flexible or not

[mick54321]

Hi Iam just preping my sigma 33, and noticed that the flexible coupling was cracked, its a plastic spider arrangement. I thought about replacing it with a vetus bullflex unit for £300 including adaptor. But do i need a flexible coupling the stern gland is floating the only other bearing on the shaft is the cutlass bearing, my land based engineering says adding a flexible coupling without the correct bearing arrangement is nonsensicle, please advise. Does the engine move that much on its mounts and if so wouldnt a flexible coupling not add to this instability without adding another bearing?

 

[mick54321]

the aquadrive system makes sense

 

[alan006]

If you don't fit a flexible coupling you risk wearing your cutlass bearing at a faster rate than otherwise. The other advantage of a flexible bearing is that it will cope with a small amount of misalingement.
If the present arrangement has worked in the past I would stick with it. I hope this helps.

 

[mikejames]

The flexible coupler absorbs the engine bounce and also impacts with the propellor are damped down a bit. It makes the need for alignment of the engine to the propshaft to thousandths of an inch less necessary.
But they do break. I have had to replace the one on Forethought where the prop shaft slipped and wore away one of the metal straps in the coupler. That resulted in extreme vibration and wear in the cutless bearings (volvo shaft seal has one , also the main P -bracket bearing. Also about a year of fiddling with the engine alignment until one day a piece of metal fell out of the coupling....

 

[roly_voya]

This arrangement would worry me as it eans the only thing between you and the sea is a bit of rubber and effectively the whole shaft is floating. Is this normal practice? I am used to seeing at leat a phosphor bronze bearing on the inboard end and maybe one at the engine as well if its a long shaft.

 

[john_morris_uk]

The arrangement is perfectly normal for small yacht engines. If there is any way that the shaft can fall out or foul the rudder, then put a jubille clip on the shaft to stop it falling through the stern tube in the event of catastrophic failure of something.

 

[saltwater_gypsy]

A simple coupling is perfectly OK for a Sigma 33. Carefully check the alignment when replacing as the crack may have been caused by the engine being out of line.
(ex Sigma 33owner, great boat!)

 

[scottie]

it sound like a R&D coupling
So change the R&D coupling for a new one and leave it at that
it is not a flexible coupling as such

you need at least one solid element in the three
IE engine mounts coupling shaft gland