Props and thru-hulls are generally made of different bronzes. Many props are made of Manganese Bronze, though some are NiBrAl.
Manganse Bronze has a considerable amount of zinc in it depending upon the composition used to make the prop. Most marine
grade Manganse Bronze is comprised of roughly 58% Copper 39% zinc 1% iron 1% tin. Though different alloy compositions of Manganese Bronze can see zinc content as low as 25% and as high as 39%. The copper in bottom paint
can lead to de-zincification of a Manganese Bronze prop which is not good for it.
Thru-hulls & seacocks are generally, and I use generally carefully as who knows what's coming out of China
these days, made from a very low zinc content bronze. This Bronze for valves and thru-hulls is usually referred to as Silicon Bronze and is comprised of roughly 96% copper 3% silicon 1% Manganese. Again, who knows what is what these days so a call to a manufacturer is sometimes a good idea. Silicon Bronze has virtually no zinc so should not suffer from de-zincification like Manganses Bronze or Tobin Bronze. Painting thru-hulls should pose minimal threat if they are in-fact made from silicon bronze but painting a Manganse Bronze prop or Tobin Bronze shaft, with copper based bottom paint
, is not generally a good idea..
Prop shafts made of bronze were mostly made from Tobin Bronze which also has a higher zinc content and also does not like copper anti-fouling
paints. If you paint
a Bronze prop shaft or prop you should really keep up with your zincs so the high content zinc in the shaft or prop does not become the sacrificial material. Better yet it may be wise to use a paint that is safe for underwater metals rateher than a copper based paint..
Bronzes are not all the same and there are many different grades of bronze that really should almost be called a brass due to the zinc content but because of the added tin or other alloys they are referred to as bronze instead of brass. I've personally never understood why most props have such a high zinc content and are made from Manganse Bronze and not something with more corrosion
My prop was painted by an un-knowlegable yard ape, or should I say "slathered" with paint, on the way back from Florida
in one of those yards that does not allow DIY. Rather than have them strip it off the PO, & good friend of mine, launched and figured he'd just keep up with the zincs. By the time I bought the boat and stripped the copper based paint off the prop it was already de-zincified and pretty much toast despite the PO keeping up with teh zincs on the prop shaft!!
I paint my thru-hulls but not my prop, shaft or strut..