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Old 02 September 2010, 15:34   #1
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Country: UK - Scotland
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Boat name: 5.8 Open Adventure
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Prop for Johnson 140

Hello,

I am looking for a new prop for our rib. Our current set up is a northcraft 6.3m with a Johnson 140 4stroke.

The prop we have on it just now is 3x14x21 but we cant even trim the engine half way and it starts to ventilate, which we think is due to the size of the prop. The anti cavitation plate is just about level with the bottom of the hull, which I think is where it should be?

I am thinking of going down the 4 blade prop option and a 17" pitch propeller has been suggested. It would be good to get a few of your opinions before I jump in and buy it incase it turns out to be the wrong choice.

The maximum speed we get out of the rib just now is about 30mph @ 4500-5000 revs and the maximum recommended revs for this engine is around 5500.

Any help would be appreciated.
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Old 02 September 2010, 18:01   #2
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There's something pretty wrong if you're only getting 30mph out of a 5.8 with 140hp on

I think the prop on my old Johnson 115 was only 13.75in diameter (x19in pitch) and did not have any problems with ventilation/cavitation - standard 3 blade alli prop and would do much more than 30 and only ventilate when it was trimmed right off the top of the trim gauge scale.

I don't know how the trim gauge is set up on yours but I think on mine there was some adjustment - is your trim gauge maybe reading wrong so when you think it is half way up it is actually much further up than that? just a thought.
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Old 03 September 2010, 03:27   #3
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Quote:
Originally Posted by BogMonster View Post
There's something pretty wrong if you're only getting 30mph out of a 5.8 with 140hp on

I think the prop on my old Johnson 115 was only 13.75in diameter (x19in pitch) and did not have any problems with ventilation/cavitation - standard 3 blade alli prop and would do much more than 30 and only ventilate when it was trimmed right off the top of the trim gauge scale.

I don't know how the trim gauge is set up on yours but I think on mine there was some adjustment - is your trim gauge maybe reading wrong so when you think it is half way up it is actually much further up than that? just a thought.
Hello,

The trim gauge is set up fine and the rib is 6.3m. I have been offered a 17" pitch 4 blade propeller from Steeldevelopments but it seems quite a considerable change from the three blade 21" pitch that we currently have.
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Old 03 September 2010, 05:58   #4
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I've got a 17" Viper you can try if you like, your based in Gourock but is the rib based at largs?
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Old 03 September 2010, 07:04   #5
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Quote:
Originally Posted by ryza00 View Post
Hello,

The trim gauge is set up fine and the rib is 6.3m. I have been offered a 17" pitch 4 blade propeller from Steeldevelopments but it seems quite a considerable change from the three blade 21" pitch that we currently have.
Oh ok sorry ... I looked at your profile info and not the post - duh! I'll shut up
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Old 03 September 2010, 09:29   #6
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I've got a 17" Viper you can try if you like, your based in Gourock but is the rib based at largs?
Yes the rib is based at large. I would nt mind given it a try!

I believe you based at largs as well?
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Old 07 September 2010, 08:21   #7
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Thanks to GordyP

Thanks to GordyP for letting us try your spare prop but unfortunately it was the wrong size for our engine.

I think I will take the gamble with the Steeldevelopments 17" pitch suggestion or go for the 18". They said if it is not the correct size then they can alter it 1" pitch in either direction, for a small fee of course.
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