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Sinkorswim
Sinkorswim

Posted: Sun Mar 31, 2019 9:05am

Post Subject: Prop Shaft

When changing gear from forward to reverse and vice versa my prop shaft moves in and out of the cuttle joint at the gearbox end by around four inches, revealing some of the prop shaft splines and causing a dull thump when engaging forward - as the prop shaft moves up to the stop on the joint with the gearbox. I've owned the boat for around ten years and this has always happened, and the thump can be decreased by packing the joint with grease. There are no screws or holes on this joint to fix the splines in place, and the whole shaft can only move by this degree as there are further joints at the prop end which stop it going any further. In effect this movement acts as a primitive extra torque converter when changing gear, but is this normal or do I have a potential problem?

Tony-B
Tony-B

Posted: Sun Mar 31, 2019 9:58am

Post Subject: Prop Shaft

I have no idea what a "cuttle joint" is. I also have no idea about the make and model of engine. gearbox and boat so I have no idea what you are talking about. There is no way a sliding splined joint can act ass a torque converter. I think I need photos, the make & model of the engine and gearbox, and a way of seeing the conversation a sit develops so I suggest that you email me some photos and the text of this and we will take it from there. Send to Tony@tb-training.co.uk

Tony-B
Tony-B

Posted: Mon Apr 01, 2019 6:22am

Post Subject: Prop Shaft

Spent a fair while last night Googling "Cuttle joint" to no avail so it must either be a very local name or the questioner has got it or typed it wrong. I know what a Cutless bearing is being the closest name related to stern gear I can think of but it has nothing to do with shaft couplings. I am afraid that this question will remain unanswered unless I get photos and more info

Sinkorswim
Sinkorswim

Posted: Mon Apr 01, 2019 9:03am

Post Subject: Prop Shaft

Hello there Tony. Yes I think I may have meant 'cutlas joint' so sorry about that. But you have it in one when you describe the joint as a 'sliding splined joint'. Now my issue is that when changing from reverse to forward the splined shaft slides into the joint and stops with the aforementioned dull thump. Similarly when moving from forward to reverse it slides out by around four inches, revealing some of the splines, though in this direction without causing any transmission 'thump'. Leave the boat in forward and the direction of spin ensures the shaft is entirely seated within the joint. This may of course not be a problem at all as it has always been the case, but I am curious to know if this is a normal mechanical set-up. I can supply photos when I am next on board but I hope this is a clear enough explanation. Many thanks for your time.

Sinkorswim
Sinkorswim

Posted: Mon Apr 01, 2019 9:29am

Post Subject: Prop Shaft

Having googled the joint a little myself, what I am trying to describe is a 'splined slip joint', I enclose a technical drawing of it. I think I'm beginning to conclude that the movement is intentional to and nothing to worry about, but I may be wrong?

Prop Shaft
Tony-B
Tony-B

Posted: Mon Apr 01, 2019 10:52am

Post Subject: Prop Shaft

I am sorry but I asked that you email me a photo and the same goes for the image you put on the forum. If you look at the forum you will see the image is too small to be of any use and I suspect it is of a vehicle component, not a marine one. Although some boats do use automotive style propeller shafts it really should have an additional component to absorb the propeller thrust. Until I can see a photo or photos of the back of the gearbox, stern gland and all the shaft I don't know what you have or have not got.It might even be that you are using an automotive gearbox. If this is the sliding joint part of an automotive style prop shaft then all that is preventing the whole of the prop shaft and propeller from sliding back into the rudder is a comparatively thin metal pressing that is threaded over the end of the splines but it does not sound like that. It is NOT a Cuttless bearing and as far as I know Cutless never made shaft couplings. Once again please email me at least one photo so I can see exactly what we are dealing with..

Sinkorswim
Sinkorswim

Posted: Mon Apr 01, 2019 4:52pm

Post Subject: Prop Shaft

Yes as I said I cannot email a photo until after I'm next on board, the drawing was the closest I could find to what is there. However, as you suggest, it looks like it may be an automotive type slip joint, and so I've now pretty much concluded that this slip joint is there to prevent binding of the u-joints on the shaft, (it's a long shaft, from a trad. engine room). Many thanks for your help with this.

Tony-B
Tony-B

Posted: Mon Apr 01, 2019 6:15pm

Post Subject: Prop Shaft

Thanks, things are becoming a little clearer now. Such shafts are often called cardan shafts although technically any intermediate shaft might be called that. Typically such shafts use spider/Hooks type universal joint at each end. In a vehicle such shafts will alter in length as the rear axle moves up and down on the suspension so a sliding joint is vital but on a boat, even with a flexible gearbox coupling (which I doubt you have because the universal joints are more than adequate to accept any slight misalignment as the engine jumps about) the shaft effectively remains a fixed length so no sliding joint is strictly required. This type of universal joint is NOT designed to take end thrust in either direction from the propeller and if there is any angle between the two joints there is always the potential for end thrust in ahead to force the shaft into a dog leg between the two joints. To use that type of cardan shaft to give optimum life and wear it should be accompanied by a trust bearing/block between the stern gland and first universal joint from the prop end. I am now sure that you do not have this and the cardan shaft is simply one taken from a vehicle. You seem to have had many years of use with it like it is and even though a thrust bearing would be ideal I suspect the simplest way to stop this sliding in and out would be to push the shaft fully into the head position, then pull it bank between 0.5" and one inch and the get a welder to tack the sliding part to the splines in a couple of places. Then give it a good test and if there are no more problems put a few more tack welds in. At present with the shaft sliding in and out by 4" you will be doing no good to the gland packing and whatever is stopping it sliding back so the prop hist the rudder is wearing away. You may have a thrust block on the actual propeller shaft that is badly worn, maladjusted or the thrust component is loose on the shaft. I await your photos when you can get them, especially one shoring the area between the stern gland and first joint.

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