The vibrations of a flexible planing craft: hydroelasticity, boat motion and noise

Abstract

The Royal National Lifeboat Institution (RNLI) is the charity that aims to save lives at sea. The RNLI D-class is a five metre inflatable lifeboat that is used near the shore in waves and surf. Anecdotal evidence indicates that the D-class has improved performance due to its unique, flexible, fabric structure, and this flexibility is highly likely to affect the vibrations generated by the D-class. The boat motion is experienced by the on-board crew, and the air and water borne noise are heard by the on-board crew and the wildlife. This thesis aims to measure these two types of vibration, predict the perception of these vibrations and measure the effects of hydroelasticity on both the vibration and perception. Three aspects of hydroelasticity were identified within the D-class: hydroelastic slamming, hydroelastic planing surfaces and global hydroelasticity. This gives a new perspective with which to view the effects of hydroelasticity. A four stage full-scale holistic hydroelastic experiment was performed with each stage aiming to trigger one aspect at a time. The four stages were: static tests, flat water trials, drop tests and wave trials. The D-class was fitted with 52 sensors to measure the boat motion, engine thrust, sponson and keel pressures, deck hinge angles, deck panel deflections and the fabric hull deformation. The static trials measured the shape of the D-class under only buoyancy and weight forces. The flat water trials measured the effect of a hydroelastic planing surface on the forward speed and investigated a phenomenon termed the pulsing motion. The drop tests were performed at full-scale and quasi-2D, and they measured the effect of hydroelastic slamming on the peak acceleration and predicted the Whole Body Vibration (WBV). The open-water wave trials investigated the global hydroelasticity.

The static tests showed that the shape of the D-class was more dependent on the keel pressure than the sponson pressure. The flat water trials proved that a flexible planing surface decreases the forward speed by 0.44 knots. The pulsing motion surprisingly exhibited the highest forward speed and it is hypothesised that the structure achieved an unstable equilibrium position of minimal potential energy. The full-scale and quasi-2D drop tests demonstrated that hydroelasticity can affect the peak accelerations and WBV, but the trend was inverted when the drop height was varied from 0.5 m to 1 m. It is believed that the keel is the dominant component during the flat water trials and drop tests, and this is coupled with the fabric hull.

No statistical difference was found in the wave trials results but this was explained through the drop test results. The predicted WBV from the wave trials does emphasises the need for a new WBV reduction strategy and incorporating an element of hydroelasticity along with other reduction methods could make a significant impact on the WBV. The airborne noise of the D-class was measured using ISO 14509. The airborne noise was above the limits set out by the European directive 2003/44/EC. A method was developed to measure the water borne noise of small High Speed Craft (HSC) in shallow waters. The water borne noise propagation was modelled using an Image source Transmission Loss (ImTL) model. The perception of the air and water borne noise by a harbour seal was predicted and it showed that the D-class is unlikely to cause damage to the auditory system at one metre but will definitely be audible to the seal at 20 m. The horizontal and vertical transmission loss through a shallow water channel was investigated.

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ORCID for P.A. Wilson: orcid.org/0000-0002-6939-682X

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