International Shipbuilding Progress - Volume 38, issue 413

Authors: Pylkkänen, J.V.

Article Type: Research Article

Abstract: The linear inverse method for ducted propellers or annular airfoils is applied to the design of the two ducts of a two-element ducted propeller. The calculated inviscid duct thrusts are corrected for viscous drag. In a parametric study, the relative lengths of the two elements were varied within a fixed total length of the installation. Also the location of the propeller in the aft duct was varied. The objective was to maximize the sum of the thrusts of the elements. As is the case with multi-element airfoils, the sum of the thrust increases compared with the single-element reference duct, provided …suitable pressure distributions for both the forward and aft ducts are prescribed. The theoretically predicted increase in thrust is of about the same magnitude as the increase in the lift of two-element airfoils in comparison with the corresponding single-element airfoil. Hydrodynamically efficient short forward ducts are thin and have a sharp leading edge, which is likely to cause flow separation in the uneven wake of a ship. The minimum acceptable ratio of the forward length to the aft length may be about 1 : 3. For a forward duct of this length, a pressure distribution with a rather long roof-top is found to be advantageous, producing a well rounded leading edge. Show more

DOI: 10.3233/ISP-1991-3841301

Citation: International Shipbuilding Progress, vol. 38, no. 413, pp. 5-23, 1991

Authors: Gearhart, W.S. | Marboe, R.C.

Article Type: Research Article

Abstract: A historical review of reaction fin development and the application of an asymmetric reaction vane system forward of a propeller on a 41 ft U.S. Coast Guard boat is presented. An analytical evaluation of the efficiency gains that can be obtained with a reaction fin system is presented and compared with experimental measurements. The use of such a system for outboard motor propulsion is proposed.

DOI: 10.3233/ISP-1991-3841302

Citation: International Shipbuilding Progress, vol. 38, no. 413, pp. 25-50, 1991

Authors: Bass, D.W. | Haddara, M.R.

Article Type: Research Article

Abstract: The determination of roll damping for non-standard hull shapes continues to be an important but difficult problem. For small fishing vessels with deep skegs and hard chines the problem is particularly difficult. In order to develop and judge suitable analytical, semi-empirical, and numerical techniques for the prediction of roll damping, the authors have embarked on an extensive series of experiments to determine roll damping characteristics for a number of small fishing vessels. Amongst the attributes investigated are the dependence of damping on amplitude and roll centre. The investigations include not only the roll damping but also the roll-sway coupled damping.

DOI: 10.3233/ISP-1991-3841303

Citation: International Shipbuilding Progress, vol. 38, no. 413, pp. 51-71, 1991

Authors: Fang, Ming-Chung

Article Type: Research Article

Abstract: The nonlinear forces, including the ‘mean’ drifting forces and added resistance, exerted on a ship advancing in regular waves are formulated in the paper. Neglecting the second-order potential, the first-order one is used to calculate the second-order forces which are expressed as products of the radiation potential, diffraction potential, the incident wave potential, and the ship motion response. Based on the well-developed strip theory, the first-order potentials of each ship section are derived by source distribution method and the related first-order ship motion responses are also obtained. Several wave headings and ship speeds are considered. Comparisons are made with other …theoretical results and some experimental data and good agreements are generally obtained. It is also found that the second-order forces obtained based on the present strip theory are generally better than those obtained by other methods. Show more

DOI: 10.3233/ISP-1991-3841304

Citation: International Shipbuilding Progress, vol. 38, no. 413, pp. 73-93, 1991