Exhaust Backpressure

We definitely have lots of oysters and barnacles in St. Andrews Bay but I've never had them cause problems in the exhaust system of any of my boats. Have you tried looking at the inside of the mufflers with an endoscope? I have one you can borrow.

Sarnico, your question is making me curious.

Did you ever come across a boat where exhaust is routed either to the main pipe or to a bypass by some sort of switchable valve?
I mean, the only bypass I'm aware of is in boats with underwater exhaust, where there's also a smallish outlet above the waterline to avoid the backpressure on the main submerged outlet while the boat is not yet at planing speed, hence not exploiting the scavenging effect on the main u/w exhaust that eliminate backpressure almost completely. But in all the boats I've seen with such exhaust arrangement, the bypass and the main u/w exhaust are always open in parallel, so to speak, with no need to switch anything and no valves anywhere.

The only exception I can think of are very fast outdrive powered boats, where the exhaust can be routed either inside the prop, or straight through the transom. But that's a wholly different kettle of fish obviously, and not applicable to a shaft+diesel powered Cabo, surely.

Yes thanks, all good here. I hope likewise at your end.

Apologies for not remembering what your old boat was, but if it came from the builder suggested by your username, I'm almost sure she had a main u/w exhaust, with an additional smaller outlet astern (bypass), between the swim platform and the waterline.
When you say that she gave you the impression of not using the bypass when cruising fast, I think that's mostly because, as I previously mentioned, the main outlet under the hull is meant not only to cut the noise, but also to "extract" the exhaust with a scavenging effect, when running at speed.
So, the main u/w outlet actually becomes the easiest way out for both the fumes and the raw water, and the bypass just above the waterline becomes irrelevant.
Besides, did you ever stick your head under the swim platform at 30+ knots, just to see if any raw water and/or fumes came out of the bypass...?
I for one wouldn't have tried that!