POSTED August 10, 2021
The Ultimate Guide To Manifolds, Risers & Exhaust Elbows On A Boat
There’s a plethora of manifolds, risers and exhaust elbows fitted to boat engines out there. It can be very confusing understanding at what intervals they require maintenance. To help keep it relevant, we’ve split this up into petrol and diesel engines. With diesel engines, you’ll typically hear about the “exhaust elbow”. Conversely, on petrol engines it’s more common to hear about the “manifolds and risers”.
Manifolds and risers are used to collect exhaust gases from the engine. They also cool them down, and allow cooling water to exit. On a petrol engine they are subjected to high temperatures and often saltwater, resulting in a very tough life. All water cooled marine engines will likely have some sort of manifold or exhaust elbow. This also includes generators. Although some can be coolant cooled as opposed to saltwater cooled.
Petrol manifolds and risers
The two most common manifolds and risers found in Australia are Mercruiser and Volvo Penta. We will cover both in this article. The two main styles are defined as “wet joint” and “dry joint” which determines where water flows. Dry joint means the water flows between the manifolds and risers implying it’s gasketed separately to the exhaust gases. Wet joint typically uses one gasket between the exhaust and water. This makes it more prone to leaking water back into the engine. To identify the correct service intervals, you need to figure out the cooling needs. Whether a) if the manifolds and risers are coolant/freshwater cooled, or b) raw/saltwater cooled.
How long do dry joint manifolds and risers last?
Mercruiser “dry joint” manifolds and risers were introduced in the early 2000s. These provide a longer lasting and lower risk alternative to “wet joint”. They’re much less likely to let water back into the engine. This is because the water and exhaust passages have a larger gap between them. But don’t let that fool you into thinking you never have to change them! They can be identified by the external casting of the water fittings where the gaskets join.
Dry joint manifolds typically last at least 5 years. These have a much smaller chance of failing and letting water back in the engine. Unless the manifolds are leaking externally, I’ve seen people run these unopened for 5 years and even getting lifespans of 7 years and more. It is still recommend they are pulled off every 3-4 years to monitor their internal condition, and if it all looks good inside they can be refitted and longer lifespans can be achieved. To my knowledge there are no “dry joint” Volvo manifolds.
How long do wet joint manifolds and risers last?
The dreaded wet joint are the exact reason why thousands of engines have failed over the years. The water galleries are within millimetres of the exhaust passages, and the gaskets are prone to failing. What typically happens, is the exhaust manifolds and risers are constantly expanding and contracting with heat, and eventually the gasket fails. This allows water either out (corroding local components), or back into the engine (damaging your engine internally). To make matters worse, add an overheat to the mix and you are destined for engine failure with distorted metal surfaces that cannot seal. The wet joint style was used all the way up until early 2000s, and Volvo is still using them. Wet joint manifolds and risers need the gasket replaced every second year, and typically don’t last as long, often failing at the gasket surfaces.
Mercruiser wet joint manifolds should be superseded/upgraded to the newer dry joint style, when due for replacement. It appears Volvo are still using wet joint style manifolds and risers on most of their petrol engines.
How to identify if I have wet joint or dry joint manifolds?
Wet joint manifolds and risers have visible mouldings on the outside of the exhaust area, sometimes with hoses joining in. Typically wet joint are square, with no mouldings.
What about catalytic converter type manifolds?
Because this style hasn’t been around for as long, there is limited information out there. Early reports are that they are holding up well, and when it’s time to be replaced the catalytic converter can be reused. Judging by the design of the manifolds and risers, it appears Volvo are running closer to the exhaust gasket, while Mercruiser has taken a slightly safer approach with some extra space between. Only time will tell how these units hold up, but I would suggest removal and inspection at regular intervals to avoid a damaged engine. It is noted that if the catalytic converter cannot be reused, this style can be the most expensive to replace.
Stainless steel manifolds and risers?
