Flexible Shaft Coupling

[Alan Wheeler]

Thanks Dave. That's a great help. Without running the numbers, I have to say I am rather shocked at the size of the prop for the boat and engine you have. Now OK, the suppliers surely are the experts of there own product and I have no experiance with this particular product, so I stand to be corrected. But seeing this first off with not running figures, the prop to me sounds plainly waaay to big. OR, there is something dramaticly wrong with one blade or the way it is deployed.
I have to go to work now, but when I get home tonight, I will run some numbers and see what I can come up with.
If anyone else wants to do that, feel free. I won't be offended. And especially anyone that is more familiar with this prop or similar folding props.

[dkall]

Thanks Alan, any assistance is appreciated.

[SkiprJohn]

Hi Dave,

I think with Wheels help you will be closing in on the problem. I truly believe there is something out of balance on your prop and I think it is oversized. You can check out the thread at Proper Prop Size on this forum for some links and graphs if you'd like. Autoprop should make it right for you once you determine what the proper diameter of the prop should be.

Gook luck

JohnL

[delmarrey]

Proper prop OD is determand by the distance between the prop and hull. My 40' boat has a 19" prop with a 1" shaft. A 21" prop doesn't sound that far out for a boat of dkall's size, especially with a 1-1/4" shaft. These are the spec's.

http://www.yanmarhelp.com/images/propaperture.gif

[SkiprJohn]

Aloha Delmarrey,
That photo you sent of the flexible coupling from R&D is the exact same one that West Marine sent me. Those things don't feel flexible at all do they? Anyway I had to return it because it was the wrong size (their packaging problem).
They must be made an extremely tough plastic that flexes just a tiny bit.
Anyway, thanks for all the good information.
Kind Regards,
JohnL

[delmarrey]

Yeah! Sometimes it's better the look up the numbers yourself, online. I got the wrong one too, from a local marine dealer (cheaper). http://www.randdmarine.com/flexiblescsearch.asp

They are suppose to be tough but allow just enough flex to absorb some of the vibiration. I believe the rubber, or what ever you call it, is the same as what is used for the new motor mounts..........................._/)

[Alan Wheeler]

Hi again Dave. I have pondered this all day today while doing other engineering jobs for freinds.
Firstly, forgive me if I ask any question that has already been asked. I read so much I forget who asked what when and where.
I guess one question I forgot(I think) to ask was, is the boat in or out of the water? If out, then making some measurements is going to be easy.
This is what I have been thinking and I may be wrong, but.
First you need to ensure two measurements are correct. Use Dels diagramme above. We need to know The distance the prop is away from the shaft bearing is correct. This is "d" in the diagramme. It sould be between 1.25" and 1.8".
The other measurement is "a". This should be no less than 3.15".
Now trying to suggest anything else on a prop that is self adjusting is next to useless. Duh! I should have thought of that this morning, but I don't think well in the morning.
There are a few things I am thinking.
1:, the prop is too big for this engine. Leave aside the fact that the boat maybe able to take a 21" prop, the issue is that I don't think the engine can spin it due to Hp, or lack of it and factored into that is the shaft speed. a 1.9:1 box is actually quite fast for a 21" prop to spin.
So I think there maybe an issue in that lot first off.
Next, even if the prop is wrong and too big, I can't see this ever bending the shaft to a point of seeing a bow in it. This is quite disconcerting. The effort to bend a 1.25" shaft from a fulcrum point of only an 1" or 2 from the bearing is huge. Humungusly huge. Plus the damage it must be doing to stern bearing and to transmission bearings must be terrible.
But the only thing I can think of that would do something like this (within this context) is that the blades are not deploying properly. So I would get that prop inspected that all is well. Which then takes me to the next possibility then.
It may be possible it is one other problem. The prop lifts itself through the water. This force is transmitted along the shaft and to the gearbox. The gearobx is connected to the engine. The engien connected to the boat and so the boat is pushed along. Now if the shaft on the inside of the boat is too long between the stern bearing and the gearbox, the "push" along the shaft will push the gearbox and motor off centre. Especially if the motor is soft mounted. This will allow a bend to occur in the shaft and as the force increases, so will the bend. It is possible to get it to bend in either forward or reverse if that motor is pivoting on a broken or faulty mount. I imagine you would notice the bow on opposing sides of the shaft between fwd and rvs if that was happening.

So I hope that gives some addition info to check that maybe you haven't checked before.
fingers crossed

[dkall]

Originally Posted by Alan Wheeler

I guess one question I forgot(I think) to ask was, is the boat in or out of the water?

It is in the water

Originally Posted by Alan Wheeler

If out, then making some measurements is going to be easy.
This is what I have been thinking and I may be wrong, but.
First you need to ensure two measurements are correct. Use Dels diagramme above. We need to know The distance the prop is away from the shaft bearing is correct. This is "d" in the diagramme. It sould be between 1.25" and 1.8". The other measurement is "a". This should be no less than 3.15".

