Shortening Drive Shaft

[topmast]

So if all else has been checked,, the dial indicator showed no wobble at the shaft end, Shaft ran true on parallel rails at the lathe,.coupling was properly faced ,cutless was good and the prop checked out good. What else could it be? May as well include a new rubber isolation coupling plate in your cutting calculations.

[a64pilot]

Originally Posted by topmast

So if all else has been checked,, the dial indicator showed no wobble at the shaft end, Shaft ran true on parallel rails at the lathe,.coupling was properly faced ,cutless was good and the prop checked out good. What else could it be? May as well include a new rubber isolation coupling plate in your cutting calculations.

Well, I paid for the shaft to be removed and run out checked, I do not believe it was done though, I know the alignment that I paid for was not. So if I go the pay someone to do it route, I want to ensure this time it's done

What is a rubber isolation coupling plate? If that is a Globe Drivesaver, I had considered that, I think my prop is extremely well protected so I do not think I need the shear feature, but if it does help lower vibrations, and electrically isolating my expensive prop is a plus, I think.
But then I would need a hull mounted anode?

[topmast]

Pilot, I think several makes available. About inch and quarter thick .Plates set in rubber ,bolts electrically isolate shaft and aborbs vibes Prop and shaft (and rudder if metal) still need anode ,if hull mounted would need a shaft brush and internal wire to connect to thru bolt If only thing hanging out there is a Maxi prop the nose anode may be enough but very very important I learned quickly to replace twice a year (old woody lots of steel close by).If you don't think engine was properly aligned as paid for ,check the mounts and start again .

[jimbunyard]

Originally Posted by a64pilot

No telling, maybe shaft has been replaced? It only sticks out 2.5" so maybe a little over an inch too long, I would have thought that 2.5" would have been OK, but it's the only thing left that can be causing vibration, all else's has been checked.
I was going to cut it off and only leave .5 " and maybe one day add a globe shaft saver and isolate the shaft from the engine electrically, and maybe dampen some vibrations.


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I realize this is a probably a stupid question, given your aviation experience, but did you have the prop balanced and checked for trueness before you started?

[a64pilot]

Prop is balanced, it was new. If you have seen an Autoprop, not sure how you could check blade tracking as the things will spin in circles. It's sort of intriguing that the thing even works.
I have been assured that the prop is good, and have been assured lately that the problem is with my excessive shaft length, but I would have thought 2.5" wasn't too bad, but an Autoprop is I believe an excessively heavy thing compared to a normal prop, maybe all that mass makes it worse?


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[Pauls]

From memory, Dockhead posted about problems he'd had with an autoprop with bearings failing quite rapidly. Just a thought, if everything else checks out OK, you might substitute another loaner prop temporarily for the autoprop. If the vibration is gone then you've found the source. If you have a prop puller changing props in the water is a practical thing to do.

[over40pirate]

Haven't read all the replies, but if you have a vibration problem with the shaft, I would take the shaft out and examine it. I'l bet there is wear at the cutlass area.
I would go to a good prop shop and get whats needed, to do it right.
Most likely a new shaft, with a keyway, new coupling, faced off on the new shaft.
I can't see how a thru bolt can hold the shaft tight. I mean, it's not sqeezing the coupling on the shaft.

Do boats really have this setup as designed?

If you don't have one, I would also get a drippless stuffing box, while it's out.
Nothing like a dry bilge!

[boatpoker]

Originally Posted by a64pilot

Prop is balanced, it was new. If you have seen an Autoprop, not sure how you could check blade tracking as the things will spin in circles. It's sort of intriguing that the thing even works.
I have been assured that the prop is good, and have been assured lately that the problem is with my excessive shaft length, but I would have thought 2.5" wasn't too bad, but an Autoprop is I believe an excessively heavy thing compared to a normal prop, maybe all that mass makes it worse?


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The space between log or strut boss and propeller should be no more than one shaft diameter. This particularly important with such a heavy unit as the Autoprop.

