Cat 3408 800 HP Black smoke

Are these electronic controls?

So the motors reach 2300 WOT? If not, being overpropped is the source of a lot of the black smoke.

I have the same engines. Black smoke typically means not enough air for mix
Are the boosting and revving up? We had a problem with bad valves where it wouldn’t boost and get past 800 RPM and just black smoked

Our 48' Sea Ray Sedan Bridge had a smoking starboard engine since we bought it. Our local guys could never solve the problem. After tens of thousands of dollars, we broke down and called RingPower. Their guy found a quarter sized hole in an access cap to the after cooler (no one else found that.). I'll post up again to update you once that repair is complete. It will cost you, but the expertise of the RingPower guys is unmatched on CATs.
Fowled injectors could cause the same problem but ours are clean.
Our props were also in perfect shape.
PS: Don't believe anyone who says that you need a bottom job weekly!!

I never run any engine over 70%. The guys telling you they re designed to be run at high load are not the ones paying the bills and are usually the guys you end up paying to repair or rebuild

Some motors such as Mans run better and last longer running 80% load as they don’t make as much torque as cats and rely on the additional boost which helps keep rpm’s And the bottom end more stable. Fuel burn per mile is generally the same whether you’re running 70 or 80% load usually. The manufacturers engineers know more than we do and they recommend 80% load. I have not seen any additional parts breakage or repairs of running 80% versus 70% load. And in all honesty, of all the yachts I’ve managed, almost never had to oversee a rebuild because the motor was worn out.

Considering the age of the 3408s, that might be somewhat related to lack of usage (like injector nozzles corroded, as an example), rather than the other way round... Where they similarly smokey, when new?
Besides, aren't you possibly talking of the 3406 instead? I'm asking because they used to be more popular, on pleasure boats.

Apologies if my memory is mixing up engines and their displays, but assuming we are indeed talking of the 3408s, I half recall that they didn't come with a display capable to show the load in real time.
I mean, 90% engine load and 10% back of max rpm are two different things.
You could indeed have 90% engine load at 90% of 2300 rpm (i.e. 2070rpm), but just by coincidence, and that would change with different props, or on another boat.

What s the WOT? With 2300 rpm motors I like to run no higher than 1900 assuming the boat isn’t underpowered and remained fully on plane.

make sure you use trim tabs and that work

probably because so many boats do do more than 100 hours a year

Rated WOT is indeed 23oo with both the 3408 and the 3406, and also with most MANs.
CJ is right about MANs, though: 2000rpm and 80% is still compliant with the factory recommendations for sustained usage, even if the boat is steadily on the plane well below that - as mine is, for instance.
I'm not aware of a recommendation to run them exactly at (or even around) 80% load, though.
It's more a matter of staying within the range where the turbos are fully operational, assuming that the hull can cope with the speed and doesn't "sit down" in the water.
Personally, I cruise anywhere between 1700 and 2000, and while the latter is indeed in the 80% load ballpark, the former is definitely lower.
I like to have a somewhat wide choice of cruising speeds, by the way.

The 3408s are out there. Not a bunch but with us. They were a ******* engine that Cat wishes was not used in the marine market.

Now the 3406, A main bearing between each rod (lung) is the Cats meow. Could make 900 reliable + Hp and not sweat or drop a beat after thousands of hours.
I wish I had a pair of 3406s now.

Back to the 3408 OP.
Black smoke is incomplete fuel combustion; Not enough air per fuel in one, some, all cylinders.. Caused by slow turbos, slow or leaking injectors, restricted air filters or air coolers. I have also been on boats with to small E R air inlets or salt encrusted air screens. If your non-computer engines do not offer any codes, you are really going to need a low pressure testing instrument installed during some running test.

Also if a non-computer engine, you are not going to easily determine % of load..
If your making RPM plus (2330+), then IMO, 2100 RPM max cruise RPM.

Yes, 2300 rpm motors. Yes, I always use the tabs and trim for best speed unless it’s very rough and I trim for ride. 80% load doesn’t hurt longevity provided they’re propped correctly even if it’s over 2000 rpms. Changing load factors does damage from the boat speeding up/ slowing down going over waves.

Modern day computers adjust all parameters and measure a heck of a lot more parameters than the old motors did so the 400 rpms off of top isn’t really relevant. Volvo doesn’t show you load and they recommend cruise as 90% of the rpms of wot.

There is reason maintenance intervals are given by hours or gallons burned which ever comes first on many motors. The faster you go, the more fuel you burn, the more friction, heat, wear is generated.

my point in this case is that if the boat stays on plane nicely at a lower power setting, try that to see if the soot issue goes away.

If you are positive that she was repowered in 2000 and with 1300hp engines, they must be the so called D2842LE404.
Pretty rare beasts, which were among the very first attempt of MAN into electronically controlled (but non-common rail) engines.
I never came across them first hand, but their reputation isn't exactly great.
Not to be confused with the following common rail version, available either with 1224 or 1360 power rating, but I'm pretty sure that neither were yet released in 2000.