed to support the engines when running at full load.

Cost of implementation

There are no investment costs, but this may impact the total revenue due to longer sailing time.

Reduction potential

The reduction potential is 10% to 50% of ship main engine fuel consumption.

The reduction potential of slow steaming is amongst others dependent on which speed the vessel sailed at before it started slow steaming and which speed it is sailing at when slow steaming.

Other References

1. DNV GL Energy efficiency finder / Holistic approach to finding applicable energy efficiency measures to a specific vessel type
2. Marine Insight / The Guide to Slow Steaming On Ships
3. Slow Steaming in Container Shipping / Meyer et. al / Overview of the slow steaming history as well as the widely assumed coherence between a ship’s speed and its fuel consumption / 2012
4. Kongsberg Vessel performance optimizer / Paper on cost efficient vessel operation
5. Regulated Slow Steaming in Maritime Transport / An assessment of Options, Costs and Benefits
6. Air pollution and energy efficiency / IMO energy efficiency appraisal tool