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Old 07-24-2014, 09:00 PM   #1
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Prop Shaft Size

Hi
I recently purchased 2x 4-236 perkins diesel engines with 23x 26 4 prop blades to repower my 34' tollycraft tri-cabin. Can I use the original 1.25 dia shafts with these engines and props? The trannys are 3:1 cr2 borg warners
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Old 07-24-2014, 09:26 PM   #2
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not sure of the horsepower or your planned cruising speeds but they sound like big props for a 1.25 shaft....especially with 3:1
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Old 07-24-2014, 09:34 PM   #3
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The 4-236 is an 85 HP engine. That is a big prop on 1.25 those engine are rated at top rpm of 2800. Might work. Check with a prop shop.
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Old 07-24-2014, 09:35 PM   #4
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What diameter are the props bored to? 23" dia is usually 1.75" minimum, maybe 1.5.

1.25 can handle 80hp at 3:1, but hanging that big wheel on it will really stress the tip of the shaft.
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Old 07-25-2014, 12:13 AM   #5
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The props have a 1.5 bore
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Old 07-25-2014, 01:29 AM   #6
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Using the Boatdiesel.com Prop. Shaft Calculator I entered the following:

Perkins 4-236 85SHP - 2,800RPM
Reduction Gear - 3:1
Shaft Safety Factor - 5 (recommended)

Here are the results:
Stainless 304 - 1.94"
Stainless 316 - 1.94"
Monel 400 - 1.54"
Monel K500 - 1.30"
Aquamet 17 - 1.28"
Aquamet 18 - 1.35"
Aquamet 19 - 1.28"
Aquamet 22 - 1.19"

So, if your 1.25" shafts are made of Aquamet XX alloy or Monel K500, you should be safe. If stainless 304 or 316, they are considerably undersized.

There is a very old "Rule of One Fourteenth" where the prop diameter is divided by 14 to roughly evaluate the prop shaft size. In your case, 23" divided by 14 = 1.64" which is sort of in the middle of the calculated results above.

Finally, using Skene's shaft size calculations:

Shaft Horsepower - 85
Shaft RPM - 933

Bronze & Mild Steel - 2-1/8"
Monel - 1-3/8"

You can draw your own conclusions from the above. They all seem to indicate that 1.25" shafting will be too small for your application. The exception would be Aquamet 22 shafts. If there is any doubt, it would be better to err on the side of safety.

Just my $.02 . . . .

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Old 07-25-2014, 06:04 AM   #7
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While I prefer paper , one of these FEEE downloads would answer your question.
  1. Elements of yacht design : Skene, Norman L. (Norman ...

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    Mar 8, 2010 - Internet Archive BookReader - Elements of yacht design ... metadata about the original images and the derived formats (OCR results, PDF etc.).

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    I recently downloaded the free PDF book, Elements of Yacht Design, by Norman L. Skene, and provided by Cornell University's Library in ...Recomend a Naval Architecture Textbook - Page 3 - Boat ...
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  3. Elements of yacht design (Open Library)

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    Apr 14, 2010 - Elements of yacht design by Norman L. Skene, 1904,Rudder publishing company edition, in English.

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    Title, Elements of Yacht Design. Author, Norman Locke Skene. Publisher, Rudder publishing Company, 1904. Original from, the New York Public Library.

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    Skene's Elements of Yacht Design, Eighth Edition [Francis S. Kinney] on Amazon.com. *FREE* shipping on qualifying offers. Eighth Edition, m Fifth printing, .


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Old 07-25-2014, 10:46 AM   #8
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For displacement speed those engines and gear ratios sound like a nice match up, the shaft size is "acceptable" if propped correctly (not the ones listed) full RPM plus an extra 100 verified by photo tach. You may have room in the strut to use a thinner walled cutless bearing to fit 1 1/2 shafts (aquamet 22) which would make a real nice setup.
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Old 07-27-2014, 11:07 AM   #9
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With a 85HP and 23x26 prop you cant turn at 2800 rpm. If you use low rpm, you need low Power and can use a 1.25" shaft. The best is 1.75"


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Old 07-27-2014, 11:40 AM   #10
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As everyone else, I agree the shafts are really small however from my limited knowledge, you should be ok if your props are balanced. Wobble will do the shafts in post haste. Keep em clean, balanced and you should be OK. Hit something, run aground better check em out. I run 250 hp Cummins, 2:1 tranny with a 1 3/8 shaft. I watch the barnacles, cause the imbalance will snap the shaft.
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Old 07-28-2014, 05:44 AM   #11
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The shafts biggest danger is docking IF the tranny is shifted quickly at high >PANIC!< rpm.
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Old 07-28-2014, 06:17 AM   #12
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Hi I checked the props that came with the 4-236 perkins and the dia is 1 3/8 so they must have been on Aquamet or Monel shafts. I am going to see what my 1 1/4 shafts are made from and go from there. I have been talking to a prop guy here and he seems to think my shafts will work so I am leaning towards using them.Thanks everyone for the info and I will let you know what happens.
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Old 07-28-2014, 07:30 AM   #13
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Quote:
Originally Posted by FF View Post
The shafts biggest danger is docking IF the tranny is shifted quickly at high >PANIC!< rpm.
Uggggg, yeah, dat too..
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Old 07-28-2014, 10:52 AM   #14
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Since this is a large 4cyl engine hooked to a big prop w a deep reduction torsional vibration should be much higher than usual.

Lots of trolling or/and very low idle speeds could very well be problematic. Also damper plates may not last long.
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Old 07-28-2014, 12:51 PM   #15
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Quote:
Originally Posted by rayman62 View Post
Hi I checked the props that came with the 4-236 perkins and the dia is 1 3/8 so they must have been on Aquamet or Monel shafts. I am going to see what my 1 1/4 shafts are made from and go from there. I have been talking to a prop guy here and he seems to think my shafts will work so I am leaning towards using them.Thanks everyone for the info and I will let you know what happens.
As you said, determining the material of shaft construction is essential. Until you know this, all is a guessing game.
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Old 07-30-2014, 05:49 PM   #16
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I have a 250 Cummins with Twin Disc and a single 2.5" shaft, 36" wheel. Mine's bigger than yours! 8^)
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Old 07-30-2014, 05:52 PM   #17
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As an aside, I recently discovered that if you move the shift lever into gear slowly, the clutches seem to take up the slack more smoothly and there is no clunk in the shifting. Sounds much less like mechanical thrashing. It seemed with a Velvet Drive, after pausing, a fast shift at idle worked best. Quite a difference. Pausing is important with both, it seems.
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Old 07-30-2014, 06:58 PM   #18
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PM me with an email address and I'll send you the ABYC shafting spec.
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Old 02-09-2015, 06:26 PM   #19
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I found two letters stamped on the shaft they say AQ and are not magnetic so I'm hoping thry are aq 22
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Old 02-09-2015, 07:13 PM   #20
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We once had a 30' Tolly that had Aquamet shafts as OEM.
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