It started as a subtle, but unusual “Thwunk, thwunk” sound when motoring in gear at idle last season just before leaving on another 1500 mile trip to the North Channel. The sound wasn’t accompanied by significant vibration, and the sound seemed to go away at higher power settings. Since this sound seemed to emanate from the propeller (far aft of the engine and transmission), I delayed any further investigation until after haulout in the fall. NOT doing anything about a problem with the drivetrain before heading out on a long cruise sure nagged at my conscience, though!
Priorities has a feathering propeller to reduce drag while sailing. After shutting the engine down, I stop the propshaft from freewheeling by putting it into forward gear. Due to their shape, the prop blades rotate on the hub into a low drag, feathered configuration that’s parallel to the water flow… especially helpful when sailing in light winds. Theoretically the blade angles adjust during motoring to find the optimum angle, too. It’s a nice luxury, but also quite complicated. A new one is also expensive… I’m glad Priorities’ previous owner bought it!
Sure enough, as Priorities’ hull was still dripping from her winter haulout and pressure wash, I moved each blade of the decade-old Autoprop H5 RBTS from forward, to reverse, and to feather, and found a possible culprit of the sound: blade number 1 was very loose. I could shake the tip of the blade about ¼”, whereas the other blades merely rotated. I guessed the looseness was from a worn blade bearing. Even if it wasn’t the bearing, I had to take it apart to get to tighten it.
According to the documentation for the Brunton’s Autoprop, the bearings for each blade are subject to wear and may require replacement in as soon as 1000 hours. I’d run it for 900 since buying the boat, and the prop was a few years old when I bought the boat, so I figured now would be a good time to replace the bearings.
I ordered a bearing replacement kit and associated tools from AB Marine, the US distributor of the UK-built Autoprop, for just over $1000 (B.O.A.T. = Bring On Another Thousand). The instructions on their website are excellent, and overall I found the procedure doable for my moderate experience.
Prop Removal Issues
Though the instructions recommended removing the prop to replace the bearings, I initially hoped to perform the procedure without removing the prop. Unfortunately, I couldn’t get enough leverage to remove the bearing caps to even begin the process, so I had to remove the prop anyway.
On two previous occasions I’ve removed the prop without much trouble, but it had been on the shaft for 5 years now, and I recently added a Shaft Shark rope cutter just in front of the hub. The Shaft Shark blocked my cheap terminal puller from gripping the prop hub. Unfortunately, I stripped the screws holding the Shaft Shark in place when I attempted to remove it… so I relented and bought the special, $200 Autoprop Puller Tool that wouldn’t require access to the front of the prop hub.
When the special Autoprop Puller arrived, I quickly figured out it also wouldn’t work exactly as expected on my propshaft. The special puller screws into the aft end of the Autoprop hub in place of the anode, then uses a giant screw to push away from the end of the propshaft. Unfortunately, my Catalina 400’s propshaft has an extra long threaded portion after the taper and extends slightly aft of the Autoprop hub… so I needed longer screws for the puller to make it work.
A quick run to the hardware store revealed their limited selection of metric screws. (D’oh!) My plan was to find screws that were strong, but not as strong as the bronze prop hub in case something stripped… better to damage a cheap screw than an expensive prop hub. The puller kit included M6 screws made from 316 Stainless (moderate strength with excellent corrosion resistance) but 316 Stainless is hard to find in a local store. I figured SAE Grade 8 would be about the right strength, but that’s not a metric grade… I found and thought metric Grade 8.8 might be similar.
Well I learned metric Grade 8.8 is not very strong at all. After over an hour of muscling my wrench and prop blade, thinking I was making progress, I noticed the hardware store screws were stripping. Though I was super frustrated at yet ANOTHER setback, I was VERY lucky to be able to remove the cheap screws from the prop hub before they broke!
I soaked the hub/shaft joint in some PB Blaster (available at Home Depot) and vowed to find a better solution on a different day.
After consulting with Geoff at AB Marine, I found I needed more of a “shock” to get the hub unstuck. A little bit of heat might help, or some careful hammering. I hoped soaking in the PB Blaster for a few days would help, too.
After a few days of soaking, I tried again with stronger Grade 10.3 steel screws. I didn’t want to hammer too hard for fear of damaging any part of the transmission farther up the propshaft. Using a pipe on a wrench to increase my leverage, and some careful hammering, the prop FINALLY came off in only a few minutes without heat!
Bearing Replacement
With the prop at home on the workbench, the actual bearing replacement went smoothly.
The instructions provided by AB Marine were excellent, and the procedure was very straightforward. Since I had been diligent about greasing the bearings each winter, all the internal parts had been bathed in grease their whole life and were free from corrosion. Therefore, the internal parts were easy to remove.
My Autoprop is an H5 RBTS, or Roller Bearing Tab Screw, and more modern Autoprops will have a slightly different procedure than mine. If you download the instructions from an online source (like AB Marine’s website), be sure you’re looking at the correct model of Autoprop instructions and diagram!
Once everything was reassembled and regreased, the blades didn’t seem to rotate as easily as before. There was a little resistance to turning, though not so much that the prop’s rotation through water wouldn’t actuate. It was as if it was newer!
Sea Trial
After over 20 hours of using the motor this season, my overhauled Autoprop has worked well. I have no strange sounds at idle, no excessive vibration, and the prop quickly transitions between forward and reverse in only a few seconds.