Questions about exhaust system features

[HPwannabe]

Pondering a couple questions for the collective mind. They are mostly academic. Has to do with design of pre-alpha exhaust system. See pictures for clarification.
1. What purpose do the small holes at the bottom of the outlet have? I know they route a portion of the exhaust through the gimbal housing where it exits externally on each side. But why? There must have been some good engineering reason. Is it just a “bypass” in case the through-hub gets clogged? That seems extremely unlikely to me. My model already allows that anyway due to the exhaust bellows being the type that allows bypass at the bell housing. What was the compelling design concern here?
2. Presumably, the flapper check valves at the Y pipe connections are to prevent water running up into the engine under some extreme case where the engine is below the waterline (which seems kind of impossible, given the geometry of the setup, unless the boat is already sinking). From what I’ve seen, these things are way more trouble than they’re worth. They get frozen, or break off and foul the exhaust pathway, etc., and they create more back-pressure. And they can’t possibly make a water-tight seal after some period of operation. Does anybody here disable them?

 

[kghost]

The exhaust Flappers are there as you say.
What happens is, say your driving at 20 mph and you pull it into nuetral. The back flush of water can and has ended up back feeding to exhaust manifold by way of the gasses and then into the cylinder heads.

The bypass holes,

Boats are design around salt water usage. The lower unit of the drive can easily get clogged up in grass, plastic bags, netting....etc...
You wont see that so much in fresh water..so it can seem like over kill...

 

[HPwannabe]

Thanks. The back-flush problems seems legitimate. Makes sense.
I don’t know that I buy the logic about fresh versus salt water use. I’ve boated in both for many years. I would expect just as much (maybe more) flotsam in fresh water lakes than in saltwater. And the whole thing seems like a solution looking for a problem. If I manage to foul my exhaust to the point that nothing is coming through the hub, I probably have bigger issues. Plus, those holes are way too small to handle the full exhaust flow. So the engine is going to be choked down to the point it probably won’t run anyway.
Maybe you’re right, but it still makes me shake my head.

 

[Droid]

The 2 ports came about in the early 70s. Also, 1st V8 with a #1 drive was 1971 888 model.
They allow for reduced back pressure that could potentially reduce performance.
Early ports were almost too small to stick a finger in. Later ones, maybe 3/4"

 

[Fun Times]

The technical name for the two ports are called idle relief exhaust ports 'Then when under more power it flows more out the lower gear housing by the prop in most cases for Merc... While the diagram below is of an OMC drive model, it sort of shows they basically had the same general concept going on there too with the ports in question with a name attached of idle relief,

 

[HPwannabe]

The 2 ports came about in the early 70s. Also, 1st V8 with a #1 drive was 1971 888 model.
They allow for reduced back pressure that could potentially reduce performance.
Early ports were almost too small to stick a finger in. Later ones,

Yep, little finger size on mine.

 

[HPwannabe]

The technical name for the two ports are called idle relief exhaust ports 'Then when under more power it flows more out the lower gear housing by the prop in most cases for Merc... While the diagram below is of an OMC drive model, it sort of shows they basically had the same general concept going on there too with the ports in question with a name attached of idle relief,
View attachment 35004

Idle relief was what I have basically assumed

 

[HPwannabe]

Idle relief was what I have basically assumed

But in my mind the logic of that seems flawed, given the size and location of the ports. They usually sit below the waterline (I realize not very far, so there’s not much back pressure), and they just seem too small to provide much relief.