Pre-Purchase Inspections
March 2009 - Prop Shaft

There's little margin for error when it comes to running gear, particularly where the interface between propellers and shafts are concerned. In most cases, there's simply one right way to properly establish this interface and virtually anything else falls outside of the guidelines established by the American Boat and Yacht Council's Standards as well as those set forth by SAE.

During an inspection of this 44-foot twin screw vessel I encountered the shaft and propeller pictured here. A casual inspection would indicate all is in order; however, this is not the case. The red dye is used as part of a process to ensure the propeller fully engages the shaft taper. This indicates the installer knew what he or she was doing. The "spooned" and radiused keyway, the recess into which the key fits, is also desirable for its load distributing ability.

The degree to which the propeller engages the tapered portion of the shaft, however, is less than ideal. If you look closely you'll notice that the diameter of the shaft continues to increase forward of the propeller, noticeable just above the red dyed area. This means that the area where the highest stress and torque load is encountered, just forward of the propeller, is at the smallest exposed shaft diameter, fully a quarter of an inch smaller than the shaft's maximum diameter. Propeller installations that suffer from this assembly error are more likely to experience catastrophic shaft failure in this highly loaded area. The propeller should fully engage the shaft taper; the leading edge of the propeller hub should end where the shaft attains its maximum diameter. The dimensions for shaft taper and matching propeller bore are clearly spelled out in ABYC and guidelines.

Finally, the overhang between the cutless bearing or strut and the leading end of the propeller hub should be no more than one shaft diameter. This example is nearly double that distance.

After being alerted to this and other problems, defects and issues, the SDMC client was able to negotiate the cost of correcting them into the purchase price of the vessel.

If you’re interested in finding out more about pre-purchase inspections, vessel selection assist, the Captains’ Club (simply click on these words to be taken to the relevant sections of this website) or other services offered by Steve D’Antonio Marine Consulting, send an email or call 804-776-0219.