Check your prop!!! No, really!!! Check your prop!!!

[boatwakes]

I surveyed a 34' Sea Ray last week, twin 8.1 Mercruiser v-drives and at haul out discovered the props were not fully seated on their shafts. The same afternoon I received an insurance claim where the shaft had snapped mid taper and my investigation revealed it was from the prop not seating on the shaft correctly. If the prop is not installed correctly, too loose or not lapped to the shaft, it will do something called cyclical loading and eventually break the shaft at the woodruff key end after a crack developes. This means a new shaft and a new prop plus whatever damages were incurred when the prop hits the hull or rudder.

Flash forward to yesterday and we're out on Lake Mohave taking a swim break with Hank and I figure "Hey, lets check the prop!" and find it's loose on the shaft! I grab my trusty tool kit and tighten the nut to ludicrous level and run the rest of the day. However, it was eating at me enough to post this for the forum because my recommendation to the sea ray buyers was replace the shafts immediately and now I'm going to do the same on our Tige. Running the weight we do, the propeller speed, the fact we have 400 hours and it was cyclically loading both the keyway and the shaft taper for an unknown number of those hours, most likely since the prop was installed incorrectly before we purchased it, but I'm not taking a chance.

To help others (including dealers) know how to correctly install a propeller, please read below. I've also included a photo of the broken shaft which has a diameter of 1.5" and an aquamet 19 rating.

The following is an excerpt from "Propeller Installation" written by Steve D'Antonio and a link to his article. Everything in it is correct EXCEPT when he commits the cardinal sin and says to USE A FEW DROPS OF LUBRICANT OVER THE KEYWAY AND TAPER. NEVER EVER USE ANY LUBRICANT TO INSTALL A PROPELLER ON ITS TAPER!!!! EVER!!! ALWAYS INSTALL PROPELLERS DRY!!!

The first time a propeller is mated up with a shaft, whether both or either are new, they should be lapped to ensure a proper fit. Lapping is essentially custom fitting a prop to a shaft using an abrasive compound that’s designed for installation of intake and exhaust valves in engine cylinder heads. The process involves applying machinists dye or, “Prussian blue”, to the shaft taper, over which the lapping compound is applied; the latter has the consistency of wet sand. The propeller is then placed onto the shaft taper and rotated by hand 180° in either direction a dozen times or so. Doing so grinds away some of the material in the prop bore, establishing a custom fit. The prop is then removed and the lapping compound washed off. The dye that remains tells the installer if he or she has completed the lapping process, roughly 85-90% of the shaft taper should be dye-free or exposing the silver shaft material beneath. If this level of fit has not been achieved, then the process must be repeated until the 85-90% engagement is achieved. Once the lapping is complete all of the lapping compound should be thoroughly washed from the taper and propeller bore (it’s water soluble).

https://stevedmarineconsulting.com/p...-installation/

[cucv]

Interesting read, thanks for sharing. My number one recommendation when changing a prop is to replace the nyloc nut. They can loose holding torque over multiple uses and time. If you know the intended holding torque, you can certainly verify when reusing.
When I upgraded my prop I forgot to order an nut too. I replaced the prop anyway and ordered a new nut to install the following weekend. I could clearly tell the nyloc had lost holding torque when reinstalling it. Later in the weekend I checked and sure enough the the nut was backed off but the prop was seated on the taper. I used a chisel to deform the metal holding the nylon on the nut to give it more holding torque to get by. I assume the nut and prop backed off in reverse and the prop reseated in forward.
My assumption would be during forward movement at high and low speeds the prop would be seated on the taper. The prop would only want to back off during deceleration or reverse which is a small fraction of time and lower rpm.
For cyclic failure to occur there has to be a load, in this instance, vibration. As a conscious boater, one would expect to feel that vibration.
In simplest terms, it's likely the shaft could have infinite cyclic loading as long as that load is less than 50% of tensile strength.
I'd suspect the failure mode on a failed shaft with new prop to be the impact endured by the old damaged prop and shaft. A close second bring the high cyclic loading on the ride home with damaged prop.

Sent from my SM-G991U using Tapatalk

[chpthril]

While I would not expect a new boat to have a prop thats not fully seated, it could happen. However, when you change props, is where it CAN be an issue. The shaft need to be marked behind the old prop, prior to removing. This tells you where the old prop was seated.

[talltigeguy]

It is with great trepidation that I would say anything that would muddy the water that Boatwakes walks in, he has forgotten more about boats than most of us know, but I do think this is worthy of discussion.

Lapping a prop on a wakeboat used in fresh water is very uncommon. Almost no dealers do it, and most owners have never heard of the concept. Prop shafts almost never break unless you own a 2013-2019 G23, but that is a different subject.

There is a recommended torque on the nut, and it is only 35 pounds. Tightening it like a gorilla is going to make the removal a lot more fun.

Acme propellers used to actually RECOMMEND using a thin layer of grease on the prop. Attached is a copy of the recommendations that came with a propeller I purchased. So even the most knowledgeable guys in the business were a fan for a while. The most recent prop I purchased didn't have that recommendation, so they seem to have changed their tone. But I am pretty sure that it won't mean imminent destruction if you do put some grease on there. Having done it both ways, it is still remarkable how tight the prop gets on the shaft even with grease. Even with grease, it requires a puller and a hefty hammer.


