Discussion

International Journal of Small Craft Technology - Discussion

Comment is invited on the papers listed below papers have been accepted for publication in forthcoming issues of the International Journal of Small Craft Technology (RINA Transactions Part B).

Comments, together with the author(s) response, will be published in the Discussion section of subsequent issues of the IJSCT. Comment may also be forwarded on papers published in the current issue of the IJSCT. Comments should not normally exceed 500 words and may contain two illustrations.

Comments may be forwarded online using the Online (Written Comment) form, or by email to ijsct@rina.org.uk with the title of the paper as the Subject, and quoting the name and affiliation which will be printed with the comment. Illustrations may be forwarded as an attachment.

IJSCT discussion papers are published in PDF format and can be downloaded free. Just click on the paper title to download. These PDF files can be saved and viewed on screen but cannot be edited or printed out.

2009 IJSCT Part B2

IJSCT 90: THE ENERGY CYCLE AS A FACTOR IN PERFORMANCE

C Mudie, Naval Architect & Yacht Designer, UK

SUMMARY

This paper outlines the author's views, backed by research and experience, on the effect of energy transmission through hulls as a largely unappreciated factor in their performance.

IJSCT 92: FIFTY YEARS OF THE GAWN-BURRILL KCA PROPELLER SERIES

D Radojcic, A Simic and M Kalajdžic, University of Belgrade, Serbia

SUMMARY

The first extensive systematic tests of flat-faced segmental-section propellers were those performed by Gawn in 1953 (open-water tests) and Gawn and Burrill in 1957 (cavitating environment). Since then several attempts to develop mathematical representations of propeller hydrodynamic characteristics (thrust coefficient KT and torque coefficient KQ) have been made in order to improve computer capabilities in predicting propeller performance. The first mathematical model was that of Blount and Hubble (1981), which was soon followed by Kozhukharov's (1986) and then Radojcic's (1988). These models were developed through application of multiple regression analysis. Koushan (2007) challenged more than 20 years of the regression approach, for representing the highly non-linear Gawn-Burrill KCA propeller characteristics, and suggested application of the artificial neural network technique. This paper compares the four mathematical models mentioned above.

IJSCT 95: RESISTANCE CHARACTERISTICS OF SEMI_DISPLACEMENT MEGA YACHT HULL FORMS

D L Blount and J A McGrath, Donald L. Blount and Associates, Inc., USA

SUMMARY

Mega yachts are growing in scale. The combination of cruise and maximum speeds, along with increase in hull lengths result in operational Froude numbers, FnL, between 0.3 and 1.0. The concentration of current mega yacht projects have FnL between 0.3 and 0.6 with few approaching FnL = 1.0. Hull forms with different transverse sections show a variation in resistance characteristics for similar slenderness ratios in this range of Froude numbers. Resistance values among other geometric considerations are sensitive to slenderness ratio and longitudinal center of gravity, LCG. On occasion, LCG shifts, stern wedges and bulbous bows are techniques being employed to achieve minimum resistance for traditional displacement hull forms. However, these techniques need to be approached with caution as transverse instabilities can result at speeds greater than 22 to 25 knots. Hull form variants incorporating flow-separating spray rails become significant at high speeds. 500 mt was selected as being representative of a typical displacement for mega yachts. Model test data were scaled to make comparisons and analyses of bare hull resistance of many available experimental hull series. Thus, this paper addresses resistance variations related to different hull forms, slenderness ratio and other hull characteristics along with suggested design criteria for forecasting the threshold of dynamic transverse stability.

IJSCT 98: FORCE AND PRESSURE INVESTIGATION OF MODERN ASYMMETRIC SPINNAKERS

I M Viola and R G J Flay, Yacht Research Unit, The University of Auckland, New Zealand

SUMMARY

An innovative pressure system was used at the Yacht Research Unit's Twisted Flow Wind Tunnel (University of Auckland) to test three asymmetrical spinnakers. The sails were designed for the most recent America's Cup Rule (AC33) and tested on a large-scale model. Force measurements were used to determine the sail characteristics, optimum apparent wind angles and resulting heel angles. Pressures were firstly measured on 5 chord-wise sections with 11 pressure taps on each section, which enabled mapping of the pressure on the sail surface. Measurements were performed between apparent wind angles of 40° and 70° and heel angles between 0° and 20°. The pressure measurements are discussed and related to the flow field around the sails. In particular the pressure on the leeward side of the asymmetric spinnaker is correlated to the leading edge separation and reattachment, and to the trailing edge separation. Subsequently 34 pressure taps were used to measure the pressure on a single section of the asymmetric spinnaker. This allowed an investigation of the effect of the sail trim on the resulting pressure distribution. The results verified that the three sails were suited for their intended design purpose. Over-trimming to reduce luff flapping was also investigated. It was found to reduce both drag and rolling moment. Further successive over-trimming showed a reduction in the leeward suction on the spinnaker, with the pressure distribution becoming more uniform as the flow became more separated.

