Exhaust Backpressure

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!

Those smaller bypass lines were usually a small vent for idling.
They offer a quieter idle and less gurgling up the hull sides.
They will spill some gasses and some water at speed.

Think of them like a gas-sep for your gen-set exhaust.

I remember adding a set of 3" bypass tubes out the back of an old gas (LBC) Carver.
What a world of differences. No gurgling up the hull sides at idle. Whole boat sounded better at any speed.

I can't be positive because I might be confusing this with another boat/builder, but did you actually see any smoke coming out of those side exits?
IIRC, what you are referring to are just very small raw water drains, meant as telltales for visual check that raw water is circulating normally.

Can't remember the exact model I have in mind - possibly a 55 Maxim, but I'm not sure.
Anyway, I would have bet to have seen on that boat a full fledged u/w exhaust, plus a smaller (but much larger than the telltale anyhow) bypass on the stern sides, just under the swim platform...

Mostly to release idle exhaust pressure, greatly reducing gurgling from under the boat. Just a little water/steam comes out.
The large block chevy (LBC) , idled much nicer and had more um-pa in & out of gear while docking.

In post #3,, I remember old Roamers with Detroit power. A reduced T before the exhaust dump can diverted water out the hull sides.
A nice telltale of water flow but a tech told us it greatly reduced back pressure after the dumb cans, thru the muffler and out the saddle-bag exhaust down the dull sides.

This same theory was always on my mind and sometimes implemented during customers re-powers from gas to diesel power when the same exhaust was used.

Back to the OP, adding water to the exhaust system increases back pressure. Adding water to the exhaust keeps your boat from catching fire.
Always consult with a proper & qualified tech before any DIY mods.

We had a few surveyors that worked together and advised us on our projects thru the years.