Shipboard measurement continues to challenge ship owners and their operators. With the focus on the environment and an effort to reduce green house gases (GHG), which includes improving fuel economy, there are many approaches that can be classified as small, incremental improvements. Stern-Appended Hydrodynamic Energy Saving Devices (SAHESD) have proven to be cost-beneficial but challenging to validate their gains in service. Ships operate on the interface of 2 mediums, water and air, and as a consequence face many variables that can disrupt precise measurements hence masking these small, but positive benefits.

Efforts to validate benefits continue to improve with better measurement tools and data processing. However, as every ship designer knows the general rule of thumb is to allow a 15% sea margin when sizing a propulsion plant to address those uncontrollable forces of wind, waves, current, and degradation of hull and propeller surfaces. With such a large margin it is difficult, if not impossible, to demonstrate small improvements without a major scientifically designed approach.

Because each SAHESD is capable of providing incremental savings, realistically in the range of 2 to 5 %, it is difficult to measure an individual device's contribution on ships in service. With sea margins of 10 to 15 %, finding, for example, a 3% improvement in service conditions requires precise data, whether it be on sea-trials or in long-term service. Therefore, combining several of these energy saving devices, especially before, at, and after the propeller, is the recommended approach in order to achieve a greater cumulative improvement which can be more readily measured (potentially 5 to 8 %) , especially on sea-trials for new-buildings where sister-ships are being built in a class.

For large ocean-going ships even the main propulsion engines are tested outside the ship not only to demonstrate performance, but also to validate fuel consumption rates. However, for most small improvements validation can only take place on full-size ships when in service. Even the hydrodynamic performance of the ship: hull, propeller, and rudder, are evaluated during newbuilding sea trials even though computerized design and model testing were done to set their form.

So what needs to be done to satisfy the skepticism of the majority of ship owners and operators about gains from SAHESD’s, even though they have been identified by IMO’s MEPC Committee’s working group as having positive benefits?

First, the concept of stipulated savings needs to be given consideration. If the design engineering is scientifically solid and it is validated in model testing, as is done for ship propellers, then at least a level of skeptical optimism should be given to the expected level of improvement. Being at a consciousness level of "Trust but Verify" is helpful when one is operating in a continuous improvement mode.

Second, serious weighting needs to be given to these stipulated savings if it is expected that the gains always will be available over the life of the ship. For example, a 3 % improvement would allow a sea margin to be reduced from 15 to 12%, or for existing ships GHG could be reduced 3%. Such an approach is necessary when retrofitting existing ships, primarily because many actions are taken at drydocking that make measurement nearly impossible for any one action. How many ship owners actually validate gains solely from propeller polishing?

Third, coordinated long-term planning by both ship owner, as buyer, and ship builder, as seller, to develop a sea trial program that affords a practicable effort to validate the achievement of these small savings is necessary. This level of cooperation seems to be rarely achieved, especially after specification creation and contract signing.

Fourth, when a series of sister-ships is being constructed, a scientific-logical approach would be to implement the improvements on half the fleet at first. This would permit sea trial comparison and also operational comparative data over several ship years of service. Having dual-axis
Doppler Sonar Velocity Logs (DSVL) would provide more precise long-term, in-service speed-through-the-water measurements. If the success of small improvements is acceptably demonstrated then installation on the outstanding portion of the fleet could be accomplished at their next dry-docking availability.

Fifth, the ship owner and operator have an overall moral responsibility to reduce the environmental impact of their operation on the planet. IMO is making an effort to address this global challenge, but formulas, such as the Energy Efficiency Design Index (EEDI), that encourage building bigger ships and slowing them down have many practical limitations. Ship design efforts should include all cost-beneficial solutions and selection of approaches should be economically tiered on a basis of fuel cost savings per capital investment, which generally would favor many incremental approaches, such as SAHESD’s. The incentives can be strong enough without economic distortions from fuel taxes or cap and trade market mechanisms.

Ship Propulsion Solutions is willing and ready to work with ship owners and operators who are motivated in meeting the environmental challenge and as a result reduce their operating costs by installing cost-beneficial stern-appended hydrodynamic energy saving devices (SAHESD).

Robert Walsh, President
Ship Propulsion Solutions, LLC
P.O. Box 910148
Lexington, KY 40591-0148 USA
T: +1-859-533-8848
F: +1-859-296-4299
E: contact@shippropulsionsolutions.com
Web Site: www.shippropulsionsolutions.com