Acme NiBro prop

[bailsout]

Three years ago I put a new 3 blade Acme prop on my Herreshoff 28 modified wooden ketch. I careened her last week and pulled the 12rh6.5v to inspect and clean. There was extreme pitting mostly in the center of the blades, no wear on the edges. One pit went clear through about a1/8” diameter. The composition is nickel bronze, not manganese bronze and came with a grooved surface, not a smooth one like my other props have been. My shaft zinc was at about 80% and a keel zinc at about 60% after a year. Any ideas of why this prop did so poorly?

[DougR]

You're not giving us much info to go on here, and a foto would be helpful, but I'll comment....

All the props that I have seen with holes burned in the center of the blade suffered from cavitation erosion of one sort or another. Granted, most of these were higher speed boats, operating in the 30 knot plus range, and many with aluminum props, which are softer and more susceptible to erosion than Nibral.

Some of the causes of this erosion are:
- poorly shaped, bent, or damaged leading edge of the blade ....this can cause a stream of vapor bubbles streaming back across the blade which then collapse at a higher pressure area and the collapse hammers particles of material from the blade surface until a hole appears. This usually happens to only one blade, the one with the damaged edge.

- If you have similar damage to all three blades, it's possible that all three blades are manufactured incorrectly, but more likely there is something disturbing water flow ahead of the prop and this " something" is causing similar problems with all of the blades. For example, a sterndrive leg that has badly shaped water pickup slots on the side of the leg ahead of the prop. These slots can cause a stream of turbulance or even vapor bubbles to flow back into the blades causing erosion in the middle of the blades, or sometimes a poorly fitting zinc anode mounted around the hub of the prop can stream bubbles into the prop near the blade root and erode so much of the root of the blade that the blades break off of the hub.

Granted, these sort of problems are most common in high speed boats, but it might be worth while looking at the area of the strut, shaft and keel ahead of the prop and see if there is something mounted there that can disturb water flow into the prop.....such as a badly eroded zinc, or depth sounder transducer, or a thru hull screen protruding into the flow.

It's also possible that you just got a bum prop that had some alloy problems. It wouldn't be the first time that has happened.

DougR

[GordMay]

Your experience is surprising, to me.

Nickel Aluminum Bronze (Nibral): This material is lighter and stronger than manganese bronze. It is more expensive than manganese bronze, and is quite repairable. It's main ingredients are copper, aluminum, and a small amount of nickel. Due to it's higher copper content, it's more corrosion resistant than manganese bronze, but left unprotected the aluminum can act as an anode to protect other underwater metals on your boat. This is indicated by dark craters on the surface of the material ringed with a green color. Left long enough, these craters will go right through the propeller, and it will have to be replaced.

Manganese Bronze: This material is moderately heavy and moderately strong. It is the least expensive of the bronze materials and is quite repairable. It's main ingredients are copper and zinc. With a lack of protection, the zinc content in the material will act as an anode to protect other underwater metals on your boat leaving oxidized copper behind. This is indicated by a pink coloring to the propeller. After enough corrosion takes place, the propeller will be brittle and need to be replaced.

Here’s an interesting study on the “Properties of bronze for marine propellers”
Which found: “The superiority of nickel aluminum bronze has been convincingly demonstrated by an in-service test.”
These results have been, more or less, duplicated many times over (other studies).
➥ Properties of bronze for marine propellers

[Wood]

Same size prop as previous? Any change in hull shape or rudder in proximity to prop? Just thinking if not galvanic could it be cavitation?

[fstbttms]

This should not have been a surprise to you. The boat absolutely should have been dived on a regular basis (by you or someone else) for cleaning and inspection.

[bailsout]

Thanks for all replies. It looks to me like a metallurgy/manufacturing issue. Zinc had been checked by a diver 6 months previously and found to be in good condition. I don’t think he got a good look at the prop because I thought the prop looked “dirty” with surface growth. Only when I cleaned it with wire brush wheel did the hole and pits become visible. This boat after 50 years of service never had a cavitation issue or pitting problem with manganese bronze. I’m now back to a manganese prop and I’ll be keeping a closer eye on it.

[GordMay]

Originally Posted by bailsout

... I’m now back to a manganese prop and I’ll be keeping a closer eye on it.

I don't know anyone who would recommend manganese bronze over NIBRAL, except for cost considerations.
Aluminum bronze, is strong, but susceptible to de-aluminification. This can be prevented with the addition of nickel, and this alloy is typically referred to as NI-BR-AL (nickel, bronze, aluminum). Propellers are often made of this materia,l and with good reason; it's strong and corrosion resistant. Manganese is sometimes added to this already mixed soup of metals to further increase strength. Prop repair shops sometimes charge a premium for reworking NIBRAL props due to their tendency to take on a set or memory. This alloy should not be confused with the inferior manganese bronze, which is actually a brass.

Dave Gerr ➥ https://www.gerrmarine.com/Articles/MarineMetalsPt2.pdf

[boatpoker]

I tend to think there was a quality issue with your Nibral prop as cavitation issues would normally start at the blade edges.

Galvanic or stray current corrosion would normally also show up first at blade edges unless there are areas of qustionable metallurgy in other areas. i.e. zinc rich areas would be first to show corrosion. I would not be surprised if this happened on a chinese prop.

Just because your anodes look "good" does not mean they are actually doing their job. There must be resistance of less than 1.0 ohm between the anode and the metal it is to protect.

[bailsout]

Gordmay, thanks for the info. I’m not sure what knifed of bronze these old sailboats carried, manganese bronze is probably not the right name, maybe it’s silicone bronze, or .... Whatever it is, it lasted for 15 years vs 3. What I do know is that I won’t be buying another Acme prop.

[boatpoker]

Originally Posted by bailsout

Gordmay, thanks for the info. I’m not sure what knifed of bronze these old sailboats carried, manganese bronze is probably not the right name, maybe it’s silicone bronze, or .... Whatever it is, it lasted for 15 years vs 3. What I do know is that I won’t be buying another Acme prop.

"Manganese bronze" is the correct term. The vast majority of pleasurecraft propellers are of this material.

Curiously "manganese bronze" is actually classed as a brass due to it's approximately 35 - 38% zinc.

[FPNC]

Me thinks that electrolysis may be on board. Consult someone who has the correct equipment to check for this.

Also, I was not aware that ACME designed and manufactured propellers suitable for a displacement vessel. I always thought they made more high speed power boat propellers. If your blades are overloaded and cavitating, this can lead to pitting. Which ACME propeller did you actually fit?

[bailsout]

As said in original post: acme 3blade 12RH 6.5 v

[FPNC]

Sounds like you have a Ski Boat Prop on your displacement vessel.

What is engine HP @ MAX RPM and gear reduction? Gas or diesel?