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  #1   IP: 71.200.119.246
Old 05-13-2013, 05:12 PM
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propeller question

I got my propeller shaft back from the machine shop this afternoon. I had a keyway cut


and had the last portion of the shaft turned down from 3/4 to 5/8 for the propeller.


I didn't want to have the shaft drilled for a traditional shear pin, because I didn't want to weaken the shaft. The propeller assembly is pretty heavy.

Suggestions as to how I can fix the propeller on the shaft? 2nd key way on the 5/8th portion and on the inside of the propeller base?

James
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Old 05-13-2013, 06:28 PM
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Exclamation Question

Most props of this sort mount on a tapered shaft. Do you have a prop that mounts straight without a spline?

Or are you speaking of the coupling?

Dave Neptune
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Old 05-13-2013, 06:36 PM
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If I understand your situation, the prop was designed to fit on a 5/8" shaft with a shear pin? If that is the original design, why do you think that would weaken the shaft excessively?
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Old 05-13-2013, 06:50 PM
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Quote:
Originally Posted by Dave Neptune View Post
Most props of this sort mount on a tapered shaft. Do you have a prop that mounts straight without a spline?

Or are you speaking of the coupling?

Dave Neptune
It is a straight shaft, not a tapered one. I am speaking of the propeller end of the shaft.

I didn't have the hole drilled for the shear pin because in this application I was afraid it would compromise the strength of the shaft, The folding propeller unit I am using is pretty heavy and I didn't want that point of connection to contribute to a bent shaft over time.
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Old 05-13-2013, 11:01 PM
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I hate to be the one but . . .

I gotta say James, a 5/8" shaft, even a section of it puckers my sphincter. I've never seen or even heard of a shaft smaller than 3/4" for an A-4. The smallest I've ever installed was 7/8" (Newport 30).

You're going to have to do something to keep the prop from spinning on the shaft, either a shear pin or a keyway. If the latter, I expect the key would be 1/4" or something close. Half of that dimension would be cut into the 5/8" portion of the shaft leaving you with 1/2" of meat.

What's bothering me is having never seen one as small, not that it won't work (can't say that with any certainty). For the record, I've seen first hand a little under 6000 A-4 installations over the entire product lines of 4 different companies.
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Old 05-14-2013, 12:32 AM
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just pulled the prop off the shaft....diameter 1"...tapered down to about 27/32...with a keyway going trough the tapered part of it.....
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Old 05-14-2013, 07:16 AM
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possible solutions

Many thanks for the comments thus far. (Neil, your opening sentence will require some mental scrubbing, but I'll be OK) I appreciate and value the experience of the folks here who have worked with these engines and systems professionally and as a recreational pursuit. I can't think of the name, but the manager of our boatyard suggested a round key; a dowel that would be less invasive on the weaker portion of the shaft, but still able to stop propeller rotation. Another possible solution might be a thick, 1/4" stainless washer, as wide as the diameter of the area at the bottom of the propeller unit that has the indentations for the shear pin. Before sliding this washer down to seat at the end of the 5/8 section, build up welding material on either side of the washer, then cut and shape the pieces into the square shape needed to mate with the bottom of the propeller unit. Slide this down the shaft, and weld them together, maybe by means of a fillet around the 3/4 portion of the shaft it will be seated against.

James
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Old 05-14-2013, 07:48 AM
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From an engineering perspective, the absence of a taper is surprising. IIRC, that abrupt change in diameter, with those two surfaces meeting at a 90 degree angle, creates a stress point from which cracks are more likely to propagate.

Bill
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Old 05-14-2013, 08:14 AM
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Quote:
Originally Posted by Hymodyne View Post
I can't think of the name, but the manager of our boatyard suggested a round key; a dowel that would be less invasive on the weaker portion of the shaft, but still able to stop propeller rotation.
Woodruff key.


Quote:
(Neil, your opening sentence will require some mental scrubbing, but I'll be OK)
Sorry. And I cleaned it up from what I originally typed!
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Last edited by ndutton; 05-14-2013 at 08:25 AM.
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Old 05-14-2013, 08:20 AM
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Well I'm going to say it. Wouldn't it be better to have enlarged to propeller hole?

Last edited by Sony2000; 05-14-2013 at 10:02 AM.
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Old 05-14-2013, 09:18 AM
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My previous boat had similar heavy prop with shear pin design and it was always a proplem. Boat had to be hauled every time shear pin broke.Stress on pin when shifting was usually the reason .
Changed over to keyway,woodruff key in shaft and this took care of problem.
Of course this was an Abbott 33 with small 7 hp. engine.
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Old 05-14-2013, 12:18 PM
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Quote:
Originally Posted by Sony2000 View Post
Well I'm going to say it. Wouldn't it be better to have enlarged to propeller hole?
Sony, I had thought of it early on, but resisted because I didn't know what it would do to the integrity of the prop unit. I will most likely get a slot cut in the shaft for a woodruff key and a keyway cut in the prop unit; the shaft is the same diameter as the shaft that the shift lever on our engine is connected to and that uses this method.
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Old 05-14-2013, 09:48 PM
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I think you will be paying to haul your boat out of the water again...only this time without a prop.

