Science of Silence


Prop Shaft Vibration

Vibration of the prop shaft stems from two directions, firstly from the propeller and secondly from the engine. Traditionally, marine engines have been coupled directly to the propeller drive shaft, via a stiff flexible coupling between the two. Therefore most of the vibration from the propeller is sent straight down the shaft, and the engine mounts can’t do their job properly because they are permanently under strain from the forward thrust of the propeller. So, vibration gets into the hull, which causes noise.

An 'Aquadrive' puts free movement between engine and shaft. It also acts as a thrust bearing, transferring the propeller thrust straight onto the hull, and allows free movement of up to 25mm between engine and shaft – up and down, sideways, or fore and aft. No vibration goes down the shaft, and the engine mounts are no longer strained forwards by the propeller’s thrust. An Aquadrive allows so much movement that you can even fit special softer mounts on engines of four cylinders or more.

Modern hulls are made from GRP or steel. Both materials carry noise badly, so structure borne noise is a particular problem. The chart shows just how much noise can be cancelled by fitting Aquadrive.