Dissimilar metals and galvanic corrosion

pjv911

pjv911

Member
Jan 25, 2014
116
Mount Sinai Long Island Sound NY
Boat Info
1998 Sundancer 330
Engines
2016 385ci 6.3
390hp 479 ft tq Modified Scorpions
4V magnums
1.5 Velvet V drives
I just picked up a Donzi 21'. I am building a 572ci big block for it.
The internet seems to be all over the place on the subject of using aluminum in a salt water cooled engine.
Here is my question....
If I were to install Aluminum performance exhaust manifolds and an aluminum intake manifold would it make sense to also use aluminum heads?
I understand that the dissimilar metals in salt water actually become a battery and causes accelerated corrosion. Does it make sense to also have cast iron heads sandwiched in between the aluminum? Im thinking that might be more of a mistake?
Lastly the new style multi-layered rubber/stainless head gaskets do a very good job to stop the galvanic corrosion between an iron block and aluminum heads.
I recognize that a closed cooling system will make problems go away but that is not an option and the boat will be trailered and flushed every use.
Hoping someone with a bit of knowledge on this subject will offer some advice.
 
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Whenever Mercruiser tried different metals for exhaust components, the results were disastrous, at least in the salt water boats. I'd be very leary. What's the advantage? Weight?
 
Weight is one advantage. The other is you can not make 650hp with a cast iron intake so if having dissimiler metals why stop there. Might be better to have the entire top end as well as exhaust be 100% aluminum.
Also aluminum cools better and allows slightly higher compression ratio to be used.
On a side note my Searay 330 has aluminum intakes bolted to cast iron heads and that seems to be the case for many 1998 and up Mercruiser engines.
I understand they may also have brass runners but its still aluminum bolted directly to cast iron.

rondds said:
Whenever Mercruiser tried different metals for exhaust components, the results were disastrous, at least in the salt water boats. I'd be very leary. What's the advantage? Weight?
Click to expand...
 
Trailered, pulled out of the water after using, and flushed and washed - no issues. You will still have corrosion but really little different than in fresh water.
You can offset it even better if you electrically bond it all together with wire and tie the bonding to a small zinc on the hull or out-drive.
The issue becomes if you are passing electrical current through a host of different metals (aluminum, stainless bolts, carbon steel) and by bonding each with a wire that corrosion between the metals greatly reduces.
 
And ensure that the bonding all ties back into your bonding system buss. I agreee that if this doesn't stay in salt water unless being used, you have a good chance to beat the corrosion problem. Bond everything!
 
Pj,

Using an insulating gasket between two dissimilar metals only works effectively under specific circumstances. For example, using an insulator between an aluminum component and a steel mounting plate would work fine, even if the "insulator" was just a good layer of paint on the steel. However using an insulator between aluminum manifolds and a cast head would have limited value. The insulator would block the circuit between the direct contact of the two metals. But depending on where the raw water for cooling enters the exhaust flow could negate the overall protection. The moisture would become part the electrical circuit bridging the two allowing the corrosion to flourish. It's the same as the problems with SS props and aluminum housing stern drives. In that case there is no direct physical contact, but still massive galvanic corrosion. So in my opinion the short version is you can't get there from here.

All that being said, I think you may be trying to achieve too much. My observation is that high performance does not equate to dependability, or longevity. You want a 650 hp boat power plant? Then resign yourself to regular engine tear downs with more frequent parts replacement. In other words, if you want a lot of ponies, be prepared to replace the aluminum exhaust parts on a regular (& frequent) basis. On the other hand if you want to only replace manifolds every five or so years, go with steel and forgo the hp benefits.

