A rough theoretical estimation suggests that adding one vessel to an existing fleet of seven on a 21 days return schedule (two sailings a week), facilitates a 14% speed reduction. This means propulsion power reduction of about 30% for the whole fleet and an increase in sailing frequency (one sailing every 3 days rather than 3.5 days) and obviously an increase in sailing time (+3 days). Crew and capital costs will increase but still a net gain is feasible. However, not every trade has such an easy gain. A smaller number of vessels on short
haul trades complicates the short-term speed regime changes. It is clear in the long-term, increasing vessel size will payoff better.
haul trades complicates the short-term speed regime changes. It is clear in the long-term, increasing vessel size will payoff better.
What does slowing down mean for the operational performance of existing vessels and for new designs? The hull design has to be optimised for the relatively lower, wave making resistance. A new optimum can be achieved in terms of block coefficient, bulbous bow and shoulder position. The calm water propeller design changes and becomes less critical from a loading point of view and even more so if a ship changes cruising speed. The added resistance in waves now becomes relatively more important and requires more attention to avoid overloading the propeller and engine.
Sustained speed also becomes an issue. If the calm water power demand is diminishing, the relative sea margin requirement could grow and the propeller design must be capable of dealing with both, quite different, operational conditions. The way forward is analysing the real operational schedule requirement through the simulation of all, or typical future voyages. These simulations will be able to precisely identify the sea margin need as a function of acceptable variations in arrival times. MARIN offers various tools to analyse the consequences at an early design stage.
If new designs reduce power levels, manoeuvring requirements can become more limiting. MARIN has seen examples of high windage vessels losing leeway due to a lack of steerage. Adjusted rudders, deliberate changes of the course stability (skegs) could help to overcome this problem. Lowering speed can cause rolling motions to increase. Parametric rolling is an important example that can even threaten cargo safety. The future will show how these challenges are faced.
If new designs reduce power levels, manoeuvring requirements can become more limiting. MARIN has seen examples of high windage vessels losing leeway due to a lack of steerage. Adjusted rudders, deliberate changes of the course stability (skegs) could help to overcome this problem. Lowering speed can cause rolling motions to increase. Parametric rolling is an important example that can even threaten cargo safety. The future will show how these challenges are faced.
More information
For more information on this subject please contact Johan de Jong.
This article was published in MARIN Report 94, August 2008.
For more information on this subject please contact Johan de Jong.
This article was published in MARIN Report 94, August 2008.
October 20, 2008