Thursday, July 28, 2011

Cutlass Bearing Replacement

Or why I walk with a limp...

You'll never think of your cutlass bearing (or your colon) the same way again.

At some point in your life, you will go to your proctologist and he will say something to you along the lines of "You are old; it's time we looked up your butt".  No doubt he'll downplay the whole thing, but the bottom line (no pun intended) is that you are going to be given "roofies", then anally raped with a camera the size of a donkey.  When you regain consciousness your doctor will try to convince you that everything went fine, then demand that you pay him for something you don't quite remember happening, which on the whole you should probably be thankful for.

The shaft log inside the prop flange, as it
protrudes the bottom of the boat. The
propeller has already been removed.
"What could this possibly have to do with boats?", I hear you ask. Well, gentle reader, if you were a vessel "of a certain age", and you notice a slight vibration at certain engine RPMs, your boat mechanic (which is sort of like a doctor, but when they rape you they don't wear a mask) will tell you that it is now time for a "cutlassoscopy", which is the marine equivalent of a colonoscopy.

That means inspecting and replacing your cutlass bearing (instead of your colon)-- but without the benefit of anesthesia.  In other words, the entire procedure will be performed while you are completely conscious and able to feel pain, and no amount of gin and tonic will ever be enough to get the image of it out of your head.   It almost makes getting polyps seem like fun.

The Cutlass Bearing Sleeve (with a
 new bearing already installed).
For those of you who haven't ever peeked at the dark underbelly of your boat, it may be useful to give a brief explanation of those bits down there.  If you pulled your boat out of the water (which you'll need to do for this particular procedure) you'll see that the bottom of an L45 catamaran has a triangular flange where the propeller exits the hull.  Inside that flange is a long tube called the log shaft and inside that is the propeller shaft.   The log shaft itself is made out of fiberglass and molded in as part of the boat.  The cutlass bearing sleeve, a stainless steel pipe with a thick, stainless steel tear-drop shaped flange welded to it at right angles (see photo on left), is inserted into the shaft at the aft-most end of the log, and bolts to the  hull.  At the opposite end, inside your engine room, the propeller shaft extends out from the log shaft  and attaches to the transmission.   Where the shaft exits the log, there is a mechanical seal (see Propeller Shaft Mechanical Seal Replacement) which allows the propeller shaft to spin freely.  Otherwise your engine compartment would flood with sea water, causing you to wonder why the bow is suddenly pointing heavenly and your insurance agent isn't returning your phone calls.
The Cutlass Bearing.

Inside the stainless steel cutlass bearing sleeve sits the cutlass bearing, which goes around the prop shaft and keeps it from wobbling (which is why they call it a bearing, although it doesn't have any balls or rollers inside it), but it does allow the propeller shaft to rotate without seizing up.

The cutlass bearing is pressed, and sometimes even glued in place, (if, like ours, your bearing sleeve is worn a bit) into the cutlass bearing sleeve.   It looks a bit like a fiberglass toilet paper tube with a rubber lining inside.  The photo on the right is the new bearing before we installed it into the sleeve.  The cutlass bearing has a series of grooves running length-wise along the inside of it, which allow sea water to circulate along the shaft as it spins.  This cools and lubricates both the shaft as well as the mechanical seal at the other end of the shaft log.
The exposed prop nut, the
bearing sleeve has already
been removed.

The "non-metallic" interior of the bearing is a composite material.  There are also metallic (brass) versions of this, but we opted with the newer style, which are supposed to last longer, and have the added advantage of not reacting to electrolysis, which can cause pitting and even mechanical failure in the shaft and sleeve.  The markings on the bearing indicate "192/11", along with "Made in U.S.A" and some obscured printing that we couldn't read, but the size was definitely a 30mm inside diameter, and approximately 45mm  outside diameter, and approximately 120mm long, although our mechanic said that had it been slightly longer, he would have just trimmed it back.
The newly installed propeller,
prop nut & sacrificial zinc.

But before we could replace the sleeve, we needed to remove the propeller and shaft, in that order.   The first step was to remove the propeller nut (which holds on the propeller) and the the propeller itself (which has effectively been "pressed" onto the prop shaft's slight conical shape, is under tension, and needs to be mechanically pulled off), which isn't as easy as it sounds.  You'll want to first remove the existing sacrificial zinc attached at the aft end of the prop nut, using a metric hex key wrench.  Save the zinc if it isn't too badly corroded.