Henry
Sent from my iPad using Tapatalk HD
 
I agree. I have also been discussing my plans with local guys and the same things have been said. So I am just gunna build it my way and deal with the consequences. Of course I will be doing a very thorough flush immediately after every use. Also this boat (unlike my Searay) will not be used for 5-10 hour trips so I doubt I will put more than 50 hours a year on it.
It was also suggested to build a mild 572 with cast iron components because my torque will be off the charts anyway and my goal is not to go 75+ mph. I am seeking strong acceleration and keeping rpm under 5400-5500 rpm.
Is it beneficial to install as many pencil anodes as possible? Will that matter much on a trailered and flushed application?


Henry Boyd said:
Pj,

Using an insulating gasket between two dissimilar metals only works effectively under specific circumstances. For example, using an insulator between an aluminum component and a steel mounting plate would work fine, even if the "insulator" was just a good layer of paint on the steel. However using an insulator between aluminum manifolds and a cast head would have limited value. The insulator would block the circuit between the direct contact of the two metals. But depending on where the raw water for cooling enters the exhaust flow could negate the overall protection. The moisture would become part the electrical circuit bridging the two allowing the corrosion to flourish. It's the same as the problems with SS props and aluminum housing stern drives. In that case there is no direct physical contact, but still massive galvanic corrosion. So in my opinion the short version is you can't get there from here.

All that being said, I think you may be trying to achieve too much. My observation is that high performance does not equate to dependability, or longevity. You want a 650 hp boat power plant? Then resign yourself to regular engine tear downs with more frequent parts replacement. In other words, if you want a lot of ponies, be prepared to replace the aluminum exhaust parts on a regular (& frequent) basis. On the other hand if you want to only replace manifolds every five or so years, go with steel and forgo the hp benefits.

Henry
Sent from my iPad using Tapatalk HD
Click to expand...
 
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Given your signature boat I assume you are a family man. Is the Donzi an older hull? Will the structure of the boat handle the HP and speed? Do you have a life insurance policy that will not exclude this type of activity?
 
It's a 1995 and Donzi is built to very high standards. No wood used in the hull anywhere.
As for life insurance? Well I do need a bigger policy.
I am a very cautious operator. I also have high respect for the ocean.

hottoddie said:
Given your signature boat I assume you are a family man. Is the Donzi an older hull? Will the structure of the boat handle the HP and speed? Do you have a life insurance policy that will not exclude this typie of activity?
Click to expand...
 
Are you putting this engine together yourself? What kind of drive are you putting behind it? Sounds like a beast.. I would be talking to a marine performance shop about your concerns. This kind of performance is being built reliably everyday for salt water. No discredit intended to any of this forums members but you will save yourself a whole lot of grief and $$ by getting it right the first time.
 
I agree. And I have been consulting performance boat people.
Yes building it myself. That's the easy part. Knowing the right parts to choose is the difficult part.
The boat has a Bravo one.


Thornton69 said:
Are you putting this engine together yourself? What kind of drive are you putting behind it? Sounds like a beast.. I would be talking to a marine performance shop about your concerns. This kind of performance is being built reliably everyday for salt water. No discredit intended to any of this forums members but you will save yourself a whole lot of grief and $$ by getting it right the first time.
Click to expand...
 
I built this boat about ten years ago. These photos were taken just before it was sold late 2015. It spent the last ten years in salt water here in east central Florida. The trick was TLC, wash, and flush right after it came out of the water. All uncoated metal was sprayed with a water displacement like WD40. But, the most important thing for corrosion prevention is closed loop cooling. All of the metal is electrically bonded; even the rub rail.

 
Wow Tom, very nice! How fast did that bad boy run?
 
fwebster said:
If the name on the transom isn't "Space Cowboy," it should have been........!
Click to expand...

Kind or leaving our corrosion discussion.... but -
This boat was a lot of fun and if you wanted to get serious and put on the safety gear it was capable of hitting the triple digits and hit those speeds in in around thirteen seconds. It demanded jurisprudence to drive. Hardly practical but what boat is?
However, after an hour or two in it you would be rattled enough to put it away; It made all of the right noises.
 
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