Prop shaft slipped off transmission when in fwd gear

[OneBoatman]

The key and keyway of both the shaft and the coupling have to be a precise slip fit. The dimples act as but a backup assisting the key which is meant to transfer the torque from your transmission to turn the shaft. Any shaft misalignment causes excess vibration which most likely contributed as to how and why your coupling came loose. Do check that your cutlass bearing is still in good condition. There should no detectable slop or side play where the shaft passes through the strut. Next pull the shaft have a new keyway cut that properly fits a new shaft coupling. Be aware that shaft couplings are specific to both the shaft size and transmission bolt circle so make sure you have the correct one for your shaft your transmission. This does not cost a lot of money. Provide the new coupling and key stock to the machine shop and they will pre-fit the coupling and spot new dimples on the shaft.

[markxengineerin]

I was interested in finding out about this item.

Agreed, I have ordered one based on this thread without even having a real need- I just think it's a better solution than a rigid coupling. It will be interesting to see if it makes any difference in noise and vibration.

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

Indeed. This would also make the shaft come loose. Nothing is supposed to be moving, everything should be tight up together. From the looks of it your shaft maybe damaged. A split coupler would help make up for this. A new coupler and new shaft would solve the problem. But just switching to a split coupler and getting a proper fitting keyway might resolve it just as well. It's a B.O.A.T. It's supposed to be stupid expensive to fix.

I've received the new split coupling and it's a tight fit to my shaft, plus the key no longer has any play. The only problem is I can't push the shaft all the way in. Heating the coupling in place is not an option so I'm wondering what I could squeeze into the split to widen the coupling a bit so that I can squeeze the shaft by hand. Or is heat the only reliable way to do it?

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[gonesail]

they fit these things .. the coupler and shaft .. at prop shops. your shaft could be distorted from previous wear. but a new shaft gonna cost you a thousand or more. if you live in the SE US there's a shop in Brunswick, GA that can do it. good luck with it.

[markxengineerin]

A machine shop is the "correct" way to do this, but, budget solution I suggest is to sand the shaft down with 220 to 400 ish grit sandpaper until there are definitely no burrs sticking out of it. Same with the key, sand until it fits. Check if it is the "height" or "width" of key causing it to be too tight either in the coupling or the shaft. More likely the width is the issue, there should be some clearance on height. The width should be as tight as possible but still fit.

Be very careful with forcing a split coupling open with any force- often made of cast iron and very brittle and easy to break almost like glass. Preparing the shaft with sandpaper is much safer. This process does require some good judgement and caution.

[Scubaseas]

Originally Posted by LifesBetterWhenYoureBeating

I've received the new split coupling and it's a tight fit to my shaft, plus the key no longer has any play. The only problem is I can't push the shaft all the way in. Heating the coupling in place is not an option so I'm wondering what I could squeeze into the split to widen the coupling a bit so that I can squeeze the shaft by hand. Or is heat the only reliable way to do it?

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First make sure your shaft is clean and has no dings, ridges or is mushroomed on the end. Clean it up with 400 grit wet/dry or similar. Knock the edge off the shoulder at the very end of the shaft with a file or sand paper.

You can heat the coupler off the shaft in an oven or with a heat gun for about 15 to 20 minutes and place it on the shaft while still hot. Captain Obvious would point out to use a good pair of gloves or oven mitts. Make sure you mark the shaft where the coupler goes as you get one shot at this. If you want to open it up some use a cold chisel or screw driver wedged into the slot but that won't affect the non slotted part of the coupler. A light coating of anti-seize compound or grease may help here. It also makes it a lot easier to remove the coupler at a later date.

Use Loctite 242 (which is blue) on everything that is a machine screw or nut.

[markxengineerin]

If you want to open it up some use a cold chisel or screw driver wedged into the slot but that won't affect the non slotted part of the coupler.

I have snapped shaft couplings using this method, do not recommend, especially if the "split" feature has a sharp internal corner as they normally do. Cast Iron looks like steel but does not act like it- no elongation before it just breaks. You might get away with it if cautious but it would be the last thing I try.

[Scubaseas]

Originally Posted by markxengineerin

I have snapped shaft couplings using this method, do not recommend, especially if the "split" feature has a sharp internal corner as they normally do. Cast Iron looks like steel but does not act like it- no elongation before it just breaks. You might get away with it if cautious but it would be the last thing I try.

I don't disagree with you especially for a novice. I keep forgetting not everyone has 50+ years wrenching experience and would know when to stop. In this instance you might be right, might be better to leave it alone.

If he can slide it on until just where the split is, opening the coupler is not going to help him anyway.