It is longer than that. I had sent this image to the autoprop people with these dimensions. They sent me a diagram to hold up to the prop shaft to make sure it would fit w/ what they needed for the prop and the room to rotate and then they're (I think) 15% or 20% margin.

Originally Posted by Alan Wheeler

Now trying to suggest anything else on a prop that is self adjusting is next to useless. Duh! I should have thought of that this morning, but I don't think well in the morning.
There are a few things I am thinking.
1:, the prop is too big for this engine. Leave aside the fact that the boat maybe able to take a 21" prop, the issue is that I don't think the engine can spin it due to Hp, or lack of it and factored into that is the shaft speed. a 1.9:1 box is actually quite fast for a 21" prop to spin.
So I think there maybe an issue in that lot first off.
Next, even if the prop is wrong and too big, I can't see this ever bending the shaft to a point of seeing a bow in it. This is quite disconcerting.

Bend may not actually be the right word. It is difficult to see spinning but the video shows that it is not running true any more. How much the rubber bearing provides a cushion for I don't know.

Originally Posted by Alan Wheeler

The effort to bend a 1.25" shaft from a fulcrum point of only an 1" or 2 from the bearing is huge. Humungusly huge. Plus the damage it must be doing to stern bearing and to transmission bearings must be terrible.
But the only thing I can think of that would do something like this (within this context) is that the blades are not deploying properly. So I would get that prop inspected that all is well. Which then takes me to the next possibility then.
It may be possible it is one other problem. The prop lifts itself through the water. This force is transmitted along the shaft and to the gearbox. The gearobx is connected to the engine. The engien connected to the boat and so the boat is pushed along. Now if the shaft on the inside of the boat is too long between the stern bearing and the gearbox, the "push" along the shaft will push the gearbox and motor off centre. Especially if the motor is soft mounted.

The engine is mounted on flexible mounts. However the movment of the shaft visually appears to be near the stern bearing more then at the engine coupling.

Originally Posted by Alan Wheeler

This will allow a bend to occur in the shaft and as the force increases, so will the bend. It is possible to get it to bend in either forward or reverse if that motor is pivoting on a broken or faulty mount. I imagine you would notice the bow on opposing sides of the shaft between fwd and rvs if that was happening.

So I hope that gives some addition info to check that maybe you haven't checked before.
fingers crossed

With all we've discussed it still seems to come down to the prop is to large for the boat. And I've yet to hear more from Autoprop. Expect I will email them this week making sure they haven't forgotten me.

[Alan Wheeler]

I think I need to see that video. I'll send you a PM.

[dkall]

Thanks, it's in your email.

-dave

[Alan Wheeler]

Thanks Dave, the last two worked just fine.
OK.
HOLY CRAP BATMAN!!!!!!!!!!
I have never seen anything like that. However, I think the issue is totaly different than the prop it's self. I think the thrust transmited up the shaft is pushing the gearbox and engine ascew. You may have a failed engine mount. Either it is broken or too light for the application.
However, I would like a second opinion and am wondering if you could send the two vids to "Never Monday". Pat may have seen something like this.

[knottybuoyz]

Send it over here too and I'll pass it by one of the engineers.

laporter at dfo-mpo.gc.ca

[dkall]

I sent it to Rick. Both versions that worked for you Alan. Funny about the engine mounts, I'll inspect closer but at the dock it doesn't begin to vibrate till about 1800 and in reverse moving it doesn't vibrate till a higer rpm too. Also if memory serves me correct the excessive vibration isn't at the engine but when I get home I'll have to check.

There is some comments on excessive vibration from another boater and he was given some blade changes by Autoprop. Also the vibration only really happens when I begin to exceed hull speed. Obviously at the dock that's not the case. Can't find "Never Monday" in the members section so I don't know who he / she is. Rick I did send it to you at the address you listed but is it really dfo-mpo or dfo.mpo? Hopefully you'll get it. I sent the last two of which Allen was able to open one.

[Alan Wheeler]

Well it could still be the prop. But if it is, it means it is even scarier than the vid shows. From what I could see, it looked as if the shaft was pushing the gearbox over to one side. Is that what was happening??? That's why I suggested checking the mounts, because that is some distance for the engine to be pushed.
So if the mounts are OK, then I think we need to go back to the possibility the prop is causing the inbalance. If it is the problem, then the stern bearing must be being wrenched around sidways to some horrific extent and it could cause severe damage to something in the end. That is some scary amount of flex.
If another person has had a vibration issue as well, then it means the prop being faulty is not out of the question. I would be interested to know what the issue was with this other guy's prop.
In your case, the only thing I can suggest is one blade is simply not adjusting the same as the others, throwing it out of balance and horificaly loading one side of the shaft.

You might have to PM Never Monday to get Pat to Email you his Address.

[never monday]

I'm here, I should have the vid any min.
BRB