Next, put the engine in reverse and hold the propeller in place while you put an adjustable wrench on the two opposing flats of the prop nut and loosen it.   If you are under twenty, have the arm strength of Popeye, and don't mind holding a propeller blade with one hand while trying to torque the blade with the other, then this approach will most likely loosen the prop nut.   I tried this first, but I did not find it enjoyable when the propeller started to cut into my fingers.   Especially since I was standing on a rickety ladder about six feet off a slippery moss-covered cement ramp, and my swain (after fifteen days of what later came to be known as "The Triton Bottom Paint Debacle") now had a vested interest in things going badly.
The installed cutlass
bearing, in its sleeve.
There is a 19mm retaining
nut holding it in place.

Discretion being the better part of valor, I solved the problem the old-fashioned way.  I asked a yard worker for help.  He placed a 2x4 underneath one of the propeller blades, close to the hub, so that as we turned the nut, the propeller blade spun downward and jammed the 2x4 against the ground, effectively locking the prop in place.  If you use this method, be careful not to damage or bend the propeller blades while you are loosening the prop nut.  If possible, put a clump of rags or hard foam to protect the propeller.  It is also theoretically possibly to lift the boat up further off the ground using this method, but I'm guessing that if you were that strong, you would have used the first approach instead.

Next, we needed to break the propeller free from the shaft.  We couldn't just pull the propeller off because it had been pressed onto the shaft by the gorilla who tightened the prop nut the last time.  To remove it, we needed to use a special tool that looks a bit like a wheel (or bearing) puller.

You can see the slight conical
shape of the shaft where the
prop itself is squeezed onto it.
   NOTE: Do not attempt this without a nut on the shaft!  The propeller is actually under significant load, and breaking it free could cause it to release with an equally significant force.  In other words, not putting a nut on the shaft could mean that the propeller might fly across the yard, landing quite far away.  Given that our boat was still pointing back at the water, this could have caused a bit of embarrassment. 

We decided that being knee-deep in the bay looking for our prop and/or having to explain why it was sticking out of our head wasn't worth the humorous story later on.  Having a retaining nut loosely threaded onto the shaft (i.e. not up tight against the prop, but allowing enough space for the prop to break free) will prevent this from happening.  The good folks at Napa Valley Marina loaned us their  prop puller (ask your boat yard, they should do the same) as this tool is something a lot of folks wouldn't normally carry around with them but is essential when pulling off a prop.  Any decent yard will always have one on hand.

It looks a bit like a giant C-clamp with a fork on one end.  We placed the large forked end around the propeller hub and the other on the prop nut itself.  To remove the propeller, we tightened the prop pulling device's threaded rod, which presses against the prop nut (make sure there is some clearance between the nut and the propeller) which then pulled the propeller's hub towards the end of the prop shaft.  We were a bit concerned about doing this, as we were worried that it might damage the conical end of the prop nut where zinc slides on, so we used a large steel nut that threaded onto the (M20 X 1.5 thread) end of the prop-shaft.  I recommend that you keep one or two of these handy. If you need to pop off a propeller, these will prevent you from damaging the shaft threads or the prop nut.  You may need to exert enormous force on the nut, BTW, and we used a sledge to pound on the prop puller's back end to help free the propeller from the shaft.   When it broke free, we knew it; the prop "popped" back against the nut and the puller fell to the ground.  So did the person holding the prop puller, much to the amusement of the yard employees.
The exposed prop nut,
without the bearing sleeve.

Once the propeller was broken free, we needed to remove the 19mm retaining bolt at the top of the bearing flange.  Be aware that there is also caulking behind the flange, which helps hold the sleeve in place, so we slid a hack-saw blade in between it and the face of the shaft log to break the seal.   Once the seal was broken (enough to spin the flange) we used a large, soft faced (i.e. padded with a rag) set of pliers to rotate the flange in place.  We continued to rotate it back and forth while carefully pried the flange away from the boat.
The Cutlass Bearing Sleeve.  You
can see the small set screw threaded
hole that holds the sleeve in place.

