Stern Gear Arrangement for Long Keelers

[DOILY]

My winter's project is to reduce vibrations my Contessa 26.
Pre launch everything was lined up and there was no play in the cutless bearing. My 2 cylinder engine on flexible mounts hardly vibrates whilst in neutral, at all revs, but the vibes in gear at all revs have always seemed excessive to me. I can't be certain if the 3/4 inch propshaft is straight (to be checked this winter!). But I think the main suspect is the long length from gearbox to cutless bearing (50 inches!). This is way over the maximum 40:1 length to dia ratio advised elsewhere in this forum.
I think the proper engineering solution would involve cv joints (but I could be wrong of course). As I can't afford an Aquadrive, I'm wondering if anyone has made their own set-up using an automobile driveshaft (much cheaper) with a thrust bearing in a bulkhead bonded to the hull?
I currently have an R & D "Flexible" coupling (I can never figure that "Flexible" tag, as the hard plastic it's made of doesn't seem very flexible to me??) and a rigid, flanged stuffing box that was bolted to a half bulkhead that supports the end of the 14 inch long stern tube. I write "was" because this season, with more motoring than usual, all the vibes have shaken it loose from the bulkhead (eeek!), allowing water in, etc., etc.!!!
Your thoughts would be much appreciated. Thank you in advance.

 

[Moodysailor]

You currently have what I would state is a suitable setup - a flexibly mounted engine, with a flexible coupling between engine & shaft, the a shaft supported by the stuffing box at one end, and cutlass at the other.

The main issues I've seen/heard about are where someone changes the stuffing box for a dripless seal, which then removes a support member.

Back to your case: You have excessive vibration - this is evident as damage has occurred (loose stuffing box). So I would start at the beginning, and work to the end. In this case, my beginning would be the prop. I'd have the prop checked for pitch & balance, check the cutlass for wear, draw the shaft and have this checked for alignment, then check the engine mounts and ensure the loading on these is even (an engine supported on 3 of 4 mounts can bounce around badly). Then when re-installing, ensure everything is fully aligned, and then check again when back in the water.

If aligned correctly the prop shaft "whipping" that you appear to suspect may have caused the damage shouldn't be an issue - unless the vessel has been re-engined, the shaft is likely to have been that length all her life, so, whilst it's not optimal, my thoughts would be that the issue is elsewhere and the unsupported length is not the cause (but is where the fault is manifesting itself).

Good luck.

 

[Tranona]

The stuffing box should provide an interim bearing, and may well have an actual bearing inside the housing. So the unsupported length is much less than you suggest. My last boat was a long keel - although the shaft was only 35 inches long, but had a rigid Stuart Turner stern tube with a white metal bearing in the inboard end lubricated part by grease and part by water with the usual gland packing for sealing. I replaced that by having the housing bored to take a short length of cutless and the outside of the housing turned down to take a Volvo seal. This was to enable the fitting of a flexibly mounted Yanmar instead of the solid ST. A Bullflex flexible coupling was used for attachment to the gearbox. Worked very well, both with the bouncy Yanmar and later a much smoother Nanni 14.

In your case as I read it the first thing to do is to beef up the inner stern tube mounting so it is really a rigid bearing. This will allow you to use a more flexible coupling like the Bullflex. Your 3/4" shaft is not ideal, well underspec for 10hp diesel which needs minimum 1", but isolating it as much as possible from the engine and supporting it well will reduce the stress on it. Pretty sure you can get the Bullflex just pilot bored so it can be bored to take 3/4" - but as you probably know few makers list fittings or seals for this size.

Hope this helps.

 

[steveeasy]

A worn cutlass bearing makes alignment easier to mess up. Ide check your prop as Ive never experienced vibration when motoring. Tighten the stuffing box mounting bolts. sort the cutlass bearing then try to align the engine with the shaft using feeler gauges. id check the shaft is not worn as well. You could have a few things all masking each other. for instance the cutlass bearing could be very worn but the alignment being out would mask any wear when looking for movement.

Steveeasy

 

[andsarkit]

Have you got a rubber cutlass bearing? The original setup on the 26 with a 3/4" shaft had a tufnol sleeve for the outer stern bearing and if this is worn it can produce severe vibration/knocking. If you pump more grease in does it change the vibration? It is easy to check for play if you waggle the propellor when out of the water. I replaced the original tufnol sleeve with a delrin one which has been fine.