Time for some planning, current setup on my twin cat is this,
Stock Merc 502's
Running Y-pipe with NO diverter @ the Y--removed the silenct choice parts over a decade ago and welded the holes.
This allows water and exhaust to go both directions.
Have Teague 4.5 muffler tips
My thought is under power the exhaust is semi quiet exiting both thru transom and thru prop.
However I also feel @ idle that most water rolls down the y-pipe and not being fully used to 'quiet' the transom exhaust.
My spring plans are to remove the open diverter,
Cap off the Y-pipe---yes I know of the added weight for now but want to 'test' this before removing the engines
Run hose straight from the manifold to my tips.
Thought about adding in a set of GGB inserts to add more db reduction.
Current setup with the tips closed I am NOT legal in MN and just barely tip legal in Wi.
And that is why I think the downward water flow is not helping me.

What I need to know, can the GGB inserts be 'slipped' in the exhaust hose?? or do they have to be inside metal?
Thought about buying stainless tube as a 'spacer' between the risers and tips---Less hose to use.

See if they make them this way still, or you maybe able to buy some stainless pipe wit correct OD and mount the inserts in them.

GGB stopped selling to the public and only went through distribution because so many people were ordering the wrong items and the return rate was ridiculously high. I am running the Competition version 5 ring through 4,5" dry pipes. Lost nothing but a lot of noise. Great product!

I don't believe they are really meant for dry exhaust. The design uses the water flow directed in a certain manner to dampen the exhaust pulses. Without the water I doubt you'd hear any measurable difference. That would be my best estimate.

They do lower the db when dry, but definitely not as much when they are wet. I can hear the difference when I start the engine for the first time when there is no water in the system. After 5-10 seconds it quiets down when the water starts flowing out of the pipes and im only running 3 discs.

I have the GGB inserts in mine. I bought them long like Trash said. I ended up just leaving them as is. On first start up, they bark pretty good, then quiet down as the water starts to flow.
My exhaust runs straight out the side of the boat. One cool thing about my setup is when the engine is shut off my exhaust manifolds pretty well empty out. I have pulled the plugs in the manifolds when I get home from the lake and there is very little water in them. Should help them last longer.

I ran the boat a little last year without the inserts, and the boat was pretty loud, especially with the Bimini top up .

Where can i buy these ggb inserts?

https://www.ebay.com/itm/153977039358