propeller question

[Hymodyne]

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

[Dave Neptune]

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

[marthur]

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?

[Hymodyne]

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.

[ndutton]

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.

[2dogsnight]

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.....

[Hymodyne]

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

[Administrator]

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

[ndutton]

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.

(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!

[Sony2000]

Well I'm going to say it. Wouldn't it be better to have enlarged to propeller hole?

[Bingy]

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.

[Hymodyne]

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.

[67c&ccorv]

I think you will be paying to haul your boat out of the water again...only this time without a prop.

Just sayin'!

[romantic comedy]

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.

[marthur]

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?