This months tip is provided courtesy of Dave Gerr, Gerr Marine, Inc., a professional Marine Surveyor located in New York. Thank you Dave for your Tip-Of-The-Month.
Proper Prop Nuts
I'll start with a minor and somewhat amusing controversy. There are two nuts
holding the propeller on the shaft: a full-height nut and half-height
locknut. Which nut goes directly against the propeller hub?
Actually, most boats have it wrong, with the full-height nut against the
prop. It seems intuitive that the larger nut against the prop would be doing
most of the work and that the smaller nut should go on second. In fact, the
smaller nut should always go against the load. This is because when the
second, outer nut is tightened down, it compresses and deforms the lower nut
a tiny bit, and rotates the lower nut a fraction of a turn. This effectively
unloads the threads of the lower nut and engages the threads of the upper
nut. Thus the top or outer nut actually takes all the load.
No matter how many times I explain this, some folks still seem dubious. You
can also see an illustration from Engineering Drawing and Design (by
Jensen/Helsel, McGraw Hill, 1985) that shows how this is standard practice
for all lock nuts; and an illustration from SAE standard J756 and J755, for
propeller shafts, which clearly shows the half-height nut against the hub.
If you go to any marina with the boats hauled in winter, my guess is that
99% of the prop nuts will be on the wrong way. I wouldn't worry too much
about the nuts being on backwards. It has proven sufficient, but you might
as well put them on correctly next time you install a prop.
Hope this helps. Call with any questions.
Gerr Marine, Inc.
838 West End Ave. - Suite BB
New York, NY 10025 USA