01 A New Prop Shaft

Cutlass Bearing Removal

Every now and then a sailboats prop shaft needs replacing. Damage to the shaft can be caused by corrosion, bending or wear at the stuffing box or cutlass bearing surfaces.

On many boats the removal of the cutlass bearing can help avoid the necessity of dropping the rudder. On this boat, an Ericson 34, the shaft just barely slid by the rudder with the cutlass bearing removed. This saved hours & hours of labor time by not needing to drop the rudder. To remove the cutlass bearing I used a Strut-Pro tool.

Before the nitpickers come out in full force the word “Cutless®” is a trademarked brand name of Duramax Marine. I therefore use the alternative and widely accepted generic spelling of “cutlass” so as not to infringe on a trademarked brand name.

From the Duramax Marine web site:Cutless® is a registered trademark of Duramax Marine® LLC.”

Cut The Old Shaft Out

If you know the shaft needs to come out why risk damage to the gear box or gear box flange by trying to press the coupling off the shaft. It is far easier and far less time consuming to simply cut the shaft out. This took all of about 45 seconds to free the 1″ shaft from the flange.

WARNING: NEVER, EVER, EVER, EVER use a “Slide Hammer” to remove a shaft from a coupling if the coupling is attached to the gear box. If you want to throw 3k out the window feel free to use a slide hammer, if not, use proper procedures for removing the shaft from the coupling such as a coupling press tool.

Only sub rate hacks, that don’t care about your wallet, your boat or your gear box, use slide hammers to remove prop shafts from the couplings when the couplings are attached to the gear.

Yes using a slide hammer is quick & dirty, and the damage to your gear box can largely goes unseen, for a period of time, but PLEASE DO NOT be fooled by hacks with slide hammers and do not allow a yard to use one on your boat.

Would you go to a dentist who used a Milwuakee Sawzall on your teeth? Sounds extreme, but this is about the same level of stupidity as using the wrong tool to remove your prop shaft..

A slide hammer is essentially a long piece of metal bar-stock that threads onto your shaft where the prop nuts go. It is about 4 feet long and has a heavy metal weight on it. The weight is slid up the bar to the prop shaft and then thrown down the bar until it hits the end and SLAMS to a sudden stop. The idea is that it breaks the coupling from the shaft when the weight comes to the sudden and abrupt stop. The reality is it destroys gear boxes in the process.

This destruction of your gear may not be readily apparent so the hacks who use them assume they get away with it and it “works“. Yes it works, it works to DESTROY gear boxes. If your boat yard tells you “we do it all the time” please do yourself a favor and find a new yard.

Slide hammers can cause brinelling of the bearings or races in the gear box. Brinelling means the shock loads imparted on the static bearings, by the “slide hammer“, create FLAT SPOTS or DIMPLES in the races or bearings. The gear may work for some time after the slide hammer event but eventually the damage rears its ugly head. The damage is often never attributed to the slide hammer and this is a mistake as the slide hammer started the failure by brinelling the bearings or races..

An Ouch Moment:

About twelve years ago I was at a yard when I overheard the summer help yard boys slamming & slamming & slamming a slide hammer to free a shaft from a coupling. All of a sudden I heard one last SLAM, then a clunk and the sounds of metal bits on fiberglass, then I heard;”OH SHIT!!!!!!“……

You guessed it, they hit it so hard they blew the case of the gear box apart and destroyed it. The slide hammer literally cracked the iron gear box wide open. The shaft, after all the beating that finally destroyed the gear box, was still firmly embedded in the coupling.

Rant over…. Please use common sense…

02 A New Prop Shaft

03 A New Prop Shaft

Double Taper Shaft

Some shafts, as can be seen here, are a double-taper on both ends. Many sailboats though lack the space for a double-taper coupling and they are most often a straight-coupling..

Bottom line?

Know what you have before trying to remove it…

New Split Coupling

For tight spaces Buck Algonquin makes a great split-coupling. I much prefer a straight split-coupling to a straight solid-coupling but both work if properly installed fitted and faced.

The nice thing about this particular model is that it is no longer than a standard solid straight-coupling and is actually a touch shorter than most. The “S” designates “short“…

04 A New Prop Shaft

05 A New Prop Shaft

A New Short Split-Coupling

This is the new split coupling from the box above. You can see how compact it is in its overall length.

Coupling Surfaces Made Parallel With Shims

Yes, even split couplings need to be properly “fitted” to the shaft. Unfortunately many yards and DIY’s incorrectly assume a split coupling is a one size fits all, and this could not be further from the truth. These coupling ship a tad undersized for the SAE shaft tolerances. This slight under-sizing allows a *competent shafting or machine shop to “fit” the shaft to the coupling.

