What is the best way to measure for a new propeller shaft?

A common problem we run into when quoting or making a customer a new prop shaft is inconsistent measuring. That isn’t to say that the customer is measuring wrong, but there may be some miscommunication in what or how they are measuring and/or the terms they use may be different than the ones we use.

The most accurate measurement and easiest for us is from the COUPLER FACE to the SMALL END OF TAPER (CF to SET). The way to get this is to put the coupler on the shaft (like it would be when it is installed in the boat) and measure from the coupler face (where the shaft coupler mates to the transmission output flange) to the small end of the taper (the smallest part of the taper right before the threads). After we have that, we simply add standard thread length* and it gives us what’s known as an “assembled length”. See the measuring diagram below.

*With this measurement, there are a couple of caveats:

• 1.This is assuming the current shaft has a standard 1:16 SAE taper.
  • 1a. If the current prop shaft is metric, or is not using a standard 1:16 SAE taper, there are a lot more measurements involved to determine the taper rate and length.
• 2.When we add the threads, we are using standard SAE thread length. If threads on the old shaft have been trimmed to fit certain brands of folding/feathering props, or they have been trimmed due to a rudder or other obstruction that is too close to the end of the shaft, that is something we will need to know so we can trim them to the proper length.

Please click on this link to see SAE taper, thread length, and other info.

We can make shafts using other measurements (overall length, output flange to forward/aft end of strut, etc.), but CF to SET creates the least amount of miscommunication.

If you need a new shaft or have any questions, please give us a call or send an email.