Just sayin'!

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Old 05-14-2013, 09:54 PM
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I am thinking about outboard motor props. if I remember correctly, my 1969 40 hp Johnson has about a 1/2 " shaft, with a shear pin. No taper. Not sure about other more modern motors.

I would go check it, but it is up north at dads house. I could go outside and check my 1940's Neptune 4hp, and 2 hp motors though.

Just as a comparison of props, shafts, and horsepower.

Last edited by romantic comedy; 05-14-2013 at 09:56 PM. Reason: sillyness, taxes, politics, religion
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Old 05-14-2013, 10:23 PM
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Quote:
I am thinking about outboard motor props. if I remember correctly, my 1969 40 hp Johnson has about a 1/2 " shaft, with a shear pin. No taper. Not sure about other more modern motors.
I had the same thought about outboards. I am not convinced that a shear pin would not work. Have other people had trouble with that set up?
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Old 05-14-2013, 10:50 PM
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shear pin (sort of)

I'm looking at a 5/32 to 3/16 shear pin for the prop system, but instead of softer metal, what about a harder one, say titanium? If not that, then something comparable to the shaft itself, (316 stainless) and resistant to corrosion.

Based on where the propeller is located in relation to the hull, there is little chance of it contacting anything that would result in a need for a traditional shear pin. Instead, if the pin was stronger, then a small diameter one could be used to secure the propeller and ensure that it does not rotate freely or have the potential to as the boat is shifted from N to forward or reverse. The stronger, small diameter shear pin will minimize the diameter of the hole drilled in the shaft, lessening the danger of the shaft being weakened by the removal of too much material.

James
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Old 05-14-2013, 10:56 PM
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energy

Quote:
Originally Posted by marthur View Post
I had the same thought about outboards. I am not convinced that a shear pin would not work. Have other people had trouble with that set up?

BTW, many thanks to you and Romantic Comedy for the positive energy regarding this problem. I'm putting an engine and drive system where there wasn't one before, making most everything myself to the highest standards I can muster, especially since my and my wife's safety hang in the balance.

If I have to, I'll get another 3/4 " stainless shaft and drill out the propeller hub, I'd just like to try and reach a successful solution to this problem without doing so if at all possible.

James
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Old 05-14-2013, 11:03 PM
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I may be missing something here?

What kind of vessel...what weight is the vessel...what sort of motoring are you intending to do?

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Old 05-15-2013, 01:40 AM
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Quote:
Originally Posted by romantic comedy View Post
I am thinking about outboard motor props. if I remember correctly, my 1969 40 hp Johnson has about a 1/2 " shaft, with a shear pin. No taper. Not sure about other more modern motors.

I would go check it, but it is up north at dads house. I could go outside and check my 1940's Neptune 4hp, and 2 hp motors though.

Just as a comparison of props, shafts, and horsepower.
RC,
I have an old neptune I restored last summer. It really shakes!

Hym,
Shear pins are for outboards only, in my humble opinion. You break one and it's(relatively) easy to tip up the kicker and do the repair. Not so on a fixed prop on a boat sitting in the water.

I'm wondering; "is this a sail drive prop?"
I would get a proper prop.
Lots of old correct props out there for sale. I don't see it necessary to reinvent the wheel. Yea, pun.
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Old 05-15-2013, 06:36 AM
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Hey, wow,
I have a old neptune "mighty might" that looks like that except it's still gold color. Doesn't run anymore.
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Old 05-15-2013, 11:23 AM
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I have been doing a little digging on this question. I do not think that a key will give you a stronger shaft than a shear pin. The material removed to fit the key could weaken the shaft as much as the hole for a shear pin depending on the size of the pin and key. The only advantage to a key is that it is much stronger in shear. The shear pin will break much easier and would save your expensive folding prop from harm.

My brother the engineer says keys are for tight, aligned, high shear applications. Shear pin can be sloppy, easy and fast by comparison.

Good luck with your choice! Mike

Last edited by marthur; 05-15-2013 at 12:15 PM.
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Old 05-15-2013, 12:05 PM
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This is again one of those times I really want to be wrong but some of my concerns are the length of the folding prop, the cantilever of its weight on the shaft attachment and Heaven forbid, the chance the blades don't open evenly.

On the other hand, I'm confident James has the analysis well covered and when complete it will be to his satisfaction. It's his satisfaction that matters, not ours. I'm hoping for a success report down the line.

edit:
now I can't remember. Folding prop?
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Old 05-15-2013, 12:12 PM
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I'd be drilling a small hole for a cotter pin to go thru a castle nut at the end of the shaft. I would not drill a large enough hole for a shear pin.
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Old 05-15-2013, 12:40 PM
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Not having thought this through completely, but here goes:
At the beginning of the old shaft drill through a hole for the appropriate brass shear pin. Then so the prop reaches the shear pin, enlarge the shaft of the propeller, for a depth of the shear pin.
Check out if cutlass bearing is far enough away.
You may have to cut a little more thread on the end.
Add cotter pin hole on end.
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Old 05-15-2013, 12:50 PM
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You could machine the shaft so the large area could act as a shear pin- engagement slot to drive the prop.
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