bronze prop shaft and sterngland

[skyflyer]

The company that winterised our boat have reported that the bronze prop shaft is showing signs of "corrosion" near the sterngland.

I was not concerned beforehand because whilst there is some green and white powdery 'gunk', it is no more than one would expect, surely, from something that - because the sterngland drips - is constantly wet!

Or should I be concerned about galvanic corrosion in a bronze shaft?

The engineers went on to suggest that a bronze shaft was a nightmare because it is relatively soft and thus overtightening of the sterngland will cause grooving around the shaft and sealing problems etc.

Removing and replacing this with a s/steel shaft however is an expensive job (I dont have time to do it myself)

Should I be concerned - what are the chances of shaft failure or a major leak? I am minded that "if it aint broke, dont fix it"!

Thanks

 

[pvb]

Should I be concerned - what are the chances of shaft failure or a major leak? I am minded that "if it aint broke, dont fix it"!

I don't think you should be overly concerned. I reckon that your engineers are looking for extra work! Bronze is a bit softer than stainless steel, but works well for propshafts (assuming it's properly sized). If you start to get excessive weeping from the stern gland, and can't fix it with adjustment or new packing, that could indicate wear. But it's relatively easy to get an existing worn shaft either rebuilt with bronze on the worn area, or sleeved with stainless steel where the gland bears on it. And these solutions may be cheaper than a new stainless shaft.

 

[vyv_cox]

Bronze shafts are normally made from a gunmetal, or another tin-based bronze. These have excellent corrosion resistance. A small amount of surface colour is typical of many bronzes (e.g. patina on statues, etc)

Galvanic corrosion is a possibility where electric currents are involved or if there is a couple with another metal, but is rare with a tin bronze.

The hardness of bronze is certainly lower than that of 18/8 stainless steel but it is widely used for shafts in conjunction with packed glands. I have never heard it described as a 'nightmare' before. You do need to be careful with overtightening the gland but one of the beauties of this type is that it is very tolerant of shaft wear.

I would not rush to change anything until there is clear evidence of a real problem.

 

[Tranona]

As said, unlikely to be serious corrosion. When I replaced my bronze shaft there was little wear (after 25 years) except the bit that ran in a white metal bearing that had probably been run short of grease at some point.

Replacement with a stainless shaft is not hugely costly in parts, it is the labour that makes it expensive, particularly if you are replacing the cutless and stuffing box as well. Job best left until you need it.

 

[skyflyer]

Thanks - that pretty much bears out what I thought! We'll keep it as is for the time being and the boatyard can look elsewhere for winter work!

 

[Spyro]

I replaced a bronze shaft 2 years ago it was leaking badly at the gland and needed constant adjustment. When I went to repack the gland I noticed the shaft was badly worn.
I reckon when under power the shaft moves forward slightly in the gland and the shaft wears at that point. When power is taken off the shaft moves back a bit until a non worn part is now in contact with the packing rings thus forcing them out a bit. when drive is applied again the worn (smaller diameter) section is now trying to seal in a larger gap.
That was my theory. The new shaft and packing cured the problem. A new shaft made up by a local engineering firm was £120. Fitted it myself.

 

[Tranona]

Unless your engine is solidly mounted (which I doubt) it will always move back and forth taking the shaft with it. anything up to +/-3mm is common on small engines, more with big engines on very soft mounts.

 

[ccscott49]

I have two bronze shafts, 2 1/2" diameter. 12 feet long. I have just had them machined from 2 5/8" down to this size, fitted new cutlass bearings and new stuffing box material, as they had worn in the seal area and one cutlass area. This is after 41 years, so I was not really surprised it needed doing. The rest of the shafts were immaculate, but of course the whole lot needed turning down, not a huge job, cost me 250 euros here in Corfu.
It is very difficult to "weld" or braze a bronze shaft and keep it straight, also never heard of sleeving with stainless although they used to be a place in lancashire who would shrink a bronze sleeve on, however they no longer do it and I couldnt find anybody to do that, anywhere, so hence the turning down.

 

[Billjratt]

OP talks about corrosion rather than wear, so possibly it's pitting in the area from the gland to the cutless if it's all in a log tube.
My 1978 boat had the same experience though the shaft is SS, probably due to an accumulation of debris from the wrong part of the periodic table in the tube.
new shaft required, although they did have a go at building up and re-turning - unsuccessful.
If I had a bronze shaft, I'd pull it out and judge the damage (or get another opinion) and only replace it if really neccessary.