By William S. Vorus

This ebook is the defining reference paintings and textual content for naval structure, and offers the rules underlying research of the vibration features of contemporary seagoing ships and the applying of these ideas in layout and challenge fixing. The classical non-stop beam version with regular country reaction to periodic excitation is gifted first. This contains traditional frequencies, mode shapes and modal growth. Discrete research is subsequent provided dependent upon finite point ideas. Examples are mentioned related to research of the total send and part components.

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Approximate formulas for evaluating propeller-induced vibratory forces are proposed in Section 3. 4 Underwater Radiated Noise. ” This is especially true with regard to noise radiated subsurface through the liquid medium. Underwater radiated noise is particularly important in vessels involved in military operations, where easy detection from far distances can be fatal. 1 Cavitation Dynamics as a Noise Source. The principal source of ship underwater noise is propeller cavitation. This occurs indirectly as structure-born noise from vessel vibration produced by propeller cavitation.

This is, again, because the size of the important pressure components is relatively greater than in the noncavitating case. However, coverage of a large area of the model surface with pressure transducers should be required in view of the very slow attenuation of the cavitation induced pressure signal. In this regard, whether forces or pressures are the interest, it is no doubt most important that boundary conditions be modeled accurately, either in analysis or experiments. Theory indicates, for example, that due to the slow spatial pressure attenuation associated with the cavitation volume source strength, surface pressure, even in the immediate propeller vicinity, can be overestimated by a factor on the order of four in typical cases if the rigid wall boundary condition is employed at the water surface.

Collapse occurs on leaving the high-wake region in a violent and unstable fashion, with the final remnants of the sheet typically trailed out behind in the blade tip vortex. The sheet may envelope almost the entire back of the outboard blade sections at its maximum extent. For large ship propellers, sheet average thicknesses are on the order of 10 cm, with maxima on the order of 25 cm occurring near the blade tip just before collapse. The type of cavitation shown in Fig. 14, while of catastrophic appearance, is usually not deleterious from the standpoint of ship propulsive performance.

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