De-Rate, De-Prop or De-Throttle Detroit

Who's doing your props? Wilmington? I see you are in Long Beach, we are in Alamitos.

Both turbos are brand new. We've run with the breathers off too. Right before COVID, we cracked the exhaust elbow on the stbd motor, which started the whole process. Sucked soot into both motors 1/2 btwn our port and Catalina on the return. Aftercoolers, blowers, turbos, injectors all rebuilt or replaced. Everything checks out fine w/out load, it's when you put a load on - we start to smoke. We played with the injector timing which did help a bit, but put it back to stock timing of 1508 vs 1484...made it worse. We can't reach 2350 loaded, so that's why I am thinking, over propped! Puffing black smoke and not making full RPM is kind of a sign we need some pitch removed - I know black smoke indicates lack of fuel or air, but could also mean the wheels.....just so frustrating!

I sent my props to Seattle, got a better price. From my experience, black smoke is too much fuel or not enough air. I set my injectors to 1.460, not a 92 guy but are you sure of the 1508 height. Prior to the cracked elbow you were making loaded rpm?

Yep, and bouncing off the governor. Actually hitting about 2400+ - but we cavitated and were not making the speed. I was like we had cardboard cut outs for props. So we added pitch back into the props, probably too much. Previous owner took an inch out of the diameter so we could not add that back.

Did the prop shop tell you to add 3 or 4 inches or did you tell the prop shop what you wanted? Without any engine numbers, I tend to agree that you are over prop'd.

Actually, we did a complete spreadsheet of numbers/specs, including speed runs and noting RPM/kts every 100 rpm from 530 up to 2350. The prop shop recommended the pitch.

I meant to ask earlier - are you in Shoreline or Alamitos?

Can you help with a better timeline? Did you change the prop pitch after you did the rebuild from losing the exhaust elbow or before? You said the smoke issue started when you did the rebuild from losing the elbow. Did you run the added pitch before this happened? Did you have the smaller pitch props after the rebuild? Or did you re-pitch the props at same time as rebuild?
And are you saying you have very visible black smoke at 800 rpm and up or is your main soot on the transom? Just trying to get a better sense of what you are dealing with and what changed, when. I'm struggling to understand the heavy smoke at 800 rpm if your only issue is being overpitched some. Yes, too much prop will cause issues throughout the range but hard to imagine that much issue at 800 rpm from 1"-2" too much pitch.

and to be clear, you only rebuilt one engine but are getting smoke now from both engines? And the only thing that changed on the non-rebuilt engine is the prop change?

Maybe share that information as well as the prop dimensions before and after the change?

Prop change was after the exhaust leak. But that said, we were smoking a bit as I stated earlier before the prop change. Stbd turbo was not making adequate boost, so we replaced the turbo. Injectors are 7425- 145 mm spray - they are not the Reliable injectors but rebuilt stock injectors. To be clear, pre re-propping, it was only the stbd that smoked, now that we are propped as close to factory spec - 24 x 27/28, both motors now smoke. Trying to upload a video, but can't for some reason - keep getting an error notice.

How short were you on RPM? And how much pitch did you add?

3.5 to 4 in. From 24 to 27/28 - about 250 to 300 rpm

What were you turning when they were 24??

I was actually over the 2350 rpm full load - 2450 and up.

100 over is not a bad place to be. 25 to 50 over is probably perfect. But if you were 100 over, why in hades would anyone recommend 4 inches of pitch be added? Well, that's a bit rhetorical.

My rule of thumb has always been that an inch is the very rough equivalent of 150-200 RPM. Now, that's a very general rule, but it's at least a mild guideline. Had I been asked, and why would you ask me?, I would have recommended adding heavy cup and no pitch change, check and balance, and splash. I'd be looking to trim you by perhaps 50 to 75 RPM, not more given what I know of you situation. You don't need to hunt maximum speed. Efficiency and longevity is your friend.

Yeah, that's clearly what I'm seeing, too. An inch and/or working with cup...but not 3-4".

I agree and many thanks! Mechanic's coming back out this week and we are going for another sea trial, but I am leaning very hard on being over propped - that's for sure! Many thanks

What size shaft do you have? 2" ? You might try looking around for a set of inexpensive used props do your testing with. 24" props are pretty common. You might find 24/24 or 24/25 so you can do some testing without having to keep altering your existing props. Once you find your sweet spot then get them dialed in.

I have 2in shafts and the props were at 24 x 24. We cavitated terribly, which is the reason we jumped up in pitch. Original spec's on the props were 25 x 28. They were cut down to 24 x 24 by the previous owner. Couldn't add diameter back so we got as close to original as we could, which is 24x27/28. I think if we take about 1.5 to 2in out of pitch like rtrafford stated, we might be as close to dialed in as we are going to get.

Maybe find a set a 24x26 to try? Again, pretty common. You might even find someone that has them as spares you could borrow just to test. Just saying, it would be easier than to keep re-pitching yours. Every time you work those props they get weaker. Are they 3 blade or 4?