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Marine Manifolds and Risers

Marine Manifolds and Risers

Marine Manifolds and Risers

This may have been answered before. But I can't see any direct posts about it.

I have a mercruiser 3.0L (4 Cylinder) inboard marine engine with a sterndrive. I am told that as the vessel is used in salt water that the manifolds and risers typically require replacement every 2-3 years at a cost of about $2000 AUD each time (labour included). It seems ludicrous.

I have seen a manufacturer claiming that they can make custom stainless ones from 304L stainless with a 1 inch Zinc annode that have a warranty of 10 years for around $3000 AUD.

Considering the consequence of failure is often water ingress through the exhaust valves, causing engine failure I hope that someone has a thought on a solution.

I would have thought that a 300 series stainless would suffer from SCC and ultimately fail without warning also.

The marine mechanic simply says. If there was a good solution Mercruiser would have implemented it and therefore his advice is to keep it original and pay the $2000 every second year.

I am wondering whether an annode in the carbon steel mercruiser manifold would help or a custom set of duplex headers with an annode would be a better thing?


RE: Marine Manifolds and Risers

I assume you mean the seawater cooling pipework?

Stainless might last a bit longer but the warranty might be interesting to see how it works in practice.

Duplex seems a bit extreme but should work. Doesn't need the anode.

Aluminium bronze is an alternative.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Marine Manifolds and Risers

It is exhaust gas presumably 200-300 degrees C being immediately cooled by Seawater at about 15-22 degrees C. It is very similar to this drawing.

RE: Marine Manifolds and Risers

John Deere uses bronze for its wet manifolds. That is by far the best route. 316 stainless could work but you would need to thoroughly flush AND dry after each use.

RE: Marine Manifolds and Risers

Yes there are a lot of these in 316SS, but they must be built to fully drain with near zero retention.
The problem is simply cost.
If you want actual corrosion resistance so seawater you need Cu/Ni, NAB (Ni Al bronze), or high alloy SS (316 and 2205 don't come close).
I have seen this done for large commercial applications but not for personal craft.

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P.E. Metallurgy, consulting work welcomed

RE: Marine Manifolds and Risers

Thanks for your input LittleInch, TugboatEng and EdStainless.

I gather the Cu/Ni and NAB are prohibitivey expensive for this application and therefore may just pay the man every few years.

Thanks again.

RE: Marine Manifolds and Risers

You need to think about your usage pattern.
I have seen manifolds fail in 6mo, and others last 6 years.
Using it a lot helps. And by 'a lot' I mean 6-10hrs a day every day.
At the other end of the spectrum is occasional use. If this is followed with freshwater flush, then long life is reasonable.

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P.E. Metallurgy, consulting work welcomed

RE: Marine Manifolds and Risers

The cost really shouldn't be prohibitive. 90/10 CuNi and 316 are fairly close in price right now. The problem is simply that nobody is doing it. The low volume of production makes acquiring materials expensive.

Heck, lots of production motorcycles come with titanium exhaust systems. That would also be an excellent material choice for marine wet manifolds. With higher production the costs become quite reasonable.

Oh, and another nail in the SS coffin, the ends of the manifolds where the exhaust hoses attach will rot out due to crevice corrosion.

RE: Marine Manifolds and Risers

Yes, Ti is the other good option.
The metal isn't all that expensive but the labor to fab is.
There is someone here is WI that makes aftermarket Merc manifolds.
Mostly ex-Merc guys.

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P.E. Metallurgy, consulting work welcomed

RE: Marine Manifolds and Risers

The useage looks a lot like 4-5 hours per week over the summer months to 4-5 hours per month over the winter months with a freshwater flush after each use.

I think Mercruiser made millions of these things and nobody bothered or were able to commercially make a better solution. So perhaps I'll remove them in 3 years and do a hydrotest each year thereafter.

Wife bought something called a salty captain engine flush kit that is suppsoed to help with the flush. However it specifies for Outboards and PWC. No idea what the active ingredients are... But I figured its probbably helpful.


RE: Marine Manifolds and Risers

Sodium nitrite makes an excellent corrosion inhibitor for flushing. It's not so good for the fishes and shouldn't be poured down drains so you would want to keep a volume of it for flushing that you can continue to recycle. It's commonly available as Diesel Engine Supplemental Coolant Additive. Mix 16:1 with water.

RE: Marine Manifolds and Risers

There is also a product called Chlor-Rid that is used for removing salt residue.
I have seen it used on industrial application where they are laying something up for storage or need to repair equipment and clean before welding.

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P.E. Metallurgy, consulting work welcomed

RE: Marine Manifolds and Risers

The SDS for the Salty Captain seems to suggest possibly something like that.

Unless mistaken I think Alkaline salts could well be a sodium nitrate. I'll take a look at chlor-rid when this other thing I have runs out.


RE: Marine Manifolds and Risers

Don't confuse sodium nitrate with sodium nitrite.

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