Improved Controllability Topic At Chesapeake Section Meeting

Members of the Chesapeake Section of The Society of Naval Architects and Marine Engineers met recently to hear the presentation of a paper titled "Evaluation of Concepts for Improved Controllability of Tank Vessels," by Eugene Miller, Vladimir Ankudinov, and Thomas Ternes.

Both Mr. Miller and Dr. Ankudinov are with Hydronautics, Inc.

of Laurel, Md., while Mr. Ternes is now with Morris Guralnick Associates of San Francisco, although at the time the work presented in the paper was accomplished he was also working for Hydronautics.

The paper was presented by Mr. Miller and Mr. Ternes, and is based on a study done by the authors for the Maritime Administration in which the reasons for tanker casualties involving collision, ramming and grounding, and concepts for improving controllability were evaluated. The U.S. Coast Guard casualty data bank was used as the source of information for the casualty analysis.

The data for a five-year period (1971-1976) were reviewed, and cases where controllability played a major role were evaluated.

Only casualties occurring in U.S. waters or involving U.S. vessels in foreign waters were contained in the Coast Guard's data bank; therefore, the study was limited to those cases.

Based on this review, it was determined that the major cause of tanker casualties involving collision, ramming, or grounding result from human error, followed by control problems in wind and current, inability to turn sharply enough, insufficient tug assistance, stopping, bank suction/ sheer, steering / propulsion failure, control while slowing, and control while backing in that general order. Human error caused casualties were not considered further although it was acknowledged that, in many cases, improved vessel controllability could have mitigated the consequences of the human error. The types of controllability required to reduce the number and severity of these casualties were evaluated and categorized into such areas as the ability to maintain control of the vessel after losing one steering or propulsion unit, the ability to maintain steerageway while slowing down, and the ability to control heading while backing. Finally, the costs incurred as a result of the casualties were investigated to form a basis for the determination of the cost effectiveness of concepts for improving tanker controllability.

The second phase of the study involved the evaluation of various methods of increasing controllability through the use of a computer simulation of ships' maneuvering characteristics and a series of model tests. A singlescrew, single-rudder, 84,000-deadweight- ton tanker with a steam propulsion plant of approximately 18,000 shp was selected as the baseline ship against which improvements in c o n t r o l l a b i l i ty would be measured. Five concepts for improved controllability were evaluated, i.e., twin propellers and rudders, increased astern power, maneuvering propulsion devices (tunnel thrusters, active rudder), high lift rudders (flapped rudders r o t a t i n g c y l i n d e r rudder), and thrust vectoring devices including steering Kort nozzle and Kitchen rudder. The Kitchen rudder is a movable shroud that surrounds the propeller and is roughly analogous to the thrust reversers found on airliners.

The authors concluded that the use of the leading concepts investigated can be attractive from a shipowner's point of view. The moderator, Comdr. James Card of the U.S. Coast Guard, supported this conclusion and referred to the fact that improving vessel safety through approaches that offer financial advantages to owners is preferable to achieving the same goals through regulation.