Stainless steel manifolds and risers bolted to aluminium heads means the cylinder head is going to be the material more likely to corrode away. The stainless steel manifolds often come with anodes and a large sticker that states “replace every 3 months”. The reality is some people aren’t even on their boat every 3 months, let alone going to change an anode they didn’t know about. Other than this I haven’t had enough experience with these manifolds to comment. I have seen some boats with them fitted and claiming 5+ years of life, but I have also seen others crack and damage an engine. Stainless manifolds eliminate the need for exhaust “elbows” and are an all-in-one unit.
Stainless steel is also used in mixing elbows on larger diesels, which we will elaborate on below.
What to consider when pricing a pair of manifolds and risers?
Whenever you are looking to replace a set of manifolds and risers, it’s important you ask a few important questions:
- Are my manifolds and risers wet joint?
- Can they be superseded to dry joint?
- Are any part of my manifolds and risers freshwater cooled?
- Are there exhaust risers fitted?
Firstly, if you have a freshwater cooled manifolds, and saltwater cooled exhaust elbows, it is unlikely you will need to replace the manifolds, and probably just the exhaust elbows.
Secondly, if you have wet joint on an older Mercruiser, you’d be crazy not to upgrade to the new style dry joint, as the cost will be similar but you will get much longer life out of the manifolds and risers, and less risk of water tracking back into the exhaust.
Lastly, if you need 3-inch or 6-inch extensions, on a twin engine setup this can add a considerable amount to the total cost of parts. A pair will likely add around $900 per engine, on top of the cost of manifolds and risers (elbows). If you find that your boat has extensions, you cannot fit them without, as they are there to keep water from back-flowing into your engine by increasing the height above waterline. The design of the hull and engine install will depend on if you need risers fitted.
Diesel exhaust elbows
Exhaust elbows come in a range of materials, such as cast iron, stainless steel and alloy. They all need replacing or inspecting at regular intervals.
Cast iron elbows are found on a number of engines, for example Volvo and Yanmar. The most common complaint is they corrode through or block up, and either let water back into the engine, or block up and overheat. You’re doing well if you get 5 years out of an elbow, but it’s worthwhile removing and inspecting every 2 years, depending on the manufacturer style and design.
Although used in some elbows, this has proven to be unsuitable and unlikely to last long. Most alloy elbows were superseded to a stainless steel or cast alternative.
Stainless steel mixing elbows are the most long-lasting, and provided they’ve been designed correctly can last in excess of 10 years. The most common place to leak is at the welds, and often near the water injection. Manufacturers have improved their design, and run sections of dry exhaust with water injection further down the elbow to avoid the chances of damaging the engine. It’s important that an adequate drop has been designed into the system, to avoid the chance of any water running back up the elbow and into the engine. For example, if the engine stalled and a wave hit, sending the elbows to an angle where water could back flow. The only downside to dry sections of exhausts are heat related, as if the exhaust temperatures get too high you can have other issues such as cracking and warping.
What about marine generator exhaust elbows?
Don’t forget about your generator elbow too! Pending it’s construction material, it will need regular inspection or replacement based on the above guide.
Not sure about what should be changed, and when?
Whilst there are a number of tell-tale signs, the most reliable method is to remove and visually inspect the internal passages of the exhaust system. If there is no history provided in the timeframes listed above, you need to assume they are going need inspection and possible replacement. If you’re not sure, we’d be happy to help with a Pre-Purchase Boat Inspection, providing our services in both NSW & QLD.
How long do exhaust manifolds and risers last?
As a general rule of thumb, when used in saltwater:
Dry Joint: 5-7 years, although recommend removal and inspection at 3-4 years
Wet Joint: Approx. 5 years, although recommend removal and inspection every 2 years
Stainless Steel: Approx. 5-8 years, provided anodes have been changed at regular intervals
When used in freshwater the lifespan can increase by 20-30%.
How long do diesel exhaust elbows last?
As a general rule of thumb, when used in saltwater:
Stainless Steel: 10+ years
Cast Iron: Approx. 5 years, although some manufacturers recommend removal and inspection at 2 year intervals
It will depend on the material and design of the elbow.
Will a low hour engine require replacement of the exhaust manifolds and risers?
Yes, it does not matter how many hours are on the exhaust manifolds and risers, they will wear out if used in saltwater and let sit for long periods. They can be removed and inspected to check before replacing, and this is the best way to check if they need replacement.