November 13 2014

Technical Meeting Report on: Prof. George Tzabiras, Professor of Ship Hydrodynamics and Director of the Laboratory for Ship and Marine Hydrodynamics (LSMH), at the School of Naval Architecture & Marine Engineering, National Technical University of Athens (NTUA) Greece, speech on the subject of:

“Experimental and Numerical Studies on Trim Effects’’, presented on 13 November 2014

 By Agapi Gerogiannaki

This presentation was the third one for the current season and covered the trim influence on the hydrodynamic efficiency of ships. It is composed of two parts.
The first part included experimental studies of six models representing various ship types. The experiments were carried out in the towing tank of the National Technical University of Athens through diploma Thesis of undergraduate students of the Department of NAME. Extrapolations at full scale were based on the standard Froude method. The first three cases, i.e.  a bulk-carrier in two loading conditions, , a single-screw passenger at full load and a twin-screw ferry at a medium draft, have a bulbous bow and a transom stern as common geometrical features. The hydrodynamic influence of the bulb and transom immersion depends on the Froude number and seems proved to be crucial when comparing various trim conditions with that of even keel. The speed range where these effects dominate the flow-field presented also to depend on the hull form. The next three models referred to special types of vessels: a semi-swath at high Froude numbers, a BOC-type sailing yacht and a traditional fishing vessel. The last one exhibited particular interest since the low length to beam ratio (3:1) led to an extended separation zone at the stern region considered responsible for the increase of resistance at the trimmed-by stern condition up to the Froude no. of 0.35. The model has also been tested in two speeds by performing self-propulsion experiments and the interesting point is that the SHP follows opposite trends than the resistance at the higher Fr. no. of 0.35.  CFD computations show the development of the separation zone at model and full scale Reynolds numbers as well as in the case of self-propulsion.
The second part of the presentation dealt with CFD calculations performed for an existing product-carrier (20000t DWT) and an effort was made to compare the required engine characteristics at various trimmed and loading conditions. The sea-trials were conducted at a lower displacement than the designed and the computed SHP and propeller RPM were compared with two cases (trimmed and zero trim) at full load for the design speed of 14 Knots. An interesting conclusion was that the influence of trim on EHP showed again different trends than SHP, which was more intense in the heavy-ballast condition. In addition, the low-steaming case of 10 Knots (full-load, trimmed) has been calculated in order to assess the SHP reduction, which was found to be almost 30% of the required at 14 Knots. In this case also, in order to have an estimate about the speed reduction in “rough” seas, the propeller was charged to overcome a hypothetical resistance increase of about 30%. All computations were carried out by CFD tools that have been developed in the Department of NAME. The propeller operation was simulated by the actuator disk model.
The performed studies have shown that the hydrodynamic effect of trim depends on the ship type, the speed range and the loading condition. Besides, all relevant investigations, either experimental or numerical should be based on self-propulsion tests.
This Technical Meeting was attended by twelve non-member and forty six SNAME members and after the presentation a small reception took place with a great success.

From the left: A. Gerogiannaki (Secretary/Treasurer), Prof. G. Tzabiras (Author), P. Lalangas (Membership Chairman)