We then removed the flange sleeve completely from the boat's log shaft, and released the hex studs on the side of the flange that help hold the cutlass bearing in place.   We had to scrape off a bit of paint, glue and caulk in order to access this area.   The next step was removing the old cutlass bearing.  The easiest way to do this is by placing the flange on a hydraulic press and using a plug just slightly smaller than 30mm to force the cutlass bearing free from the sleeve.   In some cases it may be necessary to first use a hacksaw from within the sleeve to cut the cutlass bearing lengthwise (being careful not to damage the sleeve itself) to help reduce the compression forces on it, especially if the old bearing had also been glued into the sleeve.  My recommendation is to give it to your boat yard mechanic and plead for help.   We tried pounding it out ourselves, but to no avail and resorted to begging for mercy.
Pitting on the shaft lining.

Once we had the old cutlass bearing removed from the sleeve,  we visually inspected it and discovered pitting at the inside edge nearest the mechanical seal, and what looked like a small amount of wear near the aftmost edge.   We surmise the former was caused by electrolysis, and the later because the old cutlass bearing was wearing.  Much later on we were told by our marine electrician that the pitting was most likely "surface crevice corrosion" caused by inadequate water flow past the shaft.  The claim being that if a boat sits too long without moving, the water surrounding the stainless steel becomes oxygen depleted, which then robs oxygen from the stainless, which leads to this type of corrosion.  It makes sense, given the observable evidence, but whose to say what really caused it?  Another tidbit we learned is that very often electrolysis will occur inside the shaft near the weld joint, if the alloy used in the weld didn't match exactly the shaft/plate alloy.  The propeller shaft itself looked fine in the corresponding areas.  The next time we change out the bearings, we'll check again to see if this became any worse, but for now it seems okay.

There was one problem though; our new cutlass bearings did not fit "snug" inside the sleeve (our mechanic claimed they usually have to press them in place), and he was concerned that we would not be able to tighten the bearing set screw enough to prevent the cutlass bearing from spinning freely within the sleeve, which, in his words "would be bad.  very, very bad.", and then thunder rang out and a wolf howled in the distance.   His advice was to use something like JB Weld 2 part epoxy to cement the sleeve in place.   This is what we ended up doing; we're not sure if this was the best thing to have tried, but we've only had good advice from the folks at Napa Valley, so, if this turns out badly we'll let you know.  Anyway, we smeared some epoxy on the bearing and slid it up into the sleeve and allowed it to harden.  We were also advised to pre-drill a small divot into the cutlass bearing itself such that the set-screw would fit into it, and to not tighten the set-screw so much that it would deform the shape of the cutlass bearing.

We then re-installed the new cutlass bearing in the sleeve, first applying "NEVER-SEEZ" anti-corrosive paste to the exterior shaft of the sleeve that contacted the shaft log (this wasn't absolutely necessary, but we thought it couldn't hurt), and also applied BoatLIFE's "Life-Calk" polysulfide sealant to the inside face of the flange that came in contact with the (aft-most) shaft log facing.  That part was necessary to keep water from leaking in between the shaft log and the bearing sleeve. We also filled the circular trough in the facing with caulk (you can clearly see the trough in one of the earlier photos), and then pressed the flange up against the facing, fairing out the caulk and making sure it created a good seal around the flange.  We then re-installed the set screws that hold the cutlass bearing from turning (also coating them with anti-seize compound).  Once everything was in place and tight, we painted the exposed surfaces with bottom paint and re-installed the propellers, the prop nut, and the zincs.

Of course, we first put the boat in neutral and turned the prop shafts to make sure that everything spun true and freely, before putting it back together.   Going back in the water was a bit nervous making, but nothing leaked and the boat ran fine at all engine speeds, without any vibration.   This seemed to fix the slight vibration we had noticed earlier, and it hasn't returned since.   If I had to do this all over again (which I'm assuming will be the case eventually) I think I'd have ordered (or had made) a new set of stainless steel sleeves (seems like a good thing to have on hand anyway), and perhaps taken that opportunity to add a shaft vibration dampening plate, or even a new prop shaft, but that is probably just wishful thinking on my part.  As it stands now, we're delighted with the result and are looking forward to many more years of faithful, maintenance-free service.

And having also undergone a colonoscopy around the same time (with similar results) I can honestly say that between the two, the medical procedure was faster, less painful, and covered by my insurance.  

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