Engine Alignment - Prop Vibration.

[john_206]

Recently bought a boat that has had its engine replaced (Perkins Perama M30) wich is different than was originally fitted.
The replacement job doesn't look the best in the World and the engine has been bolted to a large make up bracket on each side.
Whenever I try to put on any power from tickover I get really bad vibration almost surely due to misaligned prop.

I want to slacken the mounting bolts and can use the steel girders of the hull to line up the engine in a straight line to the stern tube. But how do I line it up vertically? ie in case the engine is sitting half inch to low or to high?? Is it just try it and see or is there a scientific way?

 

[gjeffery]

Good news is that correction of vertical mis-alignment is easier than horizontal mis-alignment.

Assuming four jacking bolts, one on each mount, calibrate the bolt pitch, ie n thou per flat. Then centre the shaft coaxially in the stern tube using same adjustment on all nuts..

Using difference in 12 o'clock and 6 o'clock gaps measured at the shaft flange, vertical mis-alignment fore and aft is calculated by basic trig, Set calculated turns on fore and aft jack nuts to correct misalignment.

Then reiterate until flange gaps are identical..

I can dig out my spreadsheet on an E&OE basis, if that helps.

 

[mikejames]

And dont forget the possibility of use of a semirigid coupling which can absorb a certain misalignment.

My 'I cant be bothered' method (** applicable to a small engine with a semirigid coupling and a Volvo shaft seal which tolerates small vertical misalignment of the shaft, with the boat in the water **) consists of trapping a feeler gauge in the flange gap , loosely bolting the flanges together, then rotating the shaft and noting if it falls out somewhere consistently as you turn the shaft. Adjust the fore and aft engine supports until the feeler gauge does not fall out.

Then check that the b***dy shaft is still the right height in the stern gland.

I did a lot of this alignment work before I discovered the reason why I couldnt align the engine to the shaft without vibration was because the coupling had a broken metal strap in it which fell out when I disconnected the engine to do a clean-up on it.

 

[Stemar]

Engine alignment is a real pain in the nether regions. Nothing will change this, but Preparation H in this case is a flexible coupling. I have a Vetus one that cost about £120 including a solid gold adapter plate. (Well, at the price it should have been)

It allows 2 degrees misalignment, and also allows for a certain amount of vertical or lateral displacement as well.

BTW, before trying to align a prop shaft, it's worth making sure it isn't bent. That really makes aligning fun! /forums/images/graemlins/frown.gif

 

[Aja]

Steve

My understanding of flexible couplings is that they do not allow for misalignment.

They only allow for engine torque on flexible mounts.

Donald

 

[fisherman]

My information is that the flanges should be within 2 thou per inch of coupling diameter, (and this can be doubled with an R&D flexible coupling, but I don't see why you would allow it to be so). It should be done with the boat afloat.

 

[Aja]

I disagree

[ QUOTE ]
My information is that the flanges should be within 2 thou per inch of coupling diameter, (and this can be doubled with an R&D flexible coupling, but I don't see why you would allow it to be so). It should be done with the boat afloat.

[/ QUOTE ]

From the R&D Website

"The R&D 910 Series couplings consist of a contoured flexible disc moulded in tough yet resilient new type polyurethane. The contoured disc gives clearance for the bolt heads, and is able to flex freely to take up any temporary misalignment of the engine and shaft, due to flexing of the boat structure or the engine moving on its rubber vibration isolation mountings...."

A better solution to vibration IMO would be a constant velocity joint type fitting. Though agree that careful alignment of engine to prop shaft is by far the best way forward.
Donald