Propeller Engine RPM Puzzle

[CT30]

I believe that my 30 by 44 Nibral Props are pushing different amounts of water. When I re furbished them and ran boat the port engine was over 100 rpms higher than std. I continue to take pitch off now 2 full inches and have it down to 30rpm at cruise and 60 rpm at WOT. To keep the load same on my CAT 3406E I just run one engine at a higher RPM than the other. Boat seems to like it and it runs good. But my STBD engine is still short RPM at WOT. Port gets 2300 Full Load but STBD only 2240. Also When boat is light Port will max out at 95% load at 2360 TBD is 100% load at 2303. Dont know if to get new props, keep taking pitch of STB or look into the engines. Any help solving the puzzle would be great.

[rslifkin]

Are both tachs confirmed accurate? And are both transmissions the same reduction ratio?

[CT30]

Thank you for reply and Yes, Cats are electronic and I have tachs on bridge and salon so tachs are good. Transmisions are same ratio. I read about that possibility but these are same.

[CT30]

One Additional piece of information that makes the engines a possible cause is that my Cat computers consistently say I have burned more fuel than the boat accepts at fillip. Last time was close to a 15% difference.

[SailingHarmonie]

Have you had the props measured by a professional prop shop? They should be identical. Have you calibrated your tachometers?

If the props are identical, and your boat is symmetrical, they MUST advance the same amount through the water at the same RPM. That is a given. The difference in RPM then must be the tuning of the engines.

But I am confused. You say there is a difference in RPM at "cruise". How are you defining "cruise"? I understand a difference at WOT, that's a fixed point. But how can there be a difference in RPM at an arbitrary point like "cruise"?

Running two different pitch props is not a good solution to the problem.

[CT30]

Thats for helping with my puzzle. Yes props have been to two different prop shops and measured and adjusted. The current RPM Differences are after the shop has taken pitch off the STBD prop in an attempt for them to run same. Cusire I mean 75% Load on my Cats so to get the same 75% Load I need to adjust the RPM of the engines. The problem"difference in RPM gets bigger the faster I go. I have had two different Cat Mechanics look at engines an say engines look good. Boost same. The current theory is that the props are 20 years old, possibly damaged over the years and that the port prop is thin and flexing the more pressure it gets. So even if it measures a certain way in the shop it flexes as soon as loaded.

[CT30]

Some more info is that it feels like the STBD prop pushes harder when engaged than port. I originally thought it was the transmission but now am starting to doubt. I have also run the boat at idle speed with the other engine in Neutral and it appears that the boat is a bit quicker when being pushed by the STBD Engine.