Nut's Got Bigger

mister p · 30 August 2013, 20:21

Chaps,Two weeks of daily buzzing off Cornwall. Pulled the SR onto the trailer. Flushed and ran dry.
Noticed play on prop, along the shaft, of about 1 cm.
the castellated brass nut is still there held by the split pin.
But the prop moves along the shaft by about 1 cm.
Initial thoughts were that a washer had disinertgrated.
While starting to remove split pin realised the nut was moving along the shaft too. Thought it had cross threaded but threads appeared sharp.
It seems to have expanded and simply slides over the male shaft thread.
If a washer had gone then threads would be unaffected.
Very puzzling esp as the prop has been held on by a split pin stopping a suddenly too large brass castellated nut from dropping off.
Eh?

spartacus · 30 August 2013, 21:37

Unusual for the castellated nut to be damaged like that. Any pictures?

What's the propeller bush like? There should be a thrust washer, then the propeller, then another washer and finally the castellated nut and split pin.

mister p · 30 August 2013, 21:44

Quote:

Originally Posted by spartacus

Unusual for the castellated nut to be damaged like that. Any pictures?

What's the propeller bush like? There should be a thrust washer, then the propeller, then another washer and finally the castellated nut and split pin.

I reckon the "another washer" has disintergrated. But how the hell did the nut get bigger? I know a stripped thread, this is as if the nut had a 20% enlarge ray zapped on it. Threads are sharp.
Sneaking suspicion is that it could have been replaced by somebody needing right size. However, the split pin was replaced.
Washer is missing hence the play. Understood, could have broken off. But why would the nut no longer fit the thread?

mick · 30 August 2013, 21:46

The nut could have split

whisper · 30 August 2013, 21:50

If the prop has play it will hammer against the nut while underway. Could this have caused the damaged nut?

willk · 30 August 2013, 21:51

Quote:

Originally Posted by mister p

Sneaking suspicion is that it could have been replaced by somebody needing right size.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

mister p · 30 August 2013, 21:52

Nut is still there. Held by the split pin. It's just that all of a sudden it is too large to tighten against the prop. The thread is there, as it is on the shaft. It's just that the nut has grown to be able to slide over ther shaft's threads.
If I were to take a picture, all you would see is an oversized castellated nut held by a split pin.

Nasher · 30 August 2013, 22:26

Are you sure you've not imagined the whole thing?

You wouldn't be the first to experience the feeling you have bigger nuts than you really do when piloting a Searider around for a few days.

Nasher

jyasaki · 31 August 2013, 02:09

Quote:

Originally Posted by spartacus

Unusual for the castellated nut to be damaged like that. Any pictures?

What's the propeller bush like? There should be a thrust washer, then the propeller, then another washer and finally the castellated nut and split pin.

Possibly a spacer in there as well, depending on motor and prop combo.

But yeah; I'd believe the stolen/replaced scenario.

jky

Leapy · 31 August 2013, 08:08

Quote:

Originally Posted by willk

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

I can't imagine how a nut could grow without damaging the threads.

In the absence of a thrust washer, fretting corrosion could have damaged the threads and made the nut ID bigger in the way you describe but then the threads would have been chewed.

I can't even imagine how the washer would disappear other than by theft. But someone would steal a thrust washer and put the split pin back? To say nothing of the prop ...

Are you sure the prop you now have is "yours"?

samt · 31 August 2013, 08:40

Incorrect thrust washer and nut?

I have seen thrust washers wear away the front of the gearbox when an incorrect (larger) one has been fitted that doesn't seat correctly on the prop shaft and instead seats on the soft alloy of the gearbox, which dissappears surprisingly quickly. Before use prop nut would have appeared at a glance to be correct.

ShinyShoe · 31 August 2013, 09:35

How possible is having the nut switched? Where does engine live?

Is it the difference between a metric and imperial nut?

Could someone have borrowed your prop? Damaged theirs and 'needed' a quick repalcement for a day or wanted to Test a different pitch?

Could someeone have wanted to whip off a prop for some other reason - compare the splines on the hub of a nackered one (theirs) to a good one (yours)?

If they had and had their nut and yours together could they have switched them by accident? Then not noticed when fitting either that one was loose and presumably the other was very tight?

Having left dinghies in sailing club dinghy parks for years I know all sorts of things get borrowed because theirs is broken or because theirs has also been borrowed or someone wants to see if a bigger or smaller XYZ makes them sail better. Somehow people think you wont mind even if they don't know you because they wouldn't mind if you asked them.

Hightower · 31 August 2013, 09:56

Have you stripped the propeller assembly apart yet? First thing to do is to eliminate all the possible parts and see what you're left with. Check the nut against the thread on the shaft by hand and see if there's play. If there is, then either the nut has been damaged or more likely the wrong nut has been used.

mister p · 03 September 2013, 09:27

And so,
nut is in one piece, clearance to end of shaft correct, thread's not stripped on shaft or nut.
My thoughts are that somebody's nicked my nut.
The boat's on hardstanding in a marina.

jyasaki · 03 September 2013, 17:12

Quote:

Originally Posted by ShinyShoe

If they had and had their nut and yours together could they have switched them by accident? Then not noticed when fitting either that one was loose and presumably the other was very tight?

I could see the scenario happening up to the last: If the wrong nut was barely or not at all engaged on MisterP's motor, his wouldn't have fit at all on the other motor.

jky

Nos4r2 · 03 September 2013, 18:21

Quote:

Originally Posted by mister p

And so,
nut is in one piece, clearance to end of shaft correct, thread's not stripped on shaft or nut.
My thoughts are that somebody's nicked my nut.
The boat's on hardstanding in a marina.

I was up and down by your boat all last week and I didn't see anyone up there.

willk · 03 September 2013, 18:21

lol- are you even sure it's your prop, P.? It could have been like that for weeks!

mister p · 04 September 2013, 18:43

Some shister has had a go.

Dirk Diggler · 05 September 2013, 10:29

Quote:

Originally Posted by willk

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

Elementary, my dear Watson.

willk · 05 September 2013, 11:26

Quote:

Originally Posted by Dirk Diggler

Elementary, my dear Watson.

Yes. Although it was your namesake that put me in mind of it:

“Sherlock Holmes observed that once you have eliminated the impossible then whatever remains, however improbable, must be the answer. I, however, do not like to eliminate the impossible.”
― Douglas Adams, Dirk Gently's Holistic Detective Agency