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Prop Keyway - Brass Vs. Stainless on 575


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2016 23LSV with 575.

 

Very familiar with inboard boats, props.  Went to swap my prop on newly acquired boat, and found a stainless keyway.

Tempted to swap it back to brass.

 

Any good reason that someone would put a stainless one in there?  Keyway is there to break in case of major contact.  Feel like a stainless one may cause more damage.

 

Only thing I can think of is potentially issues with the high torque 575 motor being able to shear brass keyways.

 

Thoughts, comments?

 

Thanks!  

 

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The key should only be under pressure in the event of a prop strike.  The taper is what holds the prop in place under normal conditions.  I'd put a brass one in there.

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On 3/18/2021 at 9:19 PM, csleaver said:

Stainless is normally recommend for 450 horsepower and above, less than that brass would be fine.  Your boat, your choice.

This is what I was thinking. Where does this recommendation come from?

first timer in the high horsepower club. 
 

thanks!

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3 hours ago, dhcomp said:

Where does this recommendation come from?

Engineers.  316 Stainless will have a shear strength roughly double that of brass (~70,000 PSI vs ~34,000 PSI). 

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I'd err on the side of caustion and use brass.  it is like those SS props - a strike is a major hit; brass, not so much.  Or would 500 ponies shear the pin? I'm not an engineer.  :)

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1 hour ago, justgary said:

Engineers.  316 Stainless will have a shear strength roughly double that of brass (~70,000 PSI vs ~34,000 PSI). 

I understand that it’s stronger. But is a prop or boat Mfg specifying one over the other based on hp?  Curious if it’s necessary. 

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Well, let's see... Assume the following because I don't have real numbers, but maybe you'll get the idea:  1/4" key, 2" long, and 1-1/8" diameter prop shaft.   Some setup calculations:

Area of key that will shear = 2" x 0.25" = 0.5 square inches

Radius of prop shaft in feet = 1.125" / 2 / 12 inches per foot = 0.046875 feet

Using the numbers I quoted above, brass would shear at about

34,000 PSI * 0.5 square inches * 0.046875 feet = ~800 foot pounds of torque

Stainless would shear at roughly double that, or ~1,600 foot pounds

Before you jump up and down yelling at me that the engine doesn't produce 800 foot pounds of torque, realize that the shear number represents where it *will* fail.  Engineering practice generally allows at least a 2:1 margin, so the working limit for brass is probably in the neighborhood of 400 foot pounds of torque on a 1-1/8" shaft.  Moving up to a 1-1/4" shaft probably gets you more like 885 foot pounds with brass.  These numbers scale with key length, so it would be less with a shorter key.  Ideally, you get a bit of torque advantage from the tapered shaft/prop interface as well, meaning that a well-lapped and fit prop could theoretically not use a key at all.  Vibration, uneven load, etc. help keep you from having an ideal situation, so use a key that is strong enough for your situation.  The goal, of course, is to have the key *never* shear, except maybe where you have a sudden impact that could instantaneously grossly exceed the safe torque for the key.

Please remember that this is just a back-of-the-envelope calculation for your entertainment value only.

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I got the recommendation for the prop shaft key material several years ago either from Larry at Indmar or from the Marine Power rep at Malibu during training when we discussed the LS3.  I can't remember exactly, but I can check my notes and get back with you.

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@justgary I copied your calculations but with the torque of the shaft applied to the center of the part of the key assuming that it sits halfway in the propeller shaft. So I had:

34,000 PSI * 0.5 in^2 * 0.04166 Feet = ~700 Ft-lbs of torque and 300 of working Ft-lbs of torque

Almost all engines that use brass keys make more torque than that. 

Since the propeller is moving through a viscous fluid it will have considerable slip and never 100% transfer all of its torque to the water. These numbers we are calculating are only applicable to if the propeller was somehow held from spinning and the engine was brought up to 700 ft-lbs of torque.  The reason why keys shear when you hit something is the force of impact on the propeller and therefore the key, adding to the torque applied by the engine. If feel you would need an astronomically powerful engine to shear any key under normal operation.  I don't really see a reason to keep it because I think your just opening your self up to greater damage if you hit something

I'm also worried if you were to run a NIBRAL propeller with a stainless steel key and were to hit something big enough to deform your propeller it could also strip the keyway of the propeller, eliminating one of the reasons for having a consumable key in the first place. Stainless has about double the strength as NIBRAL, but that really depends on what kind of stainless it is.

This is just me throwing my thoughts out there as an engineering student. Its an interesting topic because I'm in Material Science and Strength of Material class now. 

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7 minutes ago, roeboat said:

@justgary I copied your calculations but with the torque of the shaft applied to the center of the part of the key assuming that it sits halfway in the propeller shaft. So I had:

34,000 PSI * 0.5 in^2 * 0.04166 Feet = ~700 Ft-lbs of torque and 300 of working Ft-lbs of torque

Almost all engines that use brass keys make more torque than that. 

Since the propeller is moving through a viscous fluid it will have considerable slip and never 100% transfer all of its torque to the water. These numbers we are calculating are only applicable to if the propeller was somehow held from spinning and the engine was brought up to 700 ft-lbs of torque.  The reason why keys shear when you hit something is the force of impact on the propeller and therefore the key, adding to the torque applied by the engine. If feel you would need an astronomically powerful engine to shear any key under normal operation.  I don't really see a reason to keep it because I think your just opening your self up to greater damage if you hit something

I'm also worried if you were to run a NIBRAL propeller with a stainless steel key and were to hit something big enough to deform your propeller it could also strip the keyway of the propeller, eliminating one of the reasons for having a consumable key in the first place. Stainless has about double the strength as NIBRAL, but that really depends on what kind of stainless it is.

This is just me throwing my thoughts out there as an engineering student. Its an interesting topic because I'm in Material Science and Strength of Material class now. 

Why use a 1" diameter?  The key will shear at the interface between the shaft and the prop, which is at the shaft diameter. 

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Just now, justgary said:

Why use a 1" diameter?  The key will shear at the interface between the shaft and the prop, which is at the shaft diameter. 

Oh shoot I was thinking of it as a distributed load and not a shear stress. The force of a distributed load would have been at the center of area of the keys contact surface which would have been 0.5" from the center. Since its a shear stress the force is at the point of shear.  I guess my calc is only applicable to boats with a 1" shaft. oops

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Great rough calcs, and great references from Indmar.  While i'm a mech engineer by schooling, i'm not as inclined to jump into calcs as I once was!

 

Sounds like more than enough reason to keep the stainless one in there on the 575.  My brass one will work as a backup.

 

Thank you all!

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The key isn't supposed to be holding the load on a tapered shaft.  It is a safety that will stay in place with a moderate shock load and shear with a bigger one.  It is designed to keep you from breaking parts, not stopping the prop from spinning on the taper.

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