de-zincification of prop

[bigman1]

Hi does the prop in the picture look as though it has started to de-zincify? I was removing the prop with a brass drift and a small piece of it came off, it looks sort of pink to the eye, any thoughts.
ts of
Also i was shopping around for a new prop at the cheeper end of the market, they appear to be made from manganese bronze which also contains zinc. What i would like to know is if i bought one and installed it on the shaft and gave it several coats of epoxy resin would that protect it from the de-zincing process..

thanks ronnie

 

[Tranona]

Classic signs. Does it ring if you tap the blades with a light hammer? An anode will prevent dezincification of your new prop.

 

[Scarboroughsloop]

Looks like it to me I'm afraid. Hit one of the blades with a spanner or something, you should get a clear 'ring' out of it if it's ok.

 

[superheat6k]

Severe dezincification I would say. manganese bronze will be fine, but fit anodes and possibly a GI if you have shorepower.

 

[bigman1]

Thanks for the replie's, did not know about the "ring" test will try that when i go back to remove prop. to check the shaft taper i believe they are all different but 1:12 is the standard. Thanks. Ronnie.

 

[Tranona]

Thanks for the replie's, did not know about the "ring" test will try that when i go back to remove prop. to check the shaft taper i believe they are all different but 1:12 is the standard. Thanks. Ronnie.

Depends on whether it is a metric or an imperial shaft. Measure the diameter carefully, but also check the length and diameters of the taper as there as some *******s around.

 

[VicS]

Severe dezincification I would say. manganese bronze will be fine, but fit anodes and possibly a GI if you have shorepower.

Anodes are needed to protect any underwater fittings that would otherwise be susceptible to galvanic corrosion. This does not include true tin bronze thru hulls, and should not normally include manganese bronze props or even DZR brass fittings but due to the mix of alloys it usually includes folding props.

A galvanic isolator is just about essential if shorepower is fitted and left connected, even if not actually in use, for prolonged periods. Without it anodes may have a very short life and drive legs may be vulnerable to expensive damage.

I agree that the OP's prop appears to be suffering from dezincification. Not enough info to identify the cause.

 

[superheat6k]

Anodes are needed to protect any underwater fittings that would otherwise be susceptible to galvanic corrosion. This does not include true tin bronze thru hulls, and should not normally include manganese bronze props or even DZR brass fittings but due to the mix of alloys it usually includes folding props.

A galvanic isolator is just about essential if shorepower is fitted and left connected, even if not actually in use, for prolonged periods. Without it anodes may have a very short life and drive legs may be vulnerable to expensive damage.

I agree that the OP's prop appears to be suffering from dezincification. Not enough info to identify the cause.

What material is the cutlass bearing housing made from - that looks a bit pink too ?

 

[vyv_cox]

Looks to me that the prop is suffering quite severe dezincification but the P-bracket is painted red. Paint is quite good protection against dezincification. My P-bracket and prop are painted with Hammerite Special Metals primer, which has good adhesion to manganese bronze. The prop has its own anode, plus a shaft anode, but the P-bracket is otherwise unprotected.

 

[Tranona]

What material is the cutlass bearing housing made from - that looks a bit pink too ?

If the boat is the one in the OPs profile the cutless housing will be bronze with (hopefully) bronze locking screws into the deadwood. There are no dissimilar metals here and the bronze does not suffer from dezincification.

 

[VicS]

What material is the cutlass bearing housing made from - that looks a bit pink too ?

If the boat is the one in the OPs profile the cutless housing will be bronze with (hopefully) bronze locking screws into the deadwood. There are no dissimilar metals here and the bronze does not suffer from dezincification.

Looks to me that the prop is suffering quite severe dezincification but the P-bracket is painted red. Paint is quite good protection against dezincification. My P-bracket and prop are painted with Hammerite Special Metals primer, which has good adhesion to manganese bronze. The prop has its own anode, plus a shaft anode, but the P-bracket is otherwise unprotected.

Looks like red antifouling or paint on the bearing housing to me!

 

[GrahamM376]

Problem is, a new prop may well suffer unless you have an anode.

