Propeller question

[boatsRus]

Whilst cleaning the accumulated grime from my propeller at the weekend, I noticed that the surface has a distinct pinkish colour over about 80% of it. When tapped with a screwdriver, it emits a dull thud rather than a sharp ring. It is a fixed three-blader, 14 years old and presumably made from manganese bronze. It is fitted with a Beneteau style anode which seems to have been working well (not much of it left after a year in the water). I would be most grateful for the Forum's advice - do I need a new propeller, or will it last another year?

 

[ANDY_W]

I'm surprised no-one else has answered this.

It sounds like your propellor has little or no connection to the anode and has lost most of the zinc content in the metal.

If you have a marina berth, another cause might be stray electrical currents affecting your berth area.

The propellor will have lost much of its strength and may fail under load.

If, on checking, you find that the prop shaft is properly connected to the anode, that fact that the anode has nearly all gone
in a year suggests that it is not big enough.

If you find that the prop shaft is not properly connected then you will need a much bigger one since you are using nearly all
the present one to protect whatever is presently connected to it.

I would also suggest that you check any other underwater metal fittings ( through hulls, etc ) in case they have the same problem.

Be advised this is not a professional answer- others here may know more.

Andy

 

[Tony Cross]

I'm surprised no-one else has answered this.

It sounds like your propellor has little or no connection to the anode and has lost most of the zinc content in the metal.

If you have a marina berth, another cause might be stray electrical currents affecting your berth area.

The propellor will have lost much of its strength and may fail under load.

If, on checking, you find that the prop shaft is properly connected to the anode, that fact that the anode has nearly all gone
in a year suggests that it is not big enough.

If you find that the prop shaft is not properly connected then you will need a much bigger one since you are using nearly all
the present one to protect whatever is presently connected to it.

I would also suggest that you check any other underwater metal fittings ( through hulls, etc ) in case they have the same problem.

Be advised this is not a professional answer- others here may know more.

Andy

+1

It's dezincification. The pink colour is the copper in the bronze that remains after much of the zinc has been removed by galvanic effects. I would get someone who really knows about these things to take a look at it and give you a feel for how bad it is.

 

[Bosun Higgs]

I'm surprised no-one else has answered this.

It sounds like your propellor has little or no connection to the anode and has lost most of the zinc content in the metal.

That is a huge assumption. My bet is that if he took some emery paper to the prop he might find that the pinkish colour is only surface deep - thats certainly the case with my prop where I have been battling this issue for several years now.

The dull sound is not a good indication when the prop is on the shaft.Take it off thje shaft and give it a sharp tap with a hammer. But more to the point is it bending or distorting or with serrated edges to the blades? No? Well then its not likely to be beyond common sense saving.

The key is a good electrical connection between prop and anode. In my case thanks to the electrical insulation of the gearbox, the path between hull anode and folding prop blades has a resistance of 60 ohms which is far too much. I've only got round it this season by fitting a new shaft long enough to allow for a shaft anode next to the prop ( circuit resistance below 1 ohm) so I have hopes for the future. Do when you refit the prop and the new anode, check the resistance between the two.

Electrical paths can be funny things. I never leave my boat with the mains power connected - a small floating voltage on the engine and stern gear can negate the effect of the anode.

 

[vyv_cox]

There is a long page on dezincification on my website at http://coxengineering.sharepoint.com/Pages/BrassandBronze.aspx There are no photos of propellers, sorry, most of it is seacocks, but there are plenty of pictures of light and severe attack and several between. A bit of pinkness is not a problem, and as said may well be only the surface layer. The time to worry is when it looks like the really bad example next to last on the page, when chunks of the blades will be chipping off the prop.

 

[Jcorstorphine]

If the prop is made from "proper" marine grade bronze, you should not get any dezincification as their is no zinc in these alloys. You can of course get "dealuminication" from Aluminium bronze etc.

As said, the pink colour is not unusual and is just a higher % of copper at the surface and as also said, a quick scrub with an abrasive should get you down to better alloy. HOWEVER!!!! The dull thud is a bit of a worry as a good prop should ring like a bell.

 

[boatsRus]

Thanks to all for your helpful responses. I have bitten the bullet and bought a new propeller - better safe than sorry.