Westerbeke 4107 Exhaust Riser

[Poche]

I would like to hear some opinions on riser height after my exhaust manifold. A marine fabricator is suggesting that he build the exhaust pipe so that it rises 8" above the exit of the manifold with a anti-siphon tube just after the top bend. The exhaust will travel down about 14" to the muffler (about 6" below the waterline) then up to just below the deck then down to the transom.
My concern is that a riser height that high might let too much fluid back into the manifold when the engine is shut off. I have no feeling of how much fluid the manifold can accommodate before it spills over into the head.
I read about risers becoming plugged by corrosion and fluid getting into the engine so I am concerned.
The engine had been custom fitted without a riser and ran for twenty years. Maybe that was luck.
I am thinking of a more modest riser height of four inches.
It's a Cheoy Lee Offshore 33 by the way.

[oldcal46skipper]

Of the 300 + 4.107's & 4.108's we have rebuilt over the last 20 years, over two thirds were damaged by faulty water injection elbows. With your engine idling, look at the amount of cooling water exhausted. Pressure from the exhaust gasses push the water up and through the elbow. When you shut the engine off, that water must go somewhere. At least one exhaust valves will be partially open, thus allowing raw water to enter the cylinder =corrosion, rust, stuck rings, etc.


I recommend the exhaust elbow be below the manifold.

[Poche]

Originally Posted by oldcal46skipper

Of the 300 + 4.107's & 4.108's we have rebuilt over the last 20 years, over two thirds were damaged by faulty water injection elbows. With your engine idling, look at the amount of cooling water exhausted. Pressure from the exhaust gasses push the water up and through the elbow. When you shut the engine off, that water must go somewhere. At least one exhaust valves will be partially open, thus allowing raw water to enter the cylinder =corrosion, rust, stuck rings, etc.


I recommend the exhaust elbow be below the manifold.

If I understand you correctly, do not use a riser elbow. Route the water directly down from the manifold to an elbow.
That makes sense to me and has worked for at least the past twenty years on this engine for the previous owner.
"the water must go somewhere". I don't think the manifold has much capacity for returned water.
Thank you for your recommendation.

[DougR]

A properly designed and built riser will not let any water into the manifold unless the riser fails in some fashion, either thru cracking or corrosion, etc. That said, old Cal's comment about failures from elbows/risers could be accurate.....elbows crack or corrode and allow water back into the engine.

The reason for this is that in typical OEM production elbows (cast iron) the entire length of the elbow is frequently water jacketed and when the jacket fails, the water has a path into the exhaust stream and could make its way into the engine.

If you feel you need a riser because of the constraints of the system dimensions, I would recommend something like the simple sketch which I have attached here. This riser is something that is very commonly found in the power boat world, both as factory supplied or custom built.

As you see, the majority of the elbow is dry, with no water jacket but with dry wrap insulation, and the water jacket portion of the pipe is only at the outlet end. These are normally built of stainless steel , but could be mild steel with a stainless portion at the end. Even if the water jacket suffers some sort of failure, all of the water injection is well past the high point of the riser and any errant water will flow down to the lift muffler and not back into the engine.

I once had a 4-108 that had an exhaust system built of black iron pipe. This went up about 12" from the manifold, across 6", down 12", made a 90 degree bend and the water was injected at the 90degree bend. Over time the pipe corroded thru on the downward leg and failed, but because the entire length was dry wrapped and the water injected at the end, no water flowed back into the engine and it was easy to repair the system.

Hope this helps,
DougR

[Poche]

[QUOTE=DougR;2989932]A properly designed and built riser will not let any water into the manifold

I'm struggling to understand how the water from a 12" riser will not flow back into the manifold when the engine stops unless there is a check valve. There is none.
Like a water column the water will flow back into the manifold.

[Cheechako]

It depends a lot of the installation, sea level etc, but generally I'd go high....having had a "lack of riser" cause an engine problem on a boat that normally wouldn't have the problem. The fix was to fabricate a riser that went as high as possible. In this boat (engine directly under deck) it wasn't that high, but maybe 8".
As best as we could tell, the water in the 4-236 was caused from the boat sitting at the dock with the stern exposed to small windwaves. They must have pumped the exhaust system full and into the engine one little dollup at a time overnight as the waves lapped in the outlet.. This boat had a huge waterlift muffler too. Probably 2 ft long and 10" -12 "diameter.
We made the riser so that water injection was on the down side and used insulation on the riser. Do that! Easy Peasy fix
It's a good question though, How much, if any, water goes back to the engine after shut off from an upward U shape lift? I wonder if any does normally? The last rotation of the engine pushes air and water out, does the last rotation pump any water in?

[DougR]

The water injection point in a riser, or for that matter an elbow, is always downhill and downstream from the high point of the system. In my sketch the water mixes with the exhaust gases at the exhaust outlet point, and the only way for the water to flow is down into the muffler pot. The water won't flow uphill and back into the engine.

There is never an "anti siphon" valve (or check valve) in the exhaust riser or elbow. The anti siphon valve is found in the water hose which feeds water into the riser/elbow. When the engine stops and water flow in that hose stops, the valve opens and allows air into the hose, thus preventing water from siphoning thru the engine and into the exhaust. The water which remains in this hose past the anti siphon valve will drain into the riser/elbow and drop into the muffler.

DougR

[Compass790]

Originally Posted by DougR

The water injection point in a riser, or for that matter an elbow, is always downhill and downstream from the high point of the system. In my sketch the water mixes with the exhaust gases at the exhaust outlet point, and the only way for the water to flow is down into the muffler pot. The water won't flow uphill and back into the engine.

There is never an "anti siphon" valve (or check valve) in the exhaust riser or elbow. The anti siphon valve is found in the water hose which feeds water into the riser/elbow. When the engine stops and water flow in that hose stops, the valve opens and allows air into the hose, thus preventing water from siphoning thru the engine and into the exhaust. The water which remains in this hose past the anti siphon valve will drain into the riser/elbow and drop into the muffler.

DougR

Agree completely with this post.
And with DougR's previous post. The yanmar ys series exhaust elbows are very badly designed in that if they fail they dump water on top of the exhaust v/v.
Of course it could be a cunning plan to make you buy Yanmars drastically overpriced elbows.
It's extremely common to see water damaged pistons,rings & bores from the elbow corroding as Old cal46 skipper says
I tell people to make them out of sched 10 316L s.s. to the design as shown by Doug R
Do that & you wont have that problem.

[skipperpete]

Compass 790 nailed it with his warning about the good looking but deadly yanmar mixer elbows, even more of a risk if your engine is horizontal rather than inclined. The telltale sign is corrosion appearing where the manifold and mixer elbow are joined. Yanmar recommend replacing them at a certain number of engine hours, a very short number of hours.
Sched 10 is the way to go.

[Poche]

I took the advice of oldcal46skipper and had the exhaust fabricator install the tailpiece without a riser. His question is sound..."Where does the water go?".
Also, because the exhaust hose rises to the deck well above the waterline after the muffler (no need for a second riser. And, this design has worked for the last twenty years without harming the engine.
Also, the manifold outlet is above the waterline and goes down to the muffler anyway.
The corrosion that caused the failure at the twenty year old bend at the manifold outlet was caused by lack of maintenance and improper gasket seal.
Thanks for your inputs.