[boatbod]

How many blades does your prop have?

[boatpoker]

Originally Posted by boatbod

How many blades does your prop have?

Autoprop's have three blades and they are very heavy units. I can't imagine how you would go about balancing one of these things as each blade swivels.

Google "autoprop image".

[petter5]

I have shortened several propeller axles in my life, ranging in size from 3/4 to 3".

Do not make things to complicated. Measure accurate how much you need to cut and mark the cutting position with a ordinary plastic tape (electrical tape) Forget about using a saw if the propeller axle is fitted within the boat, use a angle grinder with a thin cutting blade, preferably for saltiness steel, but a blade for ordinary steel will do fine as well. a 5/4" axle will cut in 30 seconds if using a proper thin cutting blade.

Remount the coupling and fix in correct position, check that the clearance at the cutless bearing is proper and correct. If that's fine, fasten the coupling to the axle by screw the clamp bolts to fasten the coupling to the axle. When done, measure the thickness of the fix bolt. Use a new and good quality drill in the same size as the fix bolt and drill the hole through the axle, using the old hole in the coupling as guide. Use low speed on the drill, and plenty of cutting oil of good quality.

[a64pilot]

There are two and three blade Autoprops, mine is a three blade.
They can be balanced, but this is a harmonic vibration, meaning that all is fine, then quite rapidly if I get much over 2,000 RPM suddenly it feels like the washing machine gets out of balance, so it's a self exciting vibration, very similar I believe to flutter.
Tacoma Narrows bridge is a famous example.
Shaft whip from an excessive overhang, would explain it. I hope that what it is, but I would have thought that 2.5" overhang for a 1.25" shaft would not be excessive, but maybe it is.
I believe that a pillow block bearing mounted mid shaft would also stop it, but that's more work than cutting the shaft, so I will start there.

I really do think I could do this myself, but I could also ruin a shaft too, and truly it ought to be removed and inspected for straightness and wear at the cutlass bearing area, vibration just might cause excessive wear.
So I'm torn with what to do, one part says get off your Butt and go cut this thing this weekend and part of me says, no make sure its done right, this is something that needs doing correctly or it may come to bite you out in the Boonies


This sort of shows what an Autoprop is, a little on how it works. It is an odd ball thing

https://www.youtube.com/watch?v=xdxY5Q7wXek

[Frankly]

I just put the last coat of bottom paint on my AP Sunday. Get in the water and swap it out later this week. I just replaced the aft motor mounts with a set of 100s (normal aft on the 3GM is 75). That AP will test your drive train to the max because of the huge mass. On Cbreeze the PO had shortened the driveshaft so that to get a doughnut zinc on the shaft I had to take 1/8" off the thickness.

When I replaced the shaft I added back enough length to get a zinc on, but I had the keyway slot machined long enough that I can shorten it right down to 1/4" clearance (no shaft zinc).

Getting that shaft overhang down would seem to be one of the secrets to getting the AP to work. Good prop (way to expensive IMO) but keep the original and the tools to swap it out onboard.

[Celestialsailor]

Being a former Machinist/Toolmaker/Engineer, I highly advise to haul out, replace the coupling style to a keyway style. It has the most strength compared to a through bolt. Less of the shaft is compromised. Then fit the shaft for your final measurement, then cut the shaft and your keyway. This is the way 90%+ are done and there is a reason why. You can do all the labor yourself (minus the keyway) when you haul. The experience will do ya good...

[a64pilot]

Frankly,
You have an Autoprop? Why a shaft zinc?
What are you putting on your prop anti foul wise? Mine is bare and I'm getting tired of scraping the thing, although it does come clean easier than I would have thought, being able to flip the blades around helps.
I have the special puller coming and I do keep the old prop as a spare.


I know thru bolting compromises the shaft, but I can't imagine the torque required to break a 1.25" shaft, so I don't worry about that.
But without a taper, isn't a key going to be weak as it is taking all the force?