brass prop + alum boat

[ididit]

Droping a diesel motor into a verry old alum. boat , and I know that [ brass an alum ] the two metals don't mix in salt water . Have search all over for a alum prop with out any luck. woundering if by painting the prop this migth help any one know what I should do? thanks

 

[Texasmark]

Re: brass prop + alum boat

Just dug out my Materials Application, Dissimilar Metals chart.

You are correct in what you said. Al is Group I while Copper and alloys (Brass-Bronze) are over in Group IV along with Stainless Steel. Obviously you know that you don't mix groups if you can help it, but adjacent groups are somewhat tolerable, and the notes say "in contact with" is where the problems occur.

I know you are talking about salt water being the vehicle where the migration will take place; hence "in contact with"; but at a highly reduced rate due to the low conductivity of the water.

Butttttttttttttt Alum alloys, which most boats are made from to get 5052 T6 grade of alum for strength, is an alloy and Zinc and Al alloys show up in Group II, along with Stainless Steel (SS is in all groups but I).

In Group III we also find Zinc along with SS.

So, a strategically placed zinc bar, like between the prop and hull (assuming you are talking about an inboard setup, would protect the weaker metal. Needs to be in contact with the alum so you will need some SS screws to attach it and then seal the screws on the inside of the hull.

The prop will mount to your SS shaft so that's fine, as is your brass/bronze thru hull bushing and your SS shaft.

More buttttttttttttttts. For years and years, brass props were used on ss drive shafted, alum housing outboards, especially at OMC. I know that the Merc trim tab has been zinc for years and years. I think Merc I/O's also had one on the outer transom mounting bracket long before they added it to their OB's.

HTH

Mark

 

[tashasdaddy]

Re: brass prop + alum boat

i think Mark covered it pretty well. Zincs. how about some pics!

 

[Dunaruna]

Re: brass prop + alum boat

[colour=blue]

I know you are talking about salt water being the vehicle where the migration will take place; hence "in contact with"; but at a highly reduced rate due to the low conductivity of the water.

Mark

Objection your honor!

 

[tommays]

Re: brass prop + alum boat

You only get Bronze or Nibra in that style So there must be a lot of Alu work boats that are just useing the right anode material to keep things in control


Is The boat going to have bottom paint ?


Tommays

 

[ididit]

Re: brass prop + alum boat

The alum boat will stay on the trailer most of it's life when I go fishing it in the water for 10 to 14 days
This was a free boat with trailer it's a old de havilland with a model numder of 101 or seral number . The boat is conctucted
with very heavy alum. 1/4;inch and had a inboard installed befor
I have a 3.5 hp. diesel motor I bougth new for au 150.00 and bougth the tranmison shaft an prop for au150.00
I still have not got the motor installed it's ben a king size nightmare trying to line it all up. now waiting for new couplings.
I will send pic. when I get more advanced thanks Donnie

 

[Texasmark]

Re: brass prop + alum boat

I wouldn't even worry about that. There are millions of alum boats (like 14') that have outboards on them, some with brass props, probably all with SS shafts. Some have been in service for 50 years. Yeah I know some get painted, but paint scratches off easily so I wouldn't put any stock in that.

Just take the outboard for example. Don't know of one that isn't made of alum and even though they are treated by all sorts of anticorrosion stuff, years ago they weren't and lots used brass/bronze props. We see lots of them on here, some restored. I vividly remember seeing numerous old Rudes, 2 and 4 cyl opposed cylinder that are in great shape with their brass prop. Gotta be 60+ years old.

Haven't seen any in really bad shape and the engine(lower unit) is a casting, not high tensile marine alum like boat hulls are made from.

But zinc anodes are readily available now so there is no reason not to have one. :%

Tom. Hear you on the work boats. I know oil company crew boats that work in the Gulf of Mexico are very big and heavy. Some are steel with Detroit Diesels and others are alum with outboards. Maybe some inboard alum also. They last a long time.

What is Nibra?

Mark

 

[tommays]

Re: brass prop + alum boat

OPPS

Nibral props

Just a much better alloy


tommays

 

[Texasmark]

Re: brass prop + alum boat

Still not reading you. Don't know what OPPS means and alloy of what? Who makes them and how do they compare to the more popular materials in terms of performance props built with the material.

Thanks,

Mark

 

[tommays]

Re: brass prop + alum boat

http://www.google.com/search?hl=en&q=NIBRAL+PROPS&btnG=Google+Search

Its the high performance material for shaft drive boats as they dont use SS on them


Tommays

 

[Texasmark]

Re: brass prop + alum boat

Ok sir. Thanks. Now you opened another curiousity.

Why don't they? I mean if it is the material of choice for hp OB and SD, why not for shaft drive inboards also?

Mark

 

[Texasmark]

Re: brass prop + alum boat

Read the www. coastal prop thread. Good reading.

Interesting comment about cavitation as a function of blade tip speed.

Not a naval architect, nor a prop tester for MWC, but hull design has a whopping impact on prop requirements...doesn't take a rocket scientist to figure that one out........how that "weight" passes thru the water. He said nothing about that.

That opens another question. If that (tip velocity) is so important, why do the highest hp outboards turn the highest pitched props at the highest speeds (posted 5750 max rpm at WOT) and the lowest gear ratio....like 1.75:1 ?

But tweaking props on an inboard has to be a pain, due to the inaccessability.

Mark

 

[tommays]

Re: brass prop + alum boat

The SS prop Keyed onto a SS shaft works out real BAD

I repiar SS mixing tanks and the SS blades eat the SS taper pins over a long period of time

Tommays

 

[Texasmark]

Re: brass prop + alum boat

Tom, you think that might be it? Surely someone has tried it. I am really surprised that you (I) just don't see any SS props out there.

So now the next question......why do your blades eat your pins? Slop? Or corrosion? I know SS resists corrosion, but it is still metal and still has (some) iron in it and can and does still corrode.

Mark

 

[tommays]

Re: brass prop + alum boat

All i can tell you is from working with SS for a long time even on dry land all it takes is a littel stray current a littel bit different batch of alloy some time and it will pit :devil:

So now pretty much everything is welded in one alloy and you cut it apart to do repiars No more knocking out pins to remove a bent blade

I only buy tig wire with the 316 SS stamped in both ends of the wire if i repiar a mixer blade with 304 wire buy mistake it will pit pretty fast in a mixing tank

In our industry all the materials come with certificates of compliance sheets so you dont mix stuff up and make a mess

Tommays

 

[Texasmark]

Re: brass prop + alum boat

Thanks

Mark

 

[tommays]

Re: brass prop + alum boat

i kind of forget that SS also really likes to gouge and stick to other SS when tight fits are needed

Tommays

 

[Texasmark]

Re: brass prop + alum boat

I think that'is a problem with materials of like kind; galling.

I know alum-alum is really bad about it. Course having said that, I can't currently recall where I had the problem with alum as I know they don't make alum screws....that I know of. Must have been some kind of alum part wedging into another one or something like that.

Mark