Aquamatic Propellers - Fitting a Long Hub Propeller to A Short Hub Shaft

I go through Aquamatic props regularly. We like to explore remote areas that are often quite shallow. My 250 outdrive takes the short hub propeller – the one where the bendy tab locks the cone.

The place that I’d been getting the short hub props no longer handles them. I bought a long hub prop – the one that uses a bolt into the end of the cone to lock it, just to see if I could make it work. I couldn’t come up with a solution. That long hub propeller needs a threaded shaft and mine isn’t threaded.

My 250 drive requires the #5 bendy tab to lock the cone.

I’d heard you can thread the shaft, but it was a scary thought. Is it hardened? Does it take a special bit and tap? How do you know if you’re drilling straight? Then one day I chucked up a 17/64” Harbor Freight bit just to see if I could put a hole in the end of the shaft, and what do you know, it wasn’t difficult to start the hole.

OK, so I could drill the hole, but how do I make it straight? I’d need a guide of some kind. I was excited to find that he (external) cone threads on the shaft are American 5/8” X 11.

Damn - these are SAE Threads.

There’s a threaded rod coupler 5/8 X 11 that will screw onto the cone threads on the propeller shaft. This was a start. Now all I needed to do was figure a way to keep the drill bit centered while I was making the hole in the shaft. 

I thought I could find a sleeve that I could use inside the coupler but nothing I found would fit snugly.

I found that a 5/16 nut could be pushed into the end of the coupler. This would not be a real snug fit, but it would keep the drill bit centered in the outward end of the coupler. There is already a dimple in the propeller shaft so the other end of the bit would start correctly.

I drilled a 17/64” hole about ¾” into the propeller shaft and used a reputable 5/16 X 18 end tap to make threads a little over ½” deep.

You may need to 'clearance' the nut to get a good fit for the drill bit.

I’ve seen all kinds of spacers on both sides of the propeller so the only way to know how long the locking bolt should be was to fix the propeller and cone on the propeller shaft and push the locking bolt in until it contacted the shaft. I’d want to have just a smidgen of shoulder above the top of the cone – this would give me more than ¼” of threads to grab but wouldn’t bottom out.

If the bolt is this long before you start screwing it in, you'll grab ~3/8" of the new threads.

 Now, after all was said and done, I remembered that I could have just used the lathe to poke a hole in a 5/8" bolt to use in the end of the coupler.

At any rate, I'm no longer being held hostage to short hub propellers. Now I can use both.