On a side note, my boat partner changed a prop underwater a few weeks ago since he was 25 miles from the marina (a prop I installed with grease on the shaft). He rigged up a tube with the tube pump to stay under water longer. He had never removed a prop before. He tightened the puller until it stripped and then worked with a friend's puller. After using a 3' pry bar on the second puller, it finally came. He didn't know to hit it with a hammer. So he then gorilla tightened the spare prop on with the nut, and all was well. I asked about the key..."what key?". It turns out if you tighten the nut enough, you don't even need the key. Not the end of the story, as I figured the key should be in place. Getting the prop off was the most difficult I had ever encountered. I finally got it off when enough heat was applied, but it was way more heat and hammering than I was initially comfortable with.

I believe the mating process between the shaft and prop is extra intense if you overtighten the nut dramatically and don't use the key, but I don't think many people have tested this theory out. I do not recommend doing this.

acme instructions.jpeg

[boatwakes]

As a note to tall's reply above, there's way more to underwater driveline inspection and propeller installation than I'd care to write. The quick and dirty is this: if the propeller is installed correctly, a woodruff key is not necessary as the correctly mated taper of both the shaft and propeller bore are trading the load over the entire taper. The reason you do not install a propeller with grease, lubricant or anything other than a clean shaft is a condition called hydrolock. When the prop nut is tightened against the bore, if there's anything on the taper, it hydrolocks the propeller in place but in the wrong position with a thin film of grease between the propeller bore and shaft. As the grease disperses through use of the boat, the propeller begins to migrate on the taper and produces cyclic loading leading to woodruff key deformation and eventual propeller shaft breakage. Do not believe installing a propeller with lubricant will aid in its removal.

Removing a propeller on a shaft drive boat should ALWAYS be a very difficult task involving heat, a puller and often times a 10-15lbs slide hammer. If you can remove your prop by hand or just with a dead blow hammer, I strongly suggest lapping and installing per above and understanding why. Most of these boats have vibration it's just difficult to understand/feel when the vibration is not normal. People will drive around with what they think is a normal issue until it becomes something else and because the safety factor of the shafts on these boats is well above pleasure rating we rarely see one break. Discussion can be held on the why and how but after seeing it across many improperly installed propellers, correlation leads to causation.

Side note, when you tighten a propeller nut, it is industry standard to jam a block of wood between the hull and propeller as you torque it. Being underwater with a wrench and holding the prop by hand yielded less than what I wanted for torque but by no means would I want anything but as tight as it would go. After I replace the shaft I will be replacing the stupid nyloc nut with dual bronze nuts as per ABYC. Furthermore, and to my point, when I checked the prop today, even though the nut is tight, the prop still wiggles on the shaft indicating there is a mating problem with either the key, keyway or taper and enough damage has occurred the shaft is most likely compromised.

[USWaker]

Weird… before I read this, last night I decided to check my prop. It was just a little loose. Having been a tool maker (machinist) in the past and having dealt with a lot of taper fit projects, I decided it would be prudent to check the taper. I used some dye and found it wasn’t quite true. So… I used a bit of valve lapping compound that I had and worked it until it passed the dye test. True to form I most likely over tightened it.
My point: CHECK YOUR PROP. Hahahaha…

[talltigeguy]

Thank you for the reply.

Is there any theory why ACME recommends the lower torque on the nut? The new instructions I received still said the same torque.

Will you put 2 brass nuts on your wakeboat prop also? Do they not do that for our use just because it is easier? I assume you would use 2 nuts that are less deep (2 of the big nylock nuts wouldn't fit).

As I was removing my partner's handiwork, cursing because I have never had to work that hard at it, I thought back to the Mastercraft I used to own and how easy the job was with a splined shaft. Why don't we see more splined shafts? I did a quick search online and although it is hard to know if I was comparing apples to apples, the price didn't seem to be a factor.

[bsreid]

Check the stuff holding the prop shaft too. Entire drivetrain was shifting over an inch due to bad motor mounts.

[boatwakes]

Torque on the nyloc is recommended due to thread deformation inside the nut. The internal threads will “strip” if you put a 2’ pipe wrench on it and go full monty so an adequate torque rating is suggested for them. The double nut system is linked below and it’s installation protocol is THIN NUT FIRST BACKED BY THE FULL NUT. I’m lapping the prop next week because my new shaft is coming from our supplier and is a few weeks out plus the reunion is closing upon us.

https://www.generalpropeller.com/inb...ut-Kits/PSK112

[boatwakes]

As for splined shaft use, I have theories and could pontificate at length but I feel two main factors are involved; the machine tooling and process to produce a splined inboard shaft requires different setup than outdrives and the NiBrAl propeller material may wear away and cause cyclic loading because there’s no taper. Because the marine shaft tapers are standardized, if the prop bore increases slightly from improper installation, it will just seat slightly higher on the taper. If a splined prop bore is damaged, it cannot be lapped or repaired.