Editing Propeller (section)

===Shaft protection===
[[File:Propeller rubber bush failed.jpg|A failed rubber bushing in an [[Outboard motor|outboard]]'s propeller|right|thumb]]
For smaller engines, such as outboards, where the propeller is exposed to the risk of collision with heavy objects, the propeller often includes a device that is designed to fail when overloaded; the device or the whole propeller is sacrificed so that the more expensive transmission and engine are not damaged.

Typically in smaller (less than {{convert|10|hp|kW|abbr=on|disp=or}}) and older engines, a narrow [[shear pin]] through the drive shaft and propeller hub transmits the power of the engine at normal loads. The pin is designed to [[Shear stress|shear]] when the propeller is put under a load that could damage the engine. After the pin is sheared the engine is unable to provide propulsive power to the boat until a new shear pin is fitted.<ref>{{Citation | last= Getchell | first= David | title= The Outboard Boater's Handbook | url= https://books.google.com/books?id=YpMTd7-Mb3sC | isbn= 978-0-07023053-8 | year= 1994 | publisher= McGraw Hill Professional }}</ref>

In larger and more modern engines, a rubber [[bushing (isolator)|bushing]] transmits the [[torque]] of the drive shaft to the propeller's hub. Under a damaging load the [[friction]] of the bushing in the hub is overcome and the rotating propeller slips on the shaft, preventing overloading of the engine's components.<ref>{{Citation | title= Admiralty Manual of Seamanship | url= https://books.google.com/books?id=jUdZlpHWShkC| isbn= 978-0-11772696-3 | publisher = Ministry of Defence (Navy)| place = Great Britain| year= 1995}}</ref> After such an event the rubber bushing may be damaged. If so, it may continue to transmit reduced power at low revolutions, but may provide no power, due to reduced friction, at high revolutions. Also, the rubber bushing may perish over time leading to its failure under loads below its designed failure load.

Whether a rubber bushing can be replaced or repaired depends upon the propeller; some cannot. Some can, but need special equipment to insert the oversized bushing for an [[interference fit]]. Others can be replaced easily. The "special equipment" usually consists of a funnel, a press and rubber lubricant (soap). If one does not have access to a lathe, an improvised funnel can be made from steel tube and car body filler; as the filler is only subject to compressive forces it is able to do a good job. Often, the bushing can be drawn into place with nothing more complex than a couple of nuts, washers and a threaded rod. A more serious problem with this type of propeller is a "frozen-on" spline bushing, which makes propeller removal impossible. In such cases the propeller must be heated in order to deliberately destroy the rubber insert. Once the propeller is removed, the splined tube can be cut away with a grinder and a new spline bushing is then required. To prevent a recurrence of the problem, the splines can be coated with anti-seize anti-corrosion compound.

In some modern propellers, a hard polymer insert called a ''drive sleeve'' replaces the rubber bushing. The [[rotating spline|splined]] or other non-circular cross section of the sleeve inserted between the shaft and propeller hub transmits the engine torque to the propeller, rather than friction. The polymer is weaker than the components of the propeller and engine so it fails before they do when the propeller is overloaded.<ref>{{cite patent | inventor1-last = Karls | inventor1-first = Michael | inventor2-last = Lindgren | inventor2-first = Daniel | pubdate = 1994-03-08 | issue-date = January 16, 1996 | title = Torsionally twisting propeller drive sleeve and adapter | country = US | number = 5484264 }}</ref> This fails completely under excessive load, but can easily be replaced.