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Old 02 September 2006, 11:42   #11
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Right, prop changed.... more questions!

The prop appears to have a rubber insert - what is this for some sort of shock absorbing function I suppose? Both props are the same anyway.

On the old prop, this rubber bit looks a bit buggered - see the attached photos of the new and old for comparison.

The central splined part inside the prop doesn't seem to be loose or anything - is this still going to be OK to use if I want/need to reinstall it? The bushing looks a bit like rubber sometimes does when it has had oil on it (e.g. suspension bushes that have suffered from oil contamination). The effect seems to be only the outside layer, after I took the photos I pulled the flappy loose bit off and it seems ok underneath.

The new prop while slightly smaller diameter has slightly broader blades so I guess the overall blade surface area should be similar. Bit chilly for ribbing today but the forecast for tomorrow doesn't look too serious so I hope to be able to test the new prop and the new aux then, and see what we get

The overhaul manual says that the nut holding the prop on should only be torqued to 14nm does this sound right? I guess it shouldn't be too tight as there is a nylon bush under the nut which would obviously not stand up to being really really tight, but 14nm is not a lot... and it took a #kin big grunt to shift the old one even with lots of WD40 first (though it could have just been seized after 6 years on there).
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Old 02 September 2006, 11:55   #12
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Yes, the prop nut doesn't need much torque as you will lock it with whatever system Johnson outboards use.

The rubber bushing provides a degree of energy absorbtion in case your prop hits something. And a degree of isolation in case little dings put the prop out of balance. It is much bigger than the internal diameter of the hub, and shouldn't really be refitted if there's any sign of damage. Either use it as a spare or send it to a specialist like Propeller Revolutions or Streamlined Propellers for replacement. One key thing is that they use a water based lubricant to fit it... not oil or detergent based!
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Old 02 September 2006, 12:09   #13
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Quote:
Originally Posted by BogMonster
The overhaul manual says that the nut holding the prop on should only be torqued to 14nm does this sound right? I guess it shouldn't be too tight as there is a nylon bush under the nut which would obviously not stand up to being really really tight, but 14nm is not a lot... and it took a #kin big grunt to shift the old one even with lots of WD40 first (though it could have just been seized after 6 years on there).
Not sure if it helps you or not but my Yamaha 20HP has a specified torque of 35 Nm.

[Edit sorry richardB had a much more intellegent answer which he posted as I was one finger typing!. ]
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Old 02 September 2006, 12:31   #14
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Thanks. I'm only expecting to keep it as a spare anyway, unless there is something fundamentally wrong with the performance of the new one. Its got a few bumps and scrapes on the blades which can't do anything for the performance either, but it is sound enough I think.

Depending on the performance with the 19in prop I may even end up getting a 21in - but I'll worry about that when I see what difference this one makes as it isn't cheap having lots of props lying around and they are a bit hard to borrow round these parts

The nut is a castle type nut retained with a big split pin so there's no danger of it coming off. Guess who forgot to order any spare split pins.... oh well, another fax to Mr Oakley, ho hum...
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Old 02 September 2006, 12:41   #15
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The nut is a castle type nut retained with a big split pin...
...and you can bet that your torque wrench will click to a stop with the castellations covering the hole!
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Old 02 September 2006, 12:46   #16
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I had a dial type torque wrench but not being sure of its accuracy (it was a cheapy Draper one) I elected to make it nearer 20nm - the book says that if you miss the hole you carry on tightening the nut till you can get the split pin in so it would have ended up being the same anyway if I had done it to 14 and then to the next hole
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Old 02 September 2006, 17:14   #17
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...and you can bet that your torque wrench will click to a stop with the castellations covering the hole!
Nothing surer in life
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Old 03 September 2006, 11:54   #18
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Testing complete

Bit of a windy day today (18-20kt and quite choppy in the harbour) so I might get a little bit more out of her on a calm day but I now get 38.8kt with the new 19in prop, and it cruises much better at part throttle - I didn't actually take readings before at 3000rpm 4000rpm and so on but its definitely noticeably better, I think 4000rpm (which is a decent cruise rpm on this engine) is now giving about 30kt which is pretty much what I was aiming for.

WOT now gives about 5100rpm which is perhaps a little low, so a 21" prop is definitely going to be too much for it, but this 19" one seems just right

For some reason it also ventilates much less in the corners when the engine is trimmed out for max speed, so that is an unexpected bonus too!

The Johnson 6hp four stroke aux gives me 5.5kt with a bit of a tailwind....
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