As a surveyor I have found that there are many differing opinions on the longevity of manifolds and risers. Manifolds are of course the exhaust system of a boat. Their job is to cool the escaping exhaust gases that the engine makes before it goes overboard. Risers do the exact same thing they introduce cooling water into the exhaust exit piping system. Why are they important? Simply put they keep the temperature of the escaping exhaust gases from overheating the manifold and riser and melting the rubber exhaust bellows just after the riser mount. So tell me how long will these things last? They are expensive wear and tear items and I need to know the longevity of them before I buy my next boat or replace my current ones.
The general consensus on raw water-cooled manifolds and risers is that they will last upwards of (6-8) years. Keep in mind that most if not all engine manufacturers do not put out a service bulletin on how long these maintenance items will last. Boat US uses a probability table to guess the life expectancy of these items @6 years they estimate that the manifolds and risers have a (65%) probability of failure and @8 years they estimate a (90%) probability of failure. While this table is helpful it does not tell you exactly what you want to hear “when should I replace them?” As a surveyor I will tell you if your manifolds and risers look bad meaning they have the tell-tale signs of needing replacement such as listed below then they probably need replacement.
- Heavy rust between the manifolds and riser joint assuming they are not “dry joint” manifolds.
- Paint is peeling from the manifolds and risers indicating overheating.
- Excessive rust on the underside of the manifold.
- If you simply do not know when they have been replaced last.
- If your engine is difficult to start, produces white smoke or runs roughly.
Now onto fresh water cooled manifolds and risers. This setup uses an antifreeze coolant mixture that circulates through the engine block then through the manifolds to avoid using raw water cooling. This setup is obviously more expensive but has it perks. I wont get into just how FWC works but I will explain what it means for manifolds. Notice how I said just manifolds… On this setup and most FWC setups only the manifolds are FWC not the risers. So this leaves the risers to be cooled by seawater and susceptible to the same corrosion as a RWC setup. Risers on a FWC setup need to be replaced on the same timeline as a RWC setup, (6-8) years. For a FWC engine setup in theory the manifolds should last as long as the engine. Meaning if the boat is in a fresh body of water the engine and manifolds can last in upwards of 20 years. Now obviously I do not believe that because I see all kinds of issues in my line of work with FWC setups. In my personal opinion if the FWC manifolds are over (10-15) years old I would go ahead and replace them. Remember we are not playing the “guess who can guess the correct timeline of a manifold” game which many internet warriors tend to squabble about. Bottom line is if these WEAR AND TEAR items fail then your engine fails and the worst part is you might not have any telltale signs before it happens.
Another manifold setup that Mercruiser came out with are aluminum manifolds with stainless steel risers. You can already guess what I’m about to say here obviously two dissimilar metals next to each other will start corrosion as the anode gets eaten (aluminum manifold). This setup just randomly breaks down internally and causes internal engine damage and you will not even see signs before it happens. I recommend completely replacing these manifolds and risers with the common cast iron manifolds and risers. This has been the recommended fix by many mechanics.
So in conclusion be vigilant keep and eye on your cooling system. If you have questions don’t be afraid to call your local surveyor to come take a peek at them. The cost will be less than hiring a mechanic to inspect them and a good surveyor will generally educate you on the tell tale signs of replacement.
Travis L. Palmer S.A, ABYC
Corsica River Marine Surveys