Friday, June 12, 2020

Stern Frame

A stern frame may be cast or fabricated and its shape is influenced by the type of rudder being used and the profile of the stern. Sternframes also differ between twin and single screw ships, the single screw sternframe having a boss for the propeller shaft. Adequate clearance is essential between propeller blade tips and sternframe in order to minimise the risk of vibration. As blades rotate water immediately ahead of the blades is compressed and at the blade tips this compression can be transmitted to the hull in the form of a series of pulses which set up vibration. Adequate clearance is necessary or alternatively constant clearance, this being provided with ducted propellers such as the Kort nozzle. A rotating propeller exerts a varying force on the sternframe boss and this can result in the transmission of vibration. Rigid construction is necessary to avoid this. The stern post, of substantial section, is carried up inside the hull and opened into a palm end which connects to a floor plate, This stern post is often referred to as the vibration post as its aim is to impart rigidity and so minimise the risk of vibration. Side plating is generally provided with a Rabbet or recess in order that the plating may be fitted flush. The after most keel plate which connects with this region the structure od the ship serves no useful purpose and it is known as the 'deadwood'. This may be removed without ill effect on stability or performance and some sternframes are designed such that the deadwood is not present.

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