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

First make sure your shaft is clean and has no dings, ridges or is mushroomed on the end. Clean it up with 400 grit wet/dry or similar. Knock the edge off the shoulder at the very end of the shaft with a file or sand paper.

You can heat the coupler off the shaft in an oven or with a heat gun for about 15 to 20 minutes and place it on the shaft while still hot. Captain Obvious would point out to use a good pair of gloves or oven mitts. Make sure you mark the shaft where the coupler goes as you get one shot at this. If you want to open it up some use a cold chisel or screw driver wedged into the slot but that won't affect the non slotted part of the coupler. A light coating of anti-seize compound or grease may help here. It also makes it a lot easier to remove the coupler at a later date.

Use Loctite 242 (which is blue) on everything that is a machine screw or nut.

I've cleaned the shaft as you suggested and it feels smooth now. I've also heated the coupling to around 100°C but I was only able to slide it onto the first 2cm of the shaft, which is the same I was able to when I tried to fit it with no heat.

I'm thinking there must be a ridge on the shaft? Or the coupling is deformed. Or could there be something else at play here?

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[Scubaseas]

How and how long did you heat the coupler up for?

How far does it go on the shaft if you put it on backwards? Same or less?

Is the boat in or out of the water?

Can you get access to a good vernier caliper or a micrometer?

"I'm thinking there must be a ridge on the shaft? Or the coupling is deformed. Or could there be something else at play here?" Could be both. Or one or the other. Coupler might be a bit undersized to allow hone to fit. Shaft might be buggered or out of round.

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

How and how long did you heat the coupler up for?

In my oven broiler for around 15 min. I measured its temp after that with an infrared thermometer and it read around 100C

How far does it go on the shaft if you put it on backwards? Same or less?

Same, and that made me realize it was something on the shaft so I sanded it a bit more. After that I was able to push the coupling all the way, but without the key as the only way I could get it to move was by wiggling while pushing. I'll have to remove it and do it again though as now that it's in place I can't turn just the coupling to line both keyways up

Is the boat in or out of the water?

In

[Scubaseas]

Take a marker to the shaft. Coat/paint the shaft with marker. Put coupler on. Rotate coupler. Wherever you do not see marker is your high spot. Or you may be able to see the high spot just from it being shiny where the rest of the shaft is not shiny from sanding it. Try putting some lube on the shaft like WD40 or oil. Some grease of anti-seize would be good if you can get it there.

I'd use just the oven set to maybe 200*C and leave it in there for 20 minutes. Heating it should make it go on a lot easier.

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

Take a marker to the shaft. Coat/paint the shaft with marker. Put coupler on. Rotate coupler. Wherever you do not see marker is your high spot. Or you may be able to see the high spot just from it being shiny where the rest of the shaft is not shiny from sanding it. Try putting some lube on the shaft like WD40 or oil. Some grease of anti-seize would be good if you can get it there.

I'd use just the oven set to maybe 200*C and leave it in there for 20 minutes. Heating it should make it go on a lot easier.

I'll heat it in the oven, and for longer, next time

Once it's in place I'll need to make a new dimple for the set screw as none of the 2 existing dimples match the position on the new coupler. I guess drilling with the coupler hole as a guide would not be a good idea as I could damage the threads? Is a center punch the only option? I'm not sure how easy it'll be to find one that fits into the set screw hole in the coupler. Are there any other alternatives?

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[Scubaseas]

Originally Posted by LifesBetterWhenYoureBeating

I'll heat it in the oven, and for longer, next time

Once it's in place I'll need to make a new dimple for the set screw as none of the 2 existing dimples match the position on the new coupler. I guess drilling with the coupler hole as a guide would not be a good idea as I could damage the threads? Is a center punch the only option? I'm not sure how easy it'll be to find one that fits into the set screw hole in the coupler. Are there any other alternatives?

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See https://marinehowto.com/a-new-prop-shaft/ and consider making a donation for his knowledge.

I would use a drill bit to mark the shaft. Grind a point on the non cutting end. Use a bit that fits snuggle in the set screw bore. Use it as a center punch to spot the shaft. Then carefully drill your set screw dimple.

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

See https://marinehowto.com/a-new-prop-shaft/ and consider making a donation for his knowledge.



I would use a drill bit to mark the shaft. Grind a point on the non cutting end. Use a bit that fits snuggle in the set screw bore. Use it as a center punch to spot the shaft. Then carefully drill your set screw dimple.

Yeah, I meant to donate a while ago but wasn't able to do so as I didn't have PayPal on my phone. Done so now!

Thanks a lot for the help, @Scubaseas.