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-   -   Mirage 30 - propellor shaft strut (http://www.sailnet.com/forums/gear-maintenance/100000-mirage-30-propellor-shaft-strut.html)

Ken Bowering 05-30-2013 07:37 PM

Mirage 30 - propellor shaft strut
 
Dear Sir/Madam,

I’ve got a 1984 Mirage 30 sailboat (manufactured by Mirage Yachts in Pointe Claire, Quebec, Canada up till the late-80s when they closed the factory). My problem is that the strut that holds the cutlass bearing for my propeller shaft is slightly bent and should either be replaced or straightened.

The propeller and shaft are not on the centerline of the boat. They’re ahead of the rudder (which is on the centerline), but they’re offset to port by about 12 inches. So, the cutlass bearing strut is also offset to port by about 12 inches.

Inside the hull, and right above the strut, there’s a beehive-shaped “dome” of fiberglass and it appears as if the strut “fits” into a cavity inside that “mound”. There are two bolts that go horizontally (athwart-ship) through that “mound” and they seem to be what holds the strut in place. It seems unlikely that there’s a horizontal flange on the strut, inside the fiberglass dome.

Thus, apart from the fitting at the end of the strut which holds the cutlass bearing, it appears as if the strut may be just a flat piece of metal – no “horizontal” flange.

The shaft has been removed as have the two bolts that pass through the dome inside the boat, but the flange doesn’t move (I haven’t put much force on it to pull it out but, after almost 30 years. I would expect it to be rather tight).

The thickness of the “flat piece of metal” portion of the strut is about ½ to 7/16 inches thick (wide – athwart ship) and the overall length is about 8½ inches long (tall), the fore-and aft measurement is about 3¼ inches.

So, what I’m asking is whether anyone here has had experience removing and/or replacing the strut?

Faster 05-30-2013 08:35 PM

Re: Mirage 30 - propellor shaft strut
 
The strut in our boat seems similarly installed, no bolts visible at all.

Can't really help you with direct experience, but seems to be that if it's that solid you might try straightening it in place.. but I'd put the bolts back in first.

Bob Perry may have information on the strut install, although that may have been a builder's decision/method.

SchockT 05-30-2013 09:42 PM

Re: Mirage 30 - propellor shaft strut
 
Do you have a shaft alignment problem? Some manufacturers deliberately offset the strut so that the prop shaft can be removed without removing the rudder.

Faster 05-30-2013 10:02 PM

Re: Mirage 30 - propellor shaft strut
 
Yes, the mirage, like the older C&C 30s have the whole drivetrain angled 5-10 degrees off the centreline. But it sounds like the strut itself was a bit bent.

rugosa 05-31-2013 12:14 AM

Re: Mirage 30 - propellor shaft strut
 
If you are planning to remove the strut you will have to refair it on installation - so, grind the bottom paint off around the strut where it enters the hull. You will expose the existing fairing and determine if there is a flange that is thru-bolted through the hull, like the C&C installation. If thru-bolted remove the rest of the fairing material (might be able to pry it off with chisel) so you can access the bolt heads. Will need to do same inside to access the nuts/washers. A good telltale is if you can see a fine line in the the bottom paint around the strut.

Are you sure it is actually bent? If there are other M30's around do a visual comparison before taking on more work than necessary. Bending a cast bronze strut takes a lot of force. Have you ruled out misalignment, worn cutless?

SchockT 05-31-2013 01:19 AM

Re: Mirage 30 - propellor shaft strut
 
My strut is slightly bent, but it has been like that for as long as I've owned my boat, and the shaft lines up perfectly with the engine so I thought it best not to open that can of worms! I am guessing replacing the strut is no small project!

sony2000 05-31-2013 08:38 AM

Re: Mirage 30 - propellor shaft strut
 
If it ain't broke don't fix it. Murphy's Law. Just change the cutlass bearing.

Ken Bowering 06-02-2013 03:51 PM

Re: Mirage 30 - propellor shaft strut
 
Thanks for your comments/thoughts.

My preference is to remove the strut and have it straightened in a shop. However, I don't want to try to pull the strut out in case there are spurs or something inside that fiberglass dome, and I end up with a butcher job on the hull. So straightening it in place may seem like the only reasonable option.

I have ruled out misalignment. Had there been alignment, when the shaft was decoupled from the transmission, either the shaft or the engine would have "sprung" back into their proper place. But that didn't happen.

Have tried to contact the designer but so far haven't heard back. Expect he might not have been in on the builder's details but maybe can point me to someone else.

With the shaft removed, I can see where the shaft was rubbing on the hull. I can also see the cutlass bearing and, while it looks to be in good condition, while the shaft is out I'll probably replace it anyway. After all, it's been in there for almost 30 years.

Not sure how to go about straightening the strut in place and do so without putting a lot of lateral force on the hull. One thought is to get two steel plates - about 1/2 inch thick - and with 4 holes in each (holes about 1/2 inch in diameter. One of the plates would have a larger hole bored in it, and threaded.

Would put the plates on both sides of the strut/cutlass bearing holder, with the top of the plates against the top of the strut right where it comes out of the hull.

With the plates firmly in place, would then screw a bolt through the 5th hole in the one plate so that it pushes laterally against the strut.

Might be able to move it enough so that the shaft clears the hull.

Any thoughts on that approach?

sony2000 06-02-2013 08:59 PM

Re: Mirage 30 - propellor shaft strut
 
The owner of Mirage Co. is a long time member of my club but just left to work on his new home. I maybe able to track him down.
It would be much easier to straighten it with heat firstly, and secondly off the boat.
You can't deform the sleeve that holds the cutlass bearing, so this may sound nuts but an option is to cut off the sleeve, straighten the stem, then weld back the sleeve with the prop shaft in place to guarantee the allignment.

SchockT 06-03-2013 01:38 AM

Re: Mirage 30 - propellor shaft strut
 
If I were going to try to straighten a strut I would be inclined to make a jig that incorporates a hydraulic "porta power" ram or bottle jack, because I don't think you could get enough force out of a screw type. It all depends on where on the strut the bend is, but I would start with a thick steel plate that acts as a base for the hydraulic ram. You could then use long bolts or reddi rod to connect the base plate to flat bars that brace the opposite side of the strut on either side of the bend. The hydraulic ram could then be positioned to apply force on the bend without placing any force on the hull connection itself.

I would definitely try straightening the strut on the boat before I considered removing the strut completely. If it breaks? Well then it needed fixing anyway! ;)


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