Propeller Shaft Size Calculator | Safety Factor

Capt.Ben

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Hey all, I made a FREE propeller shaft size calculator for our website. I tried to make it as "intuitive" as possible and provide explanations of torsional yield and RPM effect in the text. This is still a work in progress and I'm hopeful for any improvement ideas from you guys.

Propeler Shaft Size Calculator – R.E. Thomas Marine Hardware

I know this topic comes up from time to time and thought it might be helpful to some looking for a better understanding of the process.
 

Diesel Jerry

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Blitzen

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Thanks Ben, very kind of you to put this out there.
 

tunafishhkg

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Thanks Ben!! one question, I see the AQ22 is the only option as in other calculators so is the AQ22HS what is normally used or called AQ22?
Thanks
 

Capt.Ben

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Thanks Ben!! one question, I see the AQ22 is the only option as in other calculators so is the AQ22HS what is normally used or called AQ22?
Thanks
It’s all just tricky marketing semantics, but I’ll try my best to explain. The only thing that “really” matters is the torsional shear strength of the shaft regardless of what it is called. “Regular” AQ22 is required to have a minimum torsion strength of 70,000 psi up to sizes 2” in diameter. In diameters above 2” (2 1/4” and up), the minimum relative torsional strength starts dropping as mentioned in the article. This is where the “HS” comes in to play. Aqualoy defines high strength as having the same torsion strength as AQ17 (which incidentally is also 70,000) for sizes 2 1/4” in diameter all the way up to 6” in diameter (in other words, the relative torsional strength doesn’t start dropping off for “HS” as its diameter gets bigger). This is why, technically, AQHS is only available for shaft diameters above 2 1/4”…but that doesn’t mean you can’t get smaller diameter shafts (<2”) that also have higher strengths. For example, it’s not uncommon to find a 1 1/2” shaft that has a 80,000 or higher strength. Hopefully this helps explain and I didn’t make matters worse. Perhaps a YouTube explainer video is in order.
 

Capt.Ben

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yep, I was sitting around in my office today thinking “God, there must be something I can do to keep John from calling me every 5 minutes to run a shaft safety factor…WAIT, I got it!!!”
I swear there was some funny emojis in here that may this look and sound more funny.
 

tunafishhkg

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Appreciate the explanation Ben Thank you! I understand now why I could not see it and the reason for question is, I am going to put a QSB Cummins in my 25 Backfin with a 1.5" shaft. The calc with my setup is giving me a 4.7 safety factor that's in the recreational range so any higher would be a plus lol.
 

chortle

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Hey all, I made a FREE propeller shaft size calculator for our website. I tried to make it as "intuitive" as possible and provide explanations of torsional yield and RPM effect in the text. This is still a work in progress and I'm hopeful for any improvement ideas from you guys.

Propeler Shaft Size Calculator – R.E. Thomas Marine Hardware

I know this topic comes up from time to time and thought it might be helpful to some looking for a better understanding of the process.
That's quite handy, going to do one for max bearing spacing too?
 

Keelboater

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Nice job Captain. I'm sure that program will get a good workout from people on this forum. Quick and easy. To avoid any confusion for the masses, torsional shaft strength has nothing to do with shaft stiffness, which is what enables shaft whip and creates the need for center bearings. Maybe you can add this calculation to your web site. I'm simply mentioning this to let people know the difference between the two properties, both of which are important for a properly designed shaft.
 

Capt.Ben

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Nice job Captain. I'm sure that program will get a good workout from people on this forum. Quick and easy. To avoid any confusion for the masses, torsional shaft strength has nothing to do with shaft stiffness, which is what enables shaft whip and creates the need for center bearings. Maybe you can add this calculation to your web site. I'm simply mentioning this to let people know the difference between the two properties, both of which are important for a properly designed shaft.
Excellent points! I’m working on the bearing spacing calculator now with a nice discussion on the “modulus of elasticity” for steel shaft alloys.
 

Capt.Ben

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Nice job Captain. I'm sure that program will get a good workout from people on this forum. Quick and easy. To avoid any confusion for the masses, torsional shaft strength has nothing to do with shaft stiffness, which is what enables shaft whip and creates the need for center bearings. Maybe you can add this calculation to your web site. I'm simply mentioning this to let people know the difference between the two properties, both of which are important for a properly designed shaft.

Here's the shaft bearing spacing formula:

Propeller Shaft Bearing Spacing Calculator – R.E. Thomas Marine Hardware
 
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