Theory and Practice of Engine/Shaft Alingnment

[Slowboat35]

I have a new engine installation that is proving to be a great deal noisier than the original. A 3GM was changed for a Beta 35 and a Featherstream prop added on the old shaft (1 ¼" by 1 metre). The old vibration coupling was retained (see seperate post).
Although the new engine runs as smooth as you'd expect in neutral once in gear it sounds as if it is stirring a barrel full of loose nuts and bolts - horrendous din.
There is no obvious vibration (in 10 ton boat not surprisinng I suspect) but the noise sounds awful. So bad anyting over ⅔ revs is too much for me to stand.
Setup is engine on 4 flexible mounts via vibration coupling through grease-packed shaft gland and cutless bearing to prop maybe 2" further out.

How can I as a capable but non-specialised mechanic check alignments are OK?
Can this be done in the water?

 

[jwfrary]

Alignments should be done in the water.

Your coupling is a centaflex m127

Curious how you were able to get it to fit to the new gearbox, yanmar flanges are different in size slighly to the beta flanges and this likely the route of your trouble.

Have you used a spacer or adapter to achieve it?

Alignment with a centaflex is fairly straight forward, install with the cone in place on the shaft, it should slide in and seat in the coupling easily if it does not make adjustments.

Your shaft looks like it has a ridgid stern gland. If its flexible in anyway you may want to support the shaft of its weight while you do this.

Another option is to release the centaflex from the gearbox and use feelers gauges to assess the engine alignment.

In this case I think the coupling is incorrect and not centred on the drive flange which is the root cause of your issue.

 

[jwfrary]

Ps. Last feather stream I fitted also used a centaflex and a volvo glad on a 110cm shaft single cutlass in the p bracket. Owner found it difficult to tell he was in or out of gear owing to the absence of any vibration at all....

Somethings up!

 

[Graham376]

Darglow sometimes under bore the Featherstream hub so the prop fits further back on the shaft, 2" seems a bit far back. I shortened our shaft to bring it further forward. Have you increased prop diameter? If so, is it too close to deadwood?

 

[fisherman]

The accepted way when I fitted out a few boats in the 70s-90s, take out the flexi element, put flanges together, use a feeler gauge between faces test all round, max deflection two thou per inch of coupling diameter. Another useful way is if you can freely slide the shaft in and out of the coupling while bolted together, but that involves dismantling it..
My worst problem was a 80hp Ford four cylinder with heavy flywheel and PRM box. The aft engine flexi mount was at the flywheel which was also the centre of gravity, so the whole engine gyrated about that point. It needed for the engine mounts to be further forward and aft. I wondered whether I could have fitted another set of mounts sideways, rather like the anti torque bar on an old Mini.

 

[jwfrary]

Could the prop be keybound?

 

[Slowboat35]

The new engine was fitted by a Beta agent who ought to know about flange sizes...so I don't know if an adaptor was used. Anyone able to state the difference is between the Yanmar and Beta flange?

Prop diameter has increased by one inch and tip clearance is now about 2 inches - less than ideal. Deadwood ahead of prop is a square cutoff
2-3 in wide which is also less than ideal.

I have considered the keybound possibility and will investigate once out of the water.

Many thanks to all for your help.