R&D Flexible couplings

[boatmike]

As usual when refitting and old boat one comes across questions regarding modifications done to the original spec. On my "new" boat, which is a 1989 Aqua-Star Oceanranger one such mod is that a previous owner has fitted R&D couplings on both Volvo TAMD41A output shafts. Unfortunately they have not moved the engines back by the thickness of the coupling (roughly 450mm) so the whole lot is now jammed so tight up to the stuffing box that it is impossible to dissassemble. He must have moved the engine to get it in and then moved it back on it's original mounts. This in itself is not ideal but it does run OK like that although clearances are minimal. The big problem is that the gap from the P bracket to the prop is now about twice what it should be at about 830mm which I am wanting to reduce to fit rope cutters.

I am therefore faced with 3 solutions.

1. Remove the couplings and bin them as I am fairly sure the boat originally didn't have them.
2. Move the engines back by the thickness of the coupling which entails re-tapping engine bearers etc which is difficult with engine in situ.
3. Shorten the shafts which entails machining etc and would still leave an assembly too tight to the stuffing box for comfort.

With all things considered alternative 1 seems the easiest but before I do is there anyone out there with a similar boat that can tell me if they have a coupling fitted or are running flange to flange and what their experience is?

Much obliged as always for constructive comments.

 

[PCUK]

Obviously don't know all the details about your installation but could you shorten the shaft tube to move the stuffing box further back. This would solve the clearance problem.
You would still need to shorten the shafts though and I think this would be the easiest if not the cheapest solution overall.

By the way, are you sure about your measurements as the R&D coupling isn't 18 inches (450mm) long and 33 1/4 inches (830mm) is a hell of a long way between the 'P' bracket and prop'.

 

[boatmike]

Obviously don't know all the details about your installation but could you shorten the shaft tube to move the stuffing box further back. This would solve the clearance problem.
You would still need to shorten the shafts though and I think this would be the easiest if not the cheapest solution overall.

By the way, are you sure about your measurements as the R&D coupling isn't 18 inches (450mm) long and 33 1/4 inches (830mm) is a hell of a long way between the 'P' bracket and prop'.

Oops! decimal point... try 45.0mm and 83.0mm. No the shaft tube can't be shortened, and as you say it wouldn't solve the overhang problem either.

 

[Tranona]

Oops! decimal point... try 45.0mm and 83.0mm. No the shaft tube can't be shortened, and as you say it wouldn't solve the overhang problem either.

Leaving aside the issue of whether you need flexible couplings, the overhang is not excessive. You could, in fact fit strippers straight into that space. You would still have about 30mm between the cutter and the prop, but it would not affect the operation of the cutter. If you removed the flexible coupling, you would then not have enough room to fit strippers (I am assuming you have 1 1/2"+ shafts) so would need to insert a spacer where the R&D is now - probably 15mm thick, which would increase the clearance to the stern glands.

 

[Latestarter1]

1. Remove the couplings and bin them as I am fairly sure the boat originally didn't have them.

JFDI

 

[boatmike]

Leaving aside the issue of whether you need flexible couplings, the overhang is not excessive. You could, in fact fit strippers straight into that space. You would still have about 30mm between the cutter and the prop, but it would not affect the operation of the cutter. If you removed the flexible coupling, you would then not have enough room to fit strippers (I am assuming you have 1 1/2"+ shafts) so would need to insert a spacer where the R&D is now - probably 15mm thick, which would increase the clearance to the stern glands.

I am not fitting strippers, I am fitting quickKutters from H4. These in my opinion are the best on the market and although expensive are thought by the RNLI to be better than scissor or knife types for various reasons, but I don't want to have a discussion over the merits of various types of rope cutters as this decision is already made. The point is they don't require the overhang of scissor types and will work better with less.

You may be correct that to fit strippers rather than quickKutters I would need to fit a spacer which is probably why the R&D couplings were fitted in the past as some form of scissor type rope cutters were fitted at some stage by a previous owner. The P brackets have mounting bolts still. The boat didn't have them on when I bought it though and I don't want to retrofit this type as I consider quickKutters better. I therefore want to reduce the overhang.

My real question was regarding the experience of other boat owners with or without R&D couplings. It is my opinion that in the setup I described they don't add value and are unnecessary. I was hoping to hear from owners of other similar boats who could confirm that they have direct flange to flange bolted drive shafts without flexible couplings without experiencing problems.

 

[volvopaul]

The max amount of shaft allowed before the shaft flexes from the end of the p bracket to the prop is 1.5 times the diameter of the shaft according to Clements .

As for the coupling you could cut the shaft and fit a new clamp type coupling and pin it to the shaft if your are taper type couplings.

 

[scottie]

I am not convinced that with the shaft being that close to the glands the r&d would do much good .

 

[Tranona]

You may be correct that to fit strippers rather than quickKutters I would need to fit a spacer which is probably why the R&D couplings were fitted in the past as some form of scissor type rope cutters were fitted at some stage by a previous owner. The P brackets have mounting bolts still. The boat didn't have them on when I bought it though and I don't want to retrofit this type as I consider quickKutters better. I therefore want to reduce the overhang.

My real question was regarding the experience of other boat owners with or without R&D couplings. It is my opinion that in the setup I described they don't add value and are unnecessary. I was hoping to hear from owners of other similar boats who could confirm that they have direct flange to flange bolted drive shafts without flexible couplings without experiencing problems.

If you are going down that route then you do need to substantially reduce the overhang, and suspect even if you choose to ditch the couplings you may still have to shorten the shaft, but no doubt Neil will advise the space you need between the P brackets and the face of the prop.

 

[boatmike]

The max amount of shaft allowed before the shaft flexes from the end of the p bracket to the prop is 1.5 times the diameter of the shaft according to Clements .

As for the coupling you could cut the shaft and fit a new clamp type coupling and pin it to the shaft if your are taper type couplings.

Totally agree Paul. Thats why I said the overhang was too much in the first place at 83mm which is 2.2 times the diameter. Taking the coupling out gives me an acceptable overhang re: Clements as you indicate.

Everything points to the conclusion that the original designed set up was direct flange to flange. In your experience would this be relatively normal for TAMD41a's ? The engine bearers seem very hard and the shaft bearings are both cutless. The most inboard being approx 18 inches back from the flange on a 1.5 inch shaft. I would have thought the coupling was only fitted to give extra distance aft of the P bracket to fit the strippers wouldn't you?