Gori prop discolouration

[Adrianmillman]

The Folding gori prop on our sail boat has got patches of pink discouration in it, after removing old surface paint. ? Dezincification . What I need to know is how relevant this is to it's function.

 

[Tranona]

That looks like surface discolouration. Scrape the pinkish areas to see how deep it goes. You may well find that you hit bright metal very quickly. You can also "ring" the blades as dezincification usually results in a dull sound. Bits breaking off the ends of blades is another sign. The critical areas for functionality are around the pivot pins where corrosion or wear can result in sloppy action. Suggest you polish the prop, renew the anode if there is one and apply an antifouling such as Prop O Dev or Trilux.

 

[vyv_cox]

Copper-zinc alloys and even copper-tin ones often have a slight pinkish surface colour. Yours doesn't appear to me to be at all serious. As Tranona suggests, abrade the affected areas lightly and it will very probably all turn to bright metal.

 

[Adrianmillman]

Thanks, I'll try the anti foul, but what do you suggest I use to polish it further?

 

[Adrianmillman]

Thanks, this is reassuring. What would you use to abrade lightly?

 

[Tranona]

Thanks, this is reassuring. What would you use to abrade lightly?

Wet and Dry (used wet) starting at probably 120 grade on the discoloured bits and finishing with 600.

 

[brianhumber]

Agree with all above. How old is the prop and when was the centre boss last off, it , the key and taper needs checking every few years. The three securing allen key grub screws are stainless so make sure you dig out the **** from the holes and they will undo, then if you still have the Gori supplied C spanner, tap the outer cone and spin off and then the use extractor puller for the inner boss to expose the taper and key - it only takes 10 mins to do this. Reassemble with grease, the Gori machining is so good the grease under the cone from 5 years previously is normally still there on mine.

Brian

 

[Jamesuk]

Looks like some paint remover has been left on, its amazing what Ospho or calcium remover does to a prop when it isnt polished off with something abrasive.

I would sand it

 

[BlueSkyNick]

I had very similar on my Flex o Fold this year, and spoke to the supplier about it at some length. He was pretty sure it was due to electrolysis even though I have an anode on the shaft. Being in a marina for a few months with all the boats plugged in to the shore supply for the winter apparently puts the prop at higher risk, even though the bronze alloy is the most noble metal on the boat (Vyv will correct me if I am wrong, no doubt)

 

[vyv_cox]

I searched a little yesterday and as far as I could tell the Gori prop uses manganese bronze in its construction. This material is a modified brass that will suffer dezincification. An anode does help but will not fully prevent dezincification, it is there primarily to protect the mixture of metals when gears, pins, shaft etc are incorporated in the design. There are plenty of manganese bronze props with a lifetime of well over 20 years, so it's not a material that fails overnight.

Abrading back with 120 or 180 wet and dry should take it back to a nice brassy colour within a very short time. If the red colour continues more than a millimetre or so you might need to consider further, especially at thin sections near the blade tips.

 

[westhinder]

Similar experience here: Gori 3-bladed showed some pitting when I bought the boat and removed antifouling from the prop. Suspected cause: boat had overwintered in a marina next to a big steel boat, both plugged into mains. Prop was returned to Gori for overhaul, and came back nice and shiny, with new rubber inserts, modified to incorporate an anode at its front end, but the pitting still in place. Gori commented that it was impossible to fill the pitting and that it would not have averse consequences.
I am contemplating what to do next. There is no evidence of vibration and the prop functions normally. I may take the prop to an industrial prop maker near me to see if they can fill and fair the pitting.