CAT Aftercooolers — and whats Real

I know that's what MAN suggests.
My point was that I've yet to come across one single MAN installation where this was actually the case.
And I'm not saying this just because by sight they didn't look isolated - I checked them with a multimeter.

Do you mean their cores? I can't see how they could be isolated, being bolted to the external box.
Which is indeed bonded with the rest of the engine, and in turn with the rest of DC ground circuit.
At least, that's what I found in many boats where I actually checked that with a multimeter.
In spite of what MAN recommend, as I said.

That's a very good point indeed.
On top of that, I have a funny feeling that Cat aren't really interested to invest in their marine segment.
Not as much as they used to, anyway.

Requires separate DC grounds back to a DC bus. No DC grounding to the engine. Bonding wire from engine and any isolated raw water components such as thru-hulls and strainers to the transom zinc plate. Many builders probably don't wire engine isolated grounds because of added cost.

The new c32 B motors are zincless. Hopefully they finally got the issues resolved.

My 3412e engines (2001) run coolant through the ACs, but I believe that other versions of this same engine run raw water. I don't know when that change was made, but the Cat tech I had on board once was very surprised to see the fresh water ACs. Obviously the tradeoff is less corrosion using coolant vs running 50-80 degree raw water (instead of ~180).

The manual does not reflect the future service bulletins. Replace the aftercoolers every 6 years regardless of hours. Some owners flush out the raw water side with fresh water when the boat will be idle for weeks which extends the life. Still I would replace them at 6 years or certainly in the 7th. Less running is more harmful to seals and O rings.

Flushing often with freshwater should help them live longer, plus keeping the drains clean. Manual also says to pull, clean and inspect aftercoolers every two years. I would stretch that to 3 years and not do again if replacing in six years. Cat needs to design an aftercooler with the right metal that will last same as other mfg.'s. I have had two boats with Mans, the aftercoolers on both lasted over twenty years.

I don't think this is too off topic--would a freshwater flush help on the 3406 a/cs given their location and/or the fact that the flush would occur when the boat is in idle at the dock? Is there a thermostat that could prevent the fresh water from reaching the a/cs when the boat isn't running hot or would they get a good flushing regardless of engine temp?

Had put flush system on my Cummins QSCs and thought it helped a lot.

Not 100% sure about 3406, but normally the thermostat opens/closes the circulation of closed cooling circuit, not of raw water.
I never came across an engine where the raw water path was restricted/deviated in any way.

Makes sense, thanks

Agreed. I would think that the exhaust will need this raw water cooling almost immediately, long before the engine itself comes up to temp...

I've never owned a boat with zincs in the ac system, definitely a good idea.