Bossing

Bossing is the faired, swelling housing of shell plating that carries the propeller shaft out from the hull on a twin-screw or multi-screw vessel where the shaft exits the main hull well inboard of the propeller. It encloses the shaft and its bearings and blends into the afterbody so the shaft line reaches the propeller with the least added drag and the cleanest inflow. The alternative arrangement is an exposed shaft on open struts (A-brackets), which gives lower wetted surface but less support; bossings suit larger or naval hulls where the shaft is long and the structural housing is preferred. Bossing shape is faired in a flow study because a poorly faired bossing separates flow and disturbs the propeller wake.