IMPORTANT: Please use extreme caution when ordering from on-line shafting shops. The vast majority of purchased on-line shaft/coupling orders I’ve seen show ZERO EVIDENCE of any fitting or facing. The owner who thinks he saved a buck or two is now looking at local machine shop costs which blows any perceived savings clean out of the water. Be an educated consumer and don’t let on-line companies sell you a bill of goods..

The proper fit for a straight-coupling is a light press fit or light interference fit. This means it does not just “slide on” and requires some light tapping, or heat to expand the coupling while it is installed over the shaft.. Getting this level of fit can be time consuming.

With split couplings the two shims are placed in the splits to make the bore ID the same at both ends before any fitting even begins.

06 A New Prop Shaft

07 A New Prop Shaft

Rough Reaming Tool

This is the rough reaming tool. The finishing is done with grinding compound or other means of honing the coupling bore.

Flange is Bored to Fit the Shaft

With the flange held tightly in the jaw of the machine, the reamer is carefully sized and rotated to remove just barely enough to start to get the perfect fit. It takes some time to make it fit just right.

08 A New Prop Shaft

09 A New Prop Shaft

Inside Finish

The inside of the bore is then honed to get the final fit. This was taken before the honing process.

Should be a Light Press Fit

Here the machinist is test fitting the coupling to the shaft. A soft lead mallet is used to tap it on. This is not pounding but a light tap fit…

Just enough interference so it won’t go on by hand is how it is done.. With a good fit you may need to heat the coupling to make the shaft slide into it when doing the install at the boat..

10 A New Prop Shaft

11 A New Prop Shaft

Shaft Is Cut To Length

For the shafting on sailboats I only spec or use Aquamet 22. Aquamet 22 shafting is a high-alloy austenitic stainless steel that offers tremendous corrosion resistance and excellent strength properties. It is some of the best shafting there is.

Here a length of 1″ Aquamet 22 is cut to length for machining.

Checking Shaft Run Out

Once the shaft is cut to length it is then tested for run out. A good shafting shop should always do this even with a brand new shaft. If it does not meet tolerance then it needs to be made true. This one was out by less than .001 for a 52″ shaft and is within spec.

This shaft was also checked for true after the machining process.

12 A New Prop Shaft

13 A New Prop Shaft

Machine for Cutting Shaft Taper

Once the shaft is cut to length it is then placed in this machine to cut the taper, or tapers, if it’s a double taper shaft.

Key-way is Cut Into Shaft

This is the machine used for cutting the key-way.

14 A New Prop Shaft

15 A New Prop Shaft

Spotting the Shaft

After the coupling has been fitted to the shaft the set screw hole needs to be “spotted” into the shaft. The drill press is spotting the shaft in this picture.

Spotting is the creation of a small dimple in the shaft for the head of the set screw to recess into. If your shaft is not spotted, for the set screw/screws, you have a poorly & improperly machined shaft. Some unscrupulous vendors cut this corner because it saves a few nickles.

No Spotting

Here is a prime example of sloppy machining. This shaft literally fell out of the coupling and the boat took on water. The coupling was clearly never “fit” correctly to the shaft adn was sloppy & loose in the coupling.  The set screws were also never spotted to act as a back up. Don’t let this happen to you.

Insist on a light interference fit and properly spotted set screws.

16 A New Prop Shaft

17 A New Prop Shaft

A Properly Spotted Shaft

This is what set screw spotting looks like. You should have dimples in your shaft to accept the set screws..

Facing The Coupling

Once the shaft and coupling have been fitted together, including the set screw spotting, the flange is mounted to the shaft and placed in the lathe.

When using a split-coupling it is very, very important to properly and evenly torque the clamping screws. Not torquing the coupling evenly and properly can result in the face being out of round with the shaft.

When facing the shaft coupling the machinist will properly & evenly torque the clamping nuts so you get a perfect facing with no run out. Once you get it to your boat it is your responsibility to once again properly & evenly torque the nuts.

18 A New Prop Shaft

19 A New Prop Shaft

Fitted & Now Faced

Just like a brake rotor lathe, the face of the coupling is made to rotate perfectly with the shafting.

If you get a new shaft and the face of the coupling does not look, and has not been freshly cut, you’ve been cheated of a proper machining job.

It would be extremely rare that a coupling would not need to be faced. They need to install it in the lathe head to check it for run out anyway so the 60 seconds it takes to actually cut the face could be a prime indicator the shop never even checked the facing run out.


Good luck & happy boating!

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