|M.Sc Student||Neuberg Or|
|Subject||Limit State Design of a Fast Boat Hull|
|Department||Department of Mechanical Engineering||Supervisor||Professor Nitai Drimer|
|Full Thesis text|
Hulls of ships, as well as fast boats, are traditionally designed using the Allowable Stress Design (ASD) methodology. The idea of design codes is to keep the stresses below certain allowable limits.
Planing is the most common concept of fast boats. Planing at head seas produces extreme loads, while the hull slams the water. Typically, these slamming loads govern the structural design. Applying the allowable stress method yields relatively heavy and rigid hulls. The design codes for high speed crafts specify a procedure for the hull design, based on empirical formulas for the loads and beam theory for the strength calculations. These applied rules further simplify the design by specifying allowable stresses which provide a service life of typically 20 years, without the need to check for fatigue.
Alternatively, a rational Limit State Design (LSD) approach provides advantages of more realistic representation of loads and load effects and may significantly reduce the scantlings. This research develops a limit state rational design method for the hull of fast boats. As oppose to rules based design, such as RINA (2009), DNV (2014), LR (2014), our method pragmatically represents the extreme load effects, the importance of hydro-elasticity and non-linearity (membrane effect); However, in this approach, fatigue limit state must be considered.
In this research we apply numerical tools developed in previous thesis for ASD, and extend the utilization to LSD. We further verify our numerical analysis with experiments of dropping a wedge section into a pool. We extend the range of parametric analyses to include more conditions, required for LSD and Fatigue Limit State (FLS). We develop a new rational FLS method for planning hulls, which assesses the boat fatigue life, based on a plan of operation conditions (distribution of speeds sea states). This new method integrates design rules with direct analysis of fluid-structure interaction and statistical representation of random sea, to a practical design procedure.
The slamming loads, which govern the hull design, are applied to the stiffened panels of the boat bottom. To solve uncertainty of the effect of stiffeners welding on the fatigue strength of the material, we also carried out some laboratory fatigue experiments.
Two design examples demonstrate the potential of the new method to reduce the scantlings of planing hulls and to predict the service life. The first example shows the differences in service life between the presented method and designing by rules, for identical scantlings. The second example shows about 20% saving in the bottom plate thickness comparing to design by rules, for boats of a service life of 20 years. The new method offers tools for designing a boat for less than 20 years and by that, saving even more.