Propeller Nut with Anode?

[Modular Hippo]

Currently building a 36' Aluminum Sailboat [https://www.youtube.com/@modularhippo] and about to order the propeller nut [M24 x 2] with built in and easily replaceable anodes. I am going in blind as I know nothing about propeller nuts so thought I tap into the collective experience here I guess 'Zink' as a sacrificial metal is a given? My propeller is a 18” diameter 3 blade made out of NiBrAl (nickel, bronze and aluminium). The Propeller shaft itself is a traditional SS 316 (35mm OD). Does anyone have experience of any model/type/brand that they are happy with and can recommend? Thanks in advance!

 

[fallguy]

I don’t profess major expertise in anodes, but zinc or aluminum are what you need for the prop. Since the prop is aluminum; I think the zinc is your best choice and less noble than aluminum. You’ll want to check that often.

 

[jehardiman]

I don't think you will find a propeller nut that includes a replaceable anode. If the prop is NIBRAL (i.e nickel aluminum bronze on this side of the pond) it is basically forever in most seawater. It is the 316 shaft and nut material you have to worry about. Make sure the shaft is 316L and don't overheat it. I would use Monel (Nickel 400) or NIBRAL for the nut. Generally, the shaft would only needs cathodic protection where it is exposed, i.e. just inboard a strut or under the rope guard at the sternpost. Then you would use a typical shaft zinc of zinc or doped aluminum.

Shaft Zinc Anodes https://zincsforboats.com/collections/shaft-zinc-anodes

Edit: Maybe I'm wrong as several manufacturers make them for their specific props (most likely aluminum props). A NIBRAL: prop shouldn't need one unless something else is going on.

Propeller Zinc Anodes https://zincsforboats.com/collections/propeller-zinc-anodes

 

[Ad Hoc]

It is more about the electrical potential that is being created, when immersed in seawater (the electrolyte) and then decide the way forward.
One should avoid any potential greater than 100mV, but most tend to opt for the upper limit of 250mV, which is taking a chance.

The SS of 316 (L only refers to Low carbon, which is important when welded and exposed to seawater) is most likely to be 'active' rather than passive....this has a potential of -180mV, and the Prop (NAB) is -310mV.
So if one sues the 100mV as the limit, is not ideal, if one uses the 250mV upper limit, it suggests it just needs care and constant attention.
Either way, there is a bimetallic corrosion cells set-up.

So, as JEH notes selecting a nut that is very close to the shaft is better as this is a mere -10mV potential from the SS 316.

You want to avoid small area elements for sacrificial purposes, thus zinc anodes carefully placed will serve you better than a nut on the prop out of zinc.
Depending upon the rest of your appendages, and their material properties, you would likely benefit form one anode for the shaft, one for the prop and one for the rudder.

 

[sdowney717]

How about putting a waterproof sealer on the threads of nut and bolt, to seal all crevices.
This stuff stays put in water.
Amazon.com: Rectorseal 23631 1/4 Pint Brush Top T Plus 2 Pipe Thread Sealant : Industrial & Scientific
I use it on all sorts of threaded fittings and rubber hoses, and it stays in place.
Or if something better use that.

About this item

• Adhesives & Sealants
• Country of manufacture: United States
• Manufacturer: Rectorseal
• Non-hardening thread sealant containing PTFE plus synthetic fibers
• These fibers dispersed in a resinous base combined with inert mineral fillers provide maximum sealing performance
• Its low coefficient of friction allows tighter joint makeup with lower torque
• It also breaks out easily without galling, seizing, or damaging threads
• It is excellent for service where immediate pressurization is necessary

*********************
You want to stop rubber hosing gluing onto metal? Just use this.
Hoses come off much easier, they do not get stuck in place
I have never had a hose pop off as long as it is normally clamped.
Including pressurized coolant hoses on engines.
Not seen it cause any reactions to whatever I put on, metal, plastic, rubber.
Keeps threaded parts from corroding together.

It will dissolve in a hydrocarbon solvent, it will wash off hands with dish soap.