Which flexible coupling

Hello all
I am choosing between two flexible couplings for my new engine installation. It's a 10hp Volvo diesel going into a 29 ft sailboat that weighs 3 metric tonnes.
I have been told that I should have a flexible coupling to cut down on vibrations. I have accepted this, and also been sharply reminded that I still need to be prudent and align properly.
I am choosing between:
The R&D coupling, white plastic, that doesn't build much in the axial direction and costs 200 dollars.
The Vetus Combiflex, that is slightly wider and bulkier, and consists of steel parts with rubber inside. Complete with tha adapter flange this will cost me 500 dollars.
Then there is the sigmadrive, but that one is out of my budget.
Then there is an australian one called Isoflex, which consists of just a piece of rubber with threaded inserts in it. It looks sturdy enough, don't get me wrong. This would probably cost more than the Vetus and seems to be out of my budget.
Now, does anyone know if the 300 dollar prize difference between the thinner, white plastic R&D type one and the bulkier Combiflex is worth it? The combiflex is said to accept a 2 degree misalignment, but if this could be avoided anyhow, are the two equal in terms of vibration reduction?
I also have a question regarding my original Volvo coupling, but that is off topic and I will post it separately.
Greetings
Per Runhammar

 

And: does anyone know of a cheaper alternative? I don't mean to sound like that, and I do realize there is money in research and developement, but people must hava made vibration reducing couplings for as long as we have been industrialized... When I google I find a large variety of all kinds of rubber filled couplings. Does anyone know if there is a way around the large brands in the marine industry?
And, getting into dire straits now, are there any thoughts in the direction of DIY? I mean, I have four screw holes in the old coupling and four screw holes in the reverse gear. Could I find some kind of heavy duty rubber or plastic and make one myself? Have axess to lathe and milling machine. Is there anything in the automotive market that one could reshape?

 

Osculati 52.290.08 €53.70 + €24 shipping = €77.70 at your door. Elastisk kopplingsfog 8 borrningar - Osculati 5229008 https://www.yachtshop.eu/se/product/5229008/Elastisk%20kopplingsfog%208%20borrningar
If you want to avoid the high shipping charge ask the local distributors if they have it in stock. Osculati - Where to buy https://www.osculati.com/en/page/retailers
This is just the rubber element, if the hole patterns don't fit with your existing couplers you need to make two adaptor rings.

If you want to use something non marine, yes it's possible, you need to choose a coupler that whithstands the HP and torque rating of your engine.

 

Thank you so much. I wonder why I did not fint that one when I googled... Osculati is well known and respected.
I am in contact with them now.

 

I read somewhere (probably on this forum) that flexible couplings could lead to shaft ‘whipping’ or something like that.

If I remember correctly, the point was that there shouldn’t be too many flexible components, and with rubber engine mounts and rubber cutless bearing, the coupling wasn’t recommended.

Not sure if there are other factors that would make a difference.

I asked my engine manufacturer, but they didn’t want to talk about anything passed the transmission.

But I’m also not sure I’d completely trust the coupling manufacturer when they’re obviously trying to sell a product.


Can anyone elaborate on when/where a flexible coupling should/shouldn’t be used?

 

Hi Runhammar, the couplings, almost any type, should not induce vibrations themselves, they are usually balanced from manufacture, or so symmetrical off lathes and mills, as to be called balanced, (except for the keyways), especially for low speed, low power machinery. Engines, gearboxes, and exhausts can supply vibrations, but it's more likely from non straight shafts. A coupling rated for slightly more HP than needed causes less stress on the 'flexible' bits. I can't comment on your choices, not being familiar with them. A common off the shelf industrial coupling, as used on 3 phase motors, with 2 cast iron flanges linked by rubber discs on several bolts may be the cheapest to buy and get spare flexible parts for around the world, if your engine room is clean, - oil, fuel and vapors damages many flexible items. Whatever you use, never rely on the coupling to withstand misalignment, unless it cost thousands of dollars in any money. However the prop shaft wants to lie naturally, as far as angle and prop position goes, also considering the weight of the shaft end drooping when unsupported, bring the gearbox, and/or engine up or down or angle or across to it, and align the two flanges with feeler gauges and straight edges, with the drive bolted down. Bolting down often moves things out of alignment, especially with wood frames. It means bolt up, feeler gauge, unbolt, grind or shim or adjust, bolt up, feeler gauge, unbolt, bit more here or there, check again, until it is as good as you can get. When it is all bolted tight with the two flange diameters aligned all around, and the faces parallel, you are done. Then put the rubbers in, with thread locker on the studs. Check your prop shaft for run out - wobble - in the middle; and dodgy propellers can cause vibration too.
A whipping shaft usually means it is bent, or a bearing has failed.

 

While I haven't tried others I've had good luck with the R&D. It's nice that it doesn't need to be unbolted to check and as yet I haven't had one fail. One of my current boats has two with a bobbin in between. I did this in order to facilitate removing the transmission as I would have to haul out and drop the rudder or unbolt and move the engine forward. An added benefit is smoother running.