The stern geometry of small ships is a difficult problem that is often accentuated by a wish to keep gross tonnage below 500 and to develop forms having minimum building cost, and model tests for such ships are being increasingly commissioned. After briefly discussing these matters and stressing the need for good inflow conditions for the propeller, the Author describes two typical examples of model tests which illustrate the importance of good stern design for small ships. The first example concerns swim sterns, which, because of preoccupation with designing for minimum ship-length for a given deadweight, frequently have such an angle that flow separation occurs. Modifying the model by limiting the cut-up angle of the swim stern to, say, 18 deg., usually in association with a modest transom immersion of about 15% of the draught, can decrease the ehp by as much as 25 to 30%, and the ship will have improved steering. The second example refers to a ferry with a round-bilge stern. It was already in service and suffering from vibration to the extent that plate repairs under the stern were an annual event, and noise levels in the accommodation were very high. Wind-tunnel tests on a model of the underwater hull showed (by means of tufts) that considerable separation of flow to the upper part of the propeller disc was occurring, and this was confirmed by a wake survey. A range of vortex generators was then evaluated with the model, and those subsequently fitted to the ship resulted in much better inflow to the propeller and were very effective in reducing vibration-induced noise and also improved the steering. It is mentioned that, though they necessitate model tests, vortex generators can, in addition to curing existing problems, possibly enable a compromise to be made between economic construction and acceptable hydrodynamics and tonnage limitation.

  • Availability:
  • Corporate Authors:

    Royal Institution of Naval Architects

    10 Upper Belgrave Street
    ,   United States 
  • Authors:
    • Flewitt, J R
  • Publication Date: 1979-11

Media Info

  • Features: References;
  • Pagination: p. 238
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00310892
  • Record Type: Publication
  • Source Agency: British Ship Research Association
  • Report/Paper Numbers: Technical Papers
  • Files: TRIS
  • Created Date: May 7 1980 12:00AM