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Guys,

I need some feedback on the appropriate side exhaust angle from the main fiberglass exhaust tube to the tube section that goes through the hull please. Here's what I'm dealing with:

Risers coming off the 454's have a 3" diameter.



Exhaust tube connected to the risers are actually 4" diameter Trident exhaust. Outboard side of the engine has two 4" 90 degree elbows and goes into a T-sweep. Inboard side of the engine has two 4" 90 degree elbows and a 45 degree elbow.

The T-sweep has two 4" inputs and a 5" output. I elected to use the T-sweep so I could put the exhaust up as close to the bulkhead behind the engines as possible per Bob Lico's comments about keeping the deck height the same when running the exhaust outside the outboard stringer to put fish boxes in the deck. This gives me the ability to easily convert to a 6" exhaust and muffler out of the engine for a diesel conversion.
Here's the T-sweep.



The Vernatube 5" exhaust is directly behind the T-sweep.

I have a 5" to 6" coupler behind the muffler that will connect to 6" fiberglass tube so the thru hull exhaust can be 6" and already set for diesel.

I'm not particularly concerned about exhaust back pressure on the 454's as the tightest point is the riser and I'm going into 4" exhaust tube now out of the riser instead of the 3.5" that was previously set up.

Where I am concerned about back pressure is at the elbow where the 6" exhaust exits the hull when there are diesel engines. I did some mock ups today using 6" plywood strips to better understand the exhaust angle and the size of hole that would need to be cut in the hull to accommodate different angles. Here are what those look like:

This is a 20 degree miter cut that leaves a 140 degree angle on the outside of the exhaust bend and creates a 8-3/4" wide faired exhaust hole in the hull.



This is a 25 degree miter cut that leaves a 130 degree angle on the outside of the exhaust bend and creates a 7-5/8" wide faired exhaust hole in the hull.



This is a 35 degree miter cut that leaves a 110 degree angle on the outside of the exhaust bend and creates a 6-1/8" wide faired exhaust hole in the hull.



This picture of the Super Mex restoration seems to indicate a fairly wide exhaust hole, which would indicate a more shallow angle around 140 degrees (based on eye-balling the width at 1.5 times the height).



Any feedback on what the miter angle of the exhaust tube cut or even the final outside angle of the exhaust bend out the side of the hull should be to prevent any back pressure issues would be greatly appreciated.

Thanks!

John

I will defer to Bruce and Bob but looking at your post, the run from the main tube run out the side is so short I don't think the back pressure would be an issue.

As far as the gas engines go, you can have too little back pressure also.

With that said our old 454's were 5 inch straight back coming out of the riser. Not sure when but I believe after a certain year Bertram made all the exhaust 5 inches. I have seen some earlier models with smaller stern ports.

Shambala is a '78. I had 5" exhaust from the wye to the thru hull from the get go. The only thing that has changed is the mechanic is telling me I need to be 4" diameter exhaust tube from the riser to the wye instead of 3.5" as he claims I had before.

Here are pictures of the old risers after they came off. Now that I look at them, they appear to be a different style and 1/8" smaller inside diameter.

All your two into one connections should be a Y design and turns should be a smooth radius and not sharp angles such as miter cuts. It's the same for any fluid dynamics.
I E when using a 90 degree fitting in a fuel line, the resistence is like adding another 10' of straight line without going up in size.

Yes older gas big blocks many times used 3.5 inch hose off the risers.

Bruce,

All of the fiberglass exhaust pieces from Centek are miter cut pieces. Are you saying I should not be converting from steel elbows to fiberglass elbows because the fiberglass elbows are miter cut pieces that do not flow as smoothly as steel?

Regarding the side exhaust, if I'm reading you correctly it sounds like you're saying don't run the exhaust out the side of the hull because any miter cut (even a double miter cut) to turn the exhaust out the side of the hull would still be a miter cut that would create back pressure.

I just want to get this right. I'll return the T-sweep and get fiberglass wyes per your comments above. However, I"m not even sure where I can get steel elbows at this point.

Thanks!

I contacted centek and followed their directions using mitre cuts, that's their business, been around for a long time. I set up the mitre saw at 22 1/2 degree to make it work in that tight space .

Thanks Marlin

Making multiple radius cuts and glassing together to me is just as much work as making smooth radius elbows.
From a production standpoint I can see a time savings on the angles.

In your application John you won't notice.

Thanks Bruce