prop shaft nut question

[cm2ncfsu2]

Okay, strange question here. I'm looking at a boat built in 1930. On the aft end of the prop shaft is what appears to be a big square nut holding the prop on. However, the aft end of the "nut" is closed over solid (sort of like a flat version of a modern acorn nut, albeit square). Anybody seen something similar? Could it be in fact a nut? Oh, last clue, on the very aft face, there is a hole in the center maybe an inch diameter. Nut's diameter (for lack of a better word, width) is about 3 inches. Thanks for any clues.

Of course it would be quite easy to figure out except of one thing. Its sitting 125 feet under water :eek:

[The Bigfella]

We need photos

[BETTY-B]

You'll have plenty of time to figure out the nuts during the restoration.

DAN

[Lew Barrett]

Back in the day (man how I hate that little euphemism) they used to include a zinc on the prop's nut. Frequently the zinc was in the shape of a cap to match the shaft and nut, and fixed through the center with a matching machined fastener.

In any case, all such arrangements have long since gone the route of high button shoes and buggy whips, and for exactly the sort of reason I surmise yours is now sleeping with the fishes. I'm not sure precisely what arrangement you're describing, but it sounds suspiciously familiar.

These days we recommend replacement with the following: castleated nuts with the shaft drilled to take a locking pin. That's what the rig now resting on the bottom should be replaced with. Use collar zincs properly placed on the shaft.

Edited for typos!

[The Bigfella]

Use collar zincs properly placed on the shaft


I've heard conflicting stories about doing that.

[Lew Barrett]

I've heard conflicting stories about doing that.

It only conflicts if you don't put them in the right places. It makes a difference where you land 'em, hence the "properly placed."

[The Bigfella]

.. and?

[Lew Barrett]

So you have to put the right amount on (which you knew) but if, for example, you need more than one, don't line both of them up on the same section of shaft. Put one in between the main strut and the first
support, and one in between that and the shaft log. If you only have one external strut, put your zinc in the center of the run, or if you have a long run and need two, start by dividing the shaft into thirds and putting the first zinc at the first division, the next at the second division and so on. Then keep an eye out for what's going on and adjust amount and/or placement
for best results. I learned this arcane bit at my last survey from the surveyor and it made perfect sense to me then.

Look for wear or thinning of the prop, and move the zinc closer if you're getting that, and check the shaft supports and struts for activity and adjust accordingly. Use the least number of zincs you can to get what you need. Best to have a silver half cell and look around the boat. Check for stray currents in the marina and your general local conditions, then measure the boats potential and add or subtract zincs as required. I'd personally call an expert to help in checking as I'm a dolt so I keep forgetting the formulas and procedures.

[pila]

You can always get a good look at that nut as the boat breaks the surface on it's way up.

[cm2ncfsu2]

Well, the 'boat' won't be breaking the surface anytime soon. It burnt to the waterline in 1931.

[pila]

Maybe you have a diver friend who could take some pix of the prop area down there.