This paper presents a method for predicting the magnitude and distribution of the slamming loads and pressures acting along the bottom of a catamaran cross structure. Comparisons with experimental model and full-scale data show good agreement. It is shown that, for a given cross structure clearance above the mean water level, there are specific ship speeds and wave conditions which will give rise to cross-structure impacts or slams. The method should provide an extremely useful tool for the design of catamaran cross structure plating.
The problem of slamming of the cross structure of a catamaran or twin-hull ship is a serious concern for the designer. The phenomenon results when the cross-structure hits the surface of the water at small or moderate impact angles giving rise to dynamic pressures and loads which can reach extremely high levels. These pressures and loads must be known since they can induce severe local damage to the cross-structure bottom plating. They can also have significant effects on the overall hull in the form of increased bending moments and shears and induced vibratory motions. The latter willusually make the master of the ship intuitively reduce the forward speed and/or change heading. Whatever the effects of slamming may be, it is extremely important that the naval architect have a design tool which will allow him to predict the magnitude and distribution of these loads.
In addition to the magnitude and distribution of the pressures and loads associated with a single impact, it is also important to know what to expect over a long period of ship operation time as far as structural and human tolerances to slamming are concerned. Thus, while the approach to the single impact situation is deterministic in nature, the long range prediction of the slamming loads must be statistical. It must be realized that one should not only be interested in the structural survivability of the ship when exposed to a large number of severe slams taking place within a small period of time. Certainly, it is just as important to be concerned with the comfort and well-being of the crew, and attempts should be made to prevent such a situation from taking place by changing speed or heading or both. Thus, both structural and human elements should be considered simultaneously when establishing the limits for operating under slamming conditions. For example, if the tolerable speed for a slam impact is below the speed free from slam damage, then the ship will be completely safe as far as slamming is concerned. If the situation is reversed, then there exists the possibility that the ship will be operated at speeds where damage may take place. In that case consideration must be given to the cross-structure bottom plating thickness to the extent that speed free from slam damage will be above the tolerable speed.