2009 IJSCT Part B1

IJSCT 77: UNITED KINGDOM REGULATION OF YACHTS

D W Ralph, Maritime and Coastguard Agency, UK

SUMMARY

The UK leads the world in the regulation of large yachts. British yachts form the majority of the world fleet, bearing in mind that this includes yachts of the wider Red Ensign Group. Historically, yachts have been built for private use but are increasingly being used, remotely from the UK, in the charter market. Vessels operating "commercially" must comply with appropriate Flag State legislation and, if of sufficient size, must also comply with and be certificated under the provisions of appropriate International Conventions.

IJSCT 86: EFFECT OF WATER DEPTH ON MOTIONS IN BEAM SEAS FOR A TYPICAL LANDING CRAFT

A Rolls, M Renilson and G Macfarlane, Australian Maritime College, Australia
S Cannon, Defence Science and Technology Organisation, Australia

SUMMARY

A set of model experiments have been conducted at the Australian Maritime College to investigate the effect of water depth on the motions of a typical landing craft in beam seas. Tests were conducted at different wave heights in two water depths: deep, corresponding to a depth to draught ratio of 9.0; and shallow, corresponding to a depth to draught ratio of 1.5. The influence of non-linearity on model motion, particularly in roll, was determined, as was the effect of water depth. It was shown that the motions in shallow water were significantly different to those in deep water.

IJSCT 87: AN INVESTIGATION INTO A METHOD FOR DETERMINING THE STABILITY OF SMALL CRAFT WITHOUT REFERENCE TO HYDROSTATIC DATA

A J Murphy and R W Birmingham, Newcastle University, UK

SUMMARY

The assessment of the stability of small craft can be difficult in the absence of hydrostatic data when it is not possible to use the normal inclining experiment. This paper provides a thorough numerical and experimental investigation in to a stability assessment method, previously proposed as a possible alternative to the inclining experiment (Birmingham, 1995; 2000). The method uses the relationship between changes in the natural roll period of vessels given changes in roll mass moment of inertia. The numerical investigation indicates that the method will be sensitive to small experimental errors although not necessarily impossible. However, the conclusions of the experimental implementation of the proposed method were that it does not ultimately provide a sufficiently accurate stability assessment as to be practically viable. Nevertheless, the work provides a comprehensive follow-up to the earlier published proposition and provides further insight in the dynamics of a rolling vessel under experimental conditions with fastidious attention to experimental accuracy.

IJSCT 88: NUMERICAL INVESTIGATION INTO THE EFFECT OF LAUNCH SKID ANGLE ON THE BEHAVIOUR OF FREE-FALL LIFEBOAT IN REGULAR WAVES

M M Karim, K S Iqbal and M R H Khondoker, Bangladesh University of Engineering and Technology, Bangladesh
S M H Rahman, American Bureau of Shipping, USA

SUMMARY

This paper presents numerical investigation into the effect of launch skid angle on the behaviour of the skid-launching free-fall lifeboat (FFLB) in regular waves. The boat has been treated as a rigid body when the differential equations of motion for the four falling phases, i.e., sliding phase, rotation phase, free-fall phase and water entry phase of the lifeboat were solved in the time domain. The behaviour of the lifeboat has been studied for different angles of launch skid, i.e., Θ= 30°, 40° and 60°. The initial falling height of the boat is fixed to 1.75m. The regular wave with amplitude of 0.5m and period of 2.0 sec. is considered in this study. From the mathematical simulation, horizontal and vertical excursions and the rotation of the axis of the boat have been obtained at different time along with its horizontal and vertical velocities. Accelerations and forces at normal and axial directions have also been computed. In each case, the behaviour of FFLB in wave is compared with that in still water.