[Chris Burkard]

Never Tune Props until you have ruled out all other sources of RPM loss.

Your Cummins should turn 3050 RPM under a full heavy load (100%, 100% Water, people and gear onboard). If you are light on fuel, people or gear, you may turn closer to 3100 or perhaps just a bit over.

Reasons for low RPM - 1) Hull Bottom Condition - Is it clean and smooth?; 2) Prop Condition - Are they clean, smooth and free of damage?; 3) Fuel Condition - Is your fuel good, less than 45 days old (use new fuel when testing for proper RPM), free of water and free of debris and algae?; 4) Fuel Filter Condition - Are your fuel filters clean and free of obstructions?; 5) Air Filