How Shaft Keys Work article on janice142

All shafts are robust solid metal. Usually they are stainless steel though some high-dollar vessels opt for monel.

I keep spare SHAFT ZINCS ↑ inside mounted on the shaft. That way they won't get lost.

Here is a photograph of a ↓ COUPLER. The coupler is between the transmission and the ↓ SHAFT.

Side Note: that is the yellow handle of a THRU HULL SHUT OFF VALVE and an 8-gauge BONDING WIRE.

The FLEXIBLE COUPLER is red. It is bolted between the transmission and the regular coupler.

A mechanic back in Carrabelle advised me to purchase the flexible coupler. The cost was approximately $100.

The Coupler has a slot cut into it. So does the shaft. The KEY ↓ (square bar stock) secures the two together.

Lower half of picture: White transmission, red flexible coupler, real coupler, shaft, and two zincs.


Diagnosing a shaft key problem. Part One.

Key for transmission: When the aft end of your transmission rotates [ RED COUPLER and THE GREEN AREA → ] and the shaft does not, verify that your shaft key is okay. At this point you should refer to Calder's for precise directions, or hire someone. I paid for an engine mechanic to replace mine.

This is not my area of expertise, thus I rely on Calder's. If I had no other option I would do the replacement myself. Still, in matters such as this I much prefer hiring someone with both strength and experience.

Please know that Calder's Boatowners Mechanical and Electrical Manual 4the Edition is amazing. It provides exact directions on how to diagnose and fix almost everything on a diesel vessel. That said, I'm not an expert at the mechanical aspects nor do I have the physical strength to do what many men can. Therefore, I hire both muscles and expertise as required.

Even though I might not do the repair, knowing how it should be done is important. This allows me to verify that the individual I hire is actually doing the job properly.

Replacing shaft keys is not a normal maintenance event. The key is generally replaced when you swap props.

NEW KEY INSTALLED ↑ just forward of the propeller.


Diagnosing a shaft key problem. Part Two.

Key for Propeller: I started the engine. After warming her up for a few minutes I put Seaweed into forward gear. Nothing happened. There was zero movement of boat. First I opened the bilge and verified that the shaft was spinning. It was, yet there was no movement of water at the transom. There are a couple possible reasons for this: The propeller has fallen off. The shaft key has broken. Note: A line wrapped around the propeller would cause the shaft to not turn and/or vibrate horribly.

When something breaks I always hope the least expensive repair will remedy the situation.

My friend Ken is a diver. He dove the boat, confirming the prop was still attached. Ken was smart and also checked for my prop key. Alas, that critical component was missing. In the process of my being anxious to have a boat that moved under her own propulsion, I made a mistake. Locally (this was in Georgia) I could find no stainless shaft keys so opted for a galvanized one. How bad could it be? Let me tell you, that was an AWFUL decision. The galvanized piece rusted and rather than dissolving sealed itself to both the shaft and propeller.

Seaweed was haul out at St. Marys Boat Services. The yard owner, Rocky, was able to heat up the rusted galvanized mess with his welding torch and remove the junk I had thought was "good enough". It was only because of his talent that I was able to get underway again, this time with a stainless key. Thanks again Rocky. Rocky→

In conclusion, I advise folks to purchase a foot or so of bar stock.

How to find your size: Put your engine in neutral. Rotate your shaft. It should turn by hand. Look next to the transmission until you see the slot with a square piece of metal in it. That is your bar stock. Measure across that span. You now know your size.

As an aside, Seaweed has a 1.25" shaft. I required 5/16" bar stock. A neighbor also has a 1.25" shaft. He uses the same diameter key as mine, though he requires a shorter length.

Where to buy: Please note that I am not being paid for this! I needed stainless bar stock, could not find it locally, searched online, found a company, and ordered via telephone. They cut it to the length I specified and shipped it promptly. That's sounds like a good company, so I share their information here:

Midwest Steel and Aluminum (763-582-1925) based in the United States.

I keep my stash in a locker, just in case a shaft key is required again. I am ready.

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