טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentHakmon Roey
SubjectVerifying a New Hydro-Elastic Design Method for Planning
Boats by Full Scale Sea Trails
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Nitai Drimer
Full Thesis textFull thesis text - English Version


Abstract

Slamming pressure is the most dominant load for the design of a planing hull sailing at head seas. The hull slams the water, undergoing loads with values that can reach hundreds of kilo-Pascal and accelerations of tens of g's. Rules for designing planing boats formulate an empirical procedure to calculate the design pressure, applied statically on a simplified structure of beams, and dictate allowable stresses. This method usually leads to rigid, heavy hulls. Design using these rules ensures a relatively long life cycle for the boat, without having to perform fatigue analysis. A rational design, however, allows a more flexible, lighter structure by increasing the length between the stiffeners and decreasing the thickness of the plates. If the allowable stresses according to the rules are satisfied, 20% of the bottom hull weight can be reduced. Performing limit state design and exceeding the allowable stresses requires performing fatigue analysis, and still proving an adequate operating life duration, with accordance to the design requirements, reducing even more of the hull weight.

This research continues previous researches in the research group, in which a new method was proposed for the rational design of fast boats while taking into consideration hydro-elasticity, non-linearity and the dynamics of the structure. A parametric model was developed for ABAQUS simulations, and sea trials were performed on a full-scale research boat, designed by the method developed in the research in order to verify the rational design procedure. This boat has one side designed by rules and one side designed by our rational design method. Comparison of stresses between design by rules, our rational design, and trial measurements shows that for the heavy and rigid side (rules design), rules, rational, and trials show similar stresses, so both rules and rational are applicable for design; however, for the light and flexible side (rational design), rules dramatically over assess the stresses, while rational and trials are similar.  We therefore expect that this study will advance the design and production of more efficient boats of higher performances.

Additional simulations were performed to cover the range of sea states in the sea trials, analysis methods were developed, and thorough, critical analysis of the measurements from all sea trials and simulation results was done, as well as comparative analysis for the verification of the rational design method in the allowable stress approach.

In a previous study, a new rational Fatigue Limit State method for planing hulls was developed, for the assessment of the expected life duration, integrating design rules with direct analysis of fluid-structure interaction and statistical representation of random sea, to a practical design procedure.

This study also examines the developed method for Fatigue Limit State design, in comparison to the sea trials, suggesting a modification to better represent the stress distribution for the assessment of fatigue service life. The modification results in a lower stress distribution, with significantly larger agreement with the measurements from the trials, while still being more conservative (higher stresses), which is fitting for fatigue design that includes high uncertainties.