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Shaft & Coupling Information

Shaft & Coupling Information


Our shafts are fabricated from special marine shafting, typically Aquamet 19. It is chosen for high strength and corrosion resistance at a (relatively) reasonable cost. It is not just generic stainless steel rod repurposed for marine use.

According to the manufacturer "AQUAMET 19 is a modified Type 304 stainless steel that is fully austenitic, non-magnetic and strengthened by nitrogen addition, with corrosion resistance better than Type 304 stainless steel. AQUAMET 19 has proven itself in demanding service on shrimpers, crabbers, scallopers and other fishing boats as well as pleasure boats."

There is an even higher strength alloy available at a higher cost. It is typically used on high performance power boats where its greater torsional strength (86,600 psi rather than 70,000psi) is important. For the tiny diesel engines used in sailboats, this is hardly a factor. The most likely scenario for a shaft to be bent on our boats is from a stray line wrapped around the shaft while motoring. Unfortunately, either shaft is likely to be bent under those conditions.

The first step in the fabrication process is to straighten the bulk shaft to a close tolerance. It is then machined using a computer numerical controlled milling machine. This shaft complies with Society for Automotive Engineers (SAE) and American Boat and Yacht Council (ABYC) standards. That means the taper, key, keyway, thread pitch, double nut and cotter pin all conform to strict standards.

When metal is removed during machining, while creating the taper for example, some internal stress is relieved. As a result, the shaft sometimes changes. So after machining it is checked once again for straightness.

When buying a new shaft, we highly recommend purchasing a new coupling at the same time. And here's why...

Whenever tubing, rod, or shafting is manufactured, it varies in size slightly from batch to batch and even from one piece to the next. Not by much, but up to several thousandths of an inch in diameter. Universal fit engine couplings must be made with a center bore diameter large enough to accept a shaft that is the largest diameter within the shaft tolerance. Although that allows you to climb into the bilge and install a new coupling onto your old shaft, it also leaves enough room to cause the installed shaft/coupling assembly to be slightly out of alignment.

The same is true when installing a new shaft. If you can slip the new shaft into your existing coupling (which you likely can), it too will be slightly out of alignment. The final result is vibration in your brand new shaft installation. This is why we offer you the option of "facing" a new coupling to your new shaft.

There are multiple steps to our "facing" procedure:

1) Our coupling #Z2736 is not a universal coupling that is bored oversize. As supplied from it's manufacturer, the center "pilot hole" of our coupler is bored undersize. This gives us the opportunity to bore the hole out to the exact size of your new shaft.
2) The coupler is undersized one or two thousands of an inch before assembly is completed. The coupler is reamed to the required specifications. The couplers exposed areas are true from the manufacture, but if found not to be true are rejected and sent back to the manufacture. The only machining done on the coupler is the inside diameter that is mated to the shaft to retain the couplers zinc plating.
3) The shaft is oversized one or two thousands of an inch before assembly is completed. The shaft is placed on an I-beam straightener and a press comes down on the spining shaft to turn to the required specifications.
4) The coupler and shaft are mated together on the I-beam straightener. The assembly is spun and a dial micrometer is used in 9 total locations of the shaft and all sides of the coupler to ensure accuracy of fit.

Whenever you replace your shaft, don't forget to install a new cutlass bearing as well.

Now, when you install your new shaft, the prop, shaft and coupling are in alignment and you have eliminated shaft induced vibration.

You have not, however, eliminated vibration caused by an engine that is out of alignment with the prop shaft. That subject is for another article...

Note: When your order a shaft, coupler and facing at the same time, the shaft and coupler will be shipped assembled. This helps protect the end of the shaft if a UPS driver treats the package carelessly and eliminates the possibility of a stray coupler damaging the shaft because it came loose inside of the shipping container. If you are replacing the shaft during a re-power, the shaft can be installed in the boat from the front while the engine is out of the way. If you are only replacing the shaft, the coupler must be removed from the shaft before the shaft can be installed from the stern.