Re: Prop Shaft Squeak

James Alton
 

James,

   I replaced the original Amel engine mounts on my Maramu #220 this year.  The part you have circled is the bronze stern tube which will extend into the hull.  It appeared that mine was bonded and glassed into the hull, it needs to be a rigid connection.  The stern tube provides the attachment for the short pc.  hose that the stuffing box gland attaches to, it is not supposed to be a bearing.  There should be relatively even clearance between the stern tube and the shaft which allows for some engine movement without the shaft touching anywhere.  As I understand things the cutlass bearing on our boats forms the aft bearing and the transmission bearing provides the front bearing to support the shaft.  Nick already mentioned the two most likely causes of your squeak but I will try to elaborate a little more.  Engine alignment is more than just getting the two faces of the coupling mated within a few thousandths, you also need to position shaft so that it is riding close to the center of the bronze stern tube so that it cannot touch.  The fancy way to set up this alignment is to remove the stuffing box and the short pc. of  hose and fit something solid like round rod of the right OD to hold rigidly hold the shaft in the very center of the stern tube, then to adjust your engine so that it is the correct height and also correctly positioned Port and Stb.  Once this is done you can do your actual alignment to get the coupling faces aligned.  An easier method that seems to work fine and can be done in the water is to decouple the shaft and by HAND force the shaft coupling to go as far as it will go up down and Port/Stb.  You will feel solid contact as you force the coupling to it’s limits as it touches the inside of the stern tube.  Note the relative displacement of the couplings to Port/Stb. and Up/Down and adjust the engine so that the displacement is the same top and bottom and side to side which will put the shaft pretty close to the center of the hole in the stern tube.  Once this is done,  align your coupling faces.  Also I initially only did a rough alignment on my engine because I knew that the new mounts would settle a bit and they did.  So after 50 hours I did a second alignment and sure enough the whole engine had settled a bit so brought it up.  

   As to the other possible causes of the squeak,  as Nick alluded is that the shaft could be bent.  You can often tell this when sailing with the engine off and the wheel turning.  If the back of the engine is moving around much in time with the shaft rotation, you could have a bent shaft,  if it is nice and smooth then probably not.

   One final possible cause of a squeak can be from having the stuffing box too tight.  With the new teflon packings you can pretty much tighten to the point where you don’t have any leakage but if you go too far the box begins to heat up, grab and squeak.  

Best of luck,

James Alton
SV Sueno
Maramu #220
Preveza,  Greece


On Nov 23, 2019, at 11:35 PM, James Lochhead <j_lochhead@...> wrote:

HI,

The attached photo is of the stuffing box and where the shaft on my maramu exits the boat.  Is the circled part another bearing that is glassed into the hull?

We have a "sqeak" or hum when the prop shaft is spinning at certain revs (both with engine and when sailing and free spinning) that we have not been able to diagnose.

The problem started following the failure of our engine mounts.  The engine mounts have been replaced and the cutless bearing was replaced at next haul out, however a squeak has remained on our last short cruise before hauling out again.  I have checked the engine alignment, the stuffing was replaced, but the squeak remained.

We will be back on the boat again in the new year and would like to eliminate the cause for this before going back in the water and if this is a bearing then possibly order one now.

--

James
Maramu #147 (1984) <Shaft photo.pdf>

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