Looking at the photo it seems you can't fit a shaft anode. If you have a flexible coupling between gearbox and shaft, does it have a bridging wire to give good continuity between the shaft/prop and the gearbox and, is the engine connected to an anode? Alternate method is one of these - http://www.anodeoutlet.co.uk/products/3/electro-eliminators/?gclid=CPrEjZua8bwCFc_MtAodfgIAPw

 

[burgundyben]

Forgive me...

I pic suggests that the prop is made of a material which includes zinc? That would be brass...

Surely the root cause of this is that the prop has been made of an unsuitable material?

A prop'er prop (sorry), would be bronze?

 

[Tranona]

Forgive me...

I pic suggests that the prop is made of a material which includes zinc? That would be brass...

Surely the root cause of this is that the prop has been made of an unsuitable material?

A prop'er prop (sorry), would be bronze?

Quite common for props to be made of "manganese bronze" - which is a bras containing zinc. Hence the need in some cases for an anode.

 

[bigman1]

Thanks for all your replies i would like to know if several coats of epoxy resin would protect a new prop. Thanks. Ronnie.

 

[VicS]

Thanks for all your replies i would like to know if several coats of epoxy resin would protect a new prop. Thanks. Ronnie.

I'd think a more diagnostic approach to the problem would be the way forward

What alloy is the existing prop? Maybe a good quality manganese bronze replacement will be all that is necessary. Fit cheap and you may simply be heading down the same route again. Manganese bronze is used for props because it is highly resistant to seawater corrosion.

If the present problem is not due to the existing prop being an unsuitable alloy or poor quality what factors may be causing the problem?
What anodes are fitted? To what are they bonded? They were not common in the days when Seadogs were being built so any fitted may be later additions
Just what is bonded to what? Current wisdom is that the one time belief that all underwater bits and pieces should be bonded together and to anodes is wrong.

Is the boat connected to a shorepower supply for prolonged periods? If so, is a galvanic isolator fitted?

What is the overall condition of the electrical system? Could some poor or inappropriate wiring be leading to electrolysis?

I am not a coatings expert so will make no comment on the advisability or otherwise of epoxy coating a prop.

 

[wee_niall]

this tap test - i take it if the prop is knackered it won't ring and sound a bit hollow/dull?

 

[vyv_cox]

Forgive me...

I pic suggests that the prop is made of a material which includes zinc? That would be brass...

Surely the root cause of this is that the prop has been made of an unsuitable material?

A prop'er prop (sorry), would be bronze?

Have you seen the price of bronze propellers? Some big fancy mobos are fitted with nickel aluminium bronze ones, by far the preferred material for cavitation and corrosion resistance, but one of them would probably cost more than your boat. Some folders and featherers have bronze parts, mostly aluminium bronze I think, but again they tend to be at the upper end of the price range. Manganese bronze is by far the most common choice for production boat props and P-brackets but, as VicS says, some precautions are necessary.

My experience is that it is difficult to bond epoxy to manganese bronze, which is why I have recommended Hammerite Special Metals primer, which has worked well for me and is cheaper, more readily available and easier to apply.

 

[Talmine]

Vyv I'm really interested in the use of special metals primer primer. Do you paint over that or do you leave as is
Jim

 

[vyv_cox]

Vyv I'm really interested in the use of special metals primer primer. Do you paint over that or do you leave as is
Jim

I bought Velox antifouling for my prop. It is quite expensive stuff and requires a primer, also quite expensive (someone on Ebay sells the two for £64 for very small tins) I had a new tin of the Hammerite primer with which I have had considerable success in the past, so I thought I would give it a try. After season 1 some of the Velox had chipped off, possibly because I had followed the instructions to apply four coats, which seemed to produce a rather thick coat. The primer beneath was perfect. For season 2 I painted the P-bracket with the Hammerite, then gave two coats of Velox to it and the prop. Results were pretty good, shown in the pics. In each case the boat was in the water for around four months in Greece.
Season 1

Season 2

After studying photo 1 it seemed that the Hammerite alone was providing some A/F protection, so in season 2 painted a couple of areas with it alone, two short lengths on the prop shaft visible in pic 2 and a small area on the keel. These turned out to be barely better than any unprotected areas, e.g. the remainder of the prop shaft in pic 2.