Under contract - 2003 Neptunus 56 w/ cat 3406e - top speed/rpm issue

I considered the props to be dialed in to a proper pitch and diameter given the RPM,s the temperatures, the pressures, and the boost. Both engines performed the same. If you added pitch to slow the props down and increase the fuel burn (wby would you?), you would load the engines, increase the temps and pressures, and stress the engines.

If you're focused upon speed and fuel burn as your primary concerns, you're staring at the wrong information. Start with RPM and temp. Don't worry about speed. Speed is more reflective of the hull or prop conditions, not the engines. Ask yourself, why aren't the engines spinning the proper RPM, an engine rotation for which they were designed to produce and perform?

The 3406E-800 should be able to spin up 2325 with light load. When you prop your boat, the aim should be more about achieving 2301 versus 2299. My engines were 1999 vintage, and at that time the spec was 2300-2325. If you over-prop, the engine will stress under load at every point of the curve, even at cruise. You'll see higher boost pressures and exhaust temps. The engine doesn't "ignore" the over-prop impacts just because it's at cruise. The same over-load follows it from the moment you put it into gear.

You're overcomplicating it and trying to build in excuses and reasons. It's simple. The boat fails to reach proper RPM. That's the only thing to know. In ideal conditions it should exceed rated RPM and it should always perform close. Do not think, talk, mention speed and fuel again until RPM is resolved.

Even on the lake and now on the coast, every service of engines, warranty or paid, concludes with a time allocation for sea trial to verify maximum RPM is reached. Only sure way to be sure service was done properly. Essential to a rebuild or overhaul. I would have never accepted a rebuild until the engines hit maximum RPM. Don't buy the subterfuge. Don't add other elements. Boat doesn't hit RPM, unacceptable. Rebuld job not complete until it does.

The fuel is just a guestimate from the CAT computer. Props are off by 3" of pitch, possibly 4". Should turn 2320-2350 with full fuel/water/etc. The overpitching is causing the loss of speed. Properly pitched 80% load will be 125-150 RPMS higher, props spinning faster will give more stern lift and more rpms= more boost and more power and higher cruise speed. The 56' Neptunus is a good boat, but price sounds high for what I've already heard about the boat.

Taking pitch out is not uncommon.....I had to take 2" pitch out of two newish boats within the first 6-8 months....... 2019 and 2021 yachts.

All this talk of RPMs at load…

Did anyone take the No Load RPMs ?
Or is that no longer part of the engine start up tests anymore ?
It use to be.
I would want to know what the no load RPMs should be from Cat for that motor ( call them your self) and see what the boat does at the dock . It may turn the rated RPMs with no load.
Then go from there to make your decisions….

A fresh bottom job, yes, if it's incredibly fouled. I don't see 3-4" of pitch. I see more like an inch or so and less cup...but there's an issue there that needs to be resolved before you buy. The purchase closing needs to be subject to full rated RPM being achieved in sea trial at the seller expense.

The no-load RPM test on an electronic engine isn't going to tell you much, other than it can turn rated RPM at no load. While that's useful, it isn't much of a test as it's only while under load that everything from boost to injection timing all comes into play. The ECU needs to be reacting to the engine loads to determine how to make the horses run. Being so close to rated RPM, I strongly doubt there is any use in no-load testing here. With a mechanical engine I'd agree. But the 3406E-800 has perhaps 40 sensors, an ECU, and chip processors on each injector. That ECU is actively doing math and making adjustments to the systems. I posted photos of the CAT displays, not for RPM, but for all of the auxiliary numbers, especially boost and fuel pressure. This low RPM rating could be prop. It could also be ECU and/or turbo and fuel. There's not enough information in this thread to sort it out.

Did the CAT tech that rebuilt perform a sea trial? And if so, did he see rated RPM? If that happened, there is less likely issues with the prop, more likely some other fuel or boost problem.

And more importantly, why wouldn't he clean the bottom BEFORE someone is sea trialing.
I'm afraid your seller is adverse to maintaining this vessel.

If it truly is a fouled bottom, which most of us doubt.

A little, but not what you're looking for (without seeing it).......maybe 20 rpms...... Now if there is 2 decades of bottom paint stacked up on the bottom, I have always gotten 2 extra knots on yachts they soda blasted back to gelcoat, new barrier coat and new paint.,...... your issue is almost certainly in the props being overpitched, provided the engines aren't lacking power and the boat isn't super heavy.

It's pretty simple. Seller, go call your diver, spend the $300-400 to have it well cleaned, and then re-trial to prove the RPM. Who the hell would try to sell a boat with a bottom so fouled that it wouldn't perform under contract?

Agreed. I've lost 2 to 3 knots to bad fouling, but I've never lost 150-200 RPM due to a barnacle.