January 25, 2015

Backward nuts to beware of

MOST BOATS with inboard engines have two nuts holding the propeller in place. And most propeller nuts are installed backwards.

I didn’t make this up. This is the word from Dave Gerr, a top U.S. boat designer. In his book, The Nature of Boats, Gerr says:

“There should always be two propeller nuts: a jam or lock nut and the main or load nut. Because it seems natural that the nut directly in contact with the prop is the one that ought to be doing most of the work, many boatyards install the full size nut here [against the prop hub]. This is the wrong way around!

“The correct procedure . . . is to install the small, half-height jam-nut first — against the hub. Tighten it up as hard as you comfortably can, by hand with a standard wrench. Then screw on the full-sized nut and tighten that down independently — again, as hard as you comfortably can, by hand with a standard wrench. Finally, fit the cotter pin, and you’re ready to go.”

Well, this obviously isn’t intuitive. It just doesn’t seem right. But here’s the expert’s explanation: As the second nut (the outside one) is tightened down, it rotates the smaller nut slightly, just a fraction of a turn. This relieves pressure on the nut against the hub, so that all the load is taken by the bigger outside nut.

“Since the top nut thus does the brunt of the work, it should be the nut with the most threads — the full-sized nut,” says Gerr.

Well, don’t just stand there. Get that boat out of the water and swop those nuts around immediately.

Today’s Thought

The bad workmen, who form the majority of the operatives in many branches of industry, are decidedly of opinion that bad workmen ought to receive the same wages as good.

— John Stuart Mill, On Liberty

 “Why so gloomy?”
“I got married three days ago.”
“So why is that making you gloomy?”
“Well, I gave all my life savings to my new husband.”
“And where is he now?”
“Dunno. I’m still waiting for him to come back from his honeymoon.”


Gregor (NZ) said...

John, if anyone is disinclined to believe Dave Gerr or wants an expanded explanation, have a look at this.

John Vigor said...

Thanks, Gregor, that's fascinating explanation of how the two-nut locking system works. There can't be any doubters after reading this.

John V.