Session: 06-08-01 Model Tests I

Paper Number: 101872

101872 - Evaluation of Seakeeping Model Tests in Open Waters Using Two Submarine Hull Forms 

Although they are mainly designed for submerged operation, there are times, when submarines must operate at the sea surface. Besides the arrival or departure of the submarine, the surfaced condition can even be the most common state of operation during peacetimes, depending on the respective operating authority. Consequently, submarines may be underway frequently at conditions, for which the design is typically not optimized. Therefore, seakeeping should not be neglected in the design process of a submarine, even if it is not the main aspect of submarine design. However, current regulations seem lacking here, due to the fact that a submarine’s surface stability is often only addressed by way of simple formulas or requirements that are aimed at minimum characteristic values such as the initial metacentric height of the vessel. The underlying assumption, that an increase in metacentric height automatically improves the situation in surfaced condition may not hold true for submarines. Therefore, model tests in open waters with free-running, remotely-controlled submarine models were performed in Winter 2021/2022 on the Bay of Eckernförde, Germany to investigate the influence of the surface stability on the seakeeping in natural seaways. In these model tests two generic hull forms strongly based on real submarine classes were used. To reduce costs and modelling effort the outer hull of the submarines is modelled as fully buoyant, i.e. the free-flow between pressure hull and outer hull is neglected in the seakeeping models. The total weight and mass distribution of each model is chosen in such a way that the initial floating position of the model corresponds to the floating position of the full-scale submarine as based on only pressure hull and pressure-resistant appendages at a given surfaced condition. Consequently, the models differ from their full-scale counterparts in regard to their mass distribution, moments of inertia and presumably also roll damping characteristics. Nonetheless, the model tests allow valuable insights into the seakeeping of two very different hull forms as depending on stability, velocity and seaway.

In this paper, the experimental procedure of the model tests is presented. The models are tested in test runs equivalent to a full-scale timespan of at least one hour in the wind-generated short-crested waves found on the bay. Hereby, various settings of stability are realized by adjustment of the vertical center of gravity and the submarine models are driven at different speeds and encounter angles between vessel and main wave propagation direction of the respective seaway. From the model test runs the model motions in all six degrees of freedom as well as the accelerations are obtained. During the test runs, seaway and wind are measured and video footage is recorded and included in the evaluation. In addition to the seakeeping tests, inclining tests are performed, as well as roll- and pitch-decay tests to assess the models’ characteristic properties.

Furthermore, this paper contains a summary of the qualitative observations along with a quantitative evaluation of the model tests by statistical analysis of selected test runs. Different heeling moments acting on the submarine in surfaced condition are identified, such as wind and a moment caused by the propeller at high forward speeds. These heeling moments can be included in the development of new regulations for the surface stability of submarines. Furthermore, the tests show that an increase in metacentric height does not necessarily lead to a better seakeeping performance of a surfaced submarine. Instead, the seakeeping of the surfaced submarine is found to be strongly dependent on the individual combination of stability, speed and environmental conditions and critical combinations can be found at any stability setting. The model tests further suggest that the initial metacentric height mainly influences the extent of static inclination of the vessel, caused by the different heeling moments.

A new surface stability regulation for submarines of the German Navy is currently being developed and will be based on the presented model tests.

Presenting Author: Wiebke Büsken Hamburg University of Technology

Presenting Author Biography: August 2020 - Present: Research Assistant at Hamburg University of Technology in seakeeping and stability of ships and submarines.

February - July 2020: Naval Architect Ship Theory and Ship Safety at shipyard MEYER WERFT GmbH & Co. KG

November 2019: M.Sc. Degree in Naval Architecture and Ocean Engineering, Hamburg University of Technology

Authors:

Wiebke Büsken Hamburg University of Technology
Stefan Krüger Hamburg University of Technology