Just picked up a Backwater 7x4 prop and when looking at it I noticed the threads are double lead threads. (FYI for those not familiar, double lead threads have two "starts" to the threads spaced 180 degrees from each other instead of the typical 1) The shaft thread is definitely a single lead 3/4" x10TPI thread which will still works fine with double lead threads, it just leaves one set of threads open. Double lead threads are not very common except in specialty equipment so I was surprised to see them on a longtail prop. Anybody know why they do this? I have been thinking about it for a day and have only a few reasons I can think of:

1. It makes the prop threads weaker so they will strip before damaging the shaft threads too severely? If I recall threading formulas right it will reduce thread strength 40%ish compare to a single lead thread. Assuming the shaft is stronger/harder this might work?
2. Create a space for debris to flow? Though where it would flow from or to I don't know.
3. Reduce the surface area of the thread engagement to reduce the force needed to unscrew the prop? Less chance for the greatest welder of all to work, Time.

I don't think it really matters much but it has been bugging me since I saw it.

-Matt

Have you figured out why we park in driveways, and drive on parkways??

That one still has me stumped...

My nose runs and my feet smell. guess i was built upside down

If the threads strip, the shaft is still garbage... I can't think of why they'd do it either and hadn't noticed it but maybe they could give you a better answer if you call or email. They're some pretty ingenious fellas up there.

My guess is that it's because the steeper helix of the thread will lessen the axial load, keeping the prop from locking on so tight that you can't unscrew it.

Just like how a coarse thread will apply less force when torqued to the same specs as a finer thread.

I finally remembered to call Backwater and they confirmed this is by design. Stated reason was to prevent the prop from sticking on the shaft.

Booyah!