Calculations for predicting propeller-induced ship vibrations at the design stage involve the determination of four essential factors: rigidity, mass, damping and excitation, which the paper reviews separately. It is concluded that, although complex structural models yield a good approximation of the rigidity as well as the structural mass of the ship, there is on the other hand a lack of fundamental knowledge of added mass, damping and excitation forces, and that this deficiency is being filled by simplified methods which are often questionable. However, where it is possible to determine the added masses with reasonable approximation, a mass-rigidity model of the structure will permit the reliable determination of the ship's natural frequencies and modes. Also, difficulties inherent to any full-scale measurement are pointed out, and serve to partly explain poor correlations between calculations and measurements as have been recently reported in the literature. It is concluded that prediction of the actual vibration behaviour of a ship under service conditions is not possible at present. However, the response of the structure to a single vertical force of given intensity applied at the proper location, for arbitrarily-chosen viscous damping, can simulate, within the approximations introduced, the results of a full-scale exciter test, and may allow potentially dangerous frequencies to be recognised so as to permit a reliable choice of the number of propeller blades. The actual case of a 122,000 cubic meters LNG carrier is studied in detail, and serves to highlight some of the problems encountered at the computational as well as the experimental level.

  • Supplemental Notes:
    • Presented at the Ship Vibration Symposium sponsored by The Ship Structure Committee and The Society of Naval Architects and Marine Engineers, Arlington, Virginia, October 16-17, 1978.
  • Corporate Authors:

    Society of Naval Architects and Marine Engineers

    601 Pavonia Avenue
    Jersey City, NJ  United States  07306-2907
  • Authors:
    • Armand, J L
    • Orsero, P
    • Robert, O
  • Publication Date: 1978

Media Info

  • Features: References;
  • Pagination: 16 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00183167
  • Record Type: Publication
  • Source Agency: Society of Naval Architects and Marine Engineers
  • Report/Paper Numbers: Proceeding
  • Files: TRIS
  • Created Date: Oct 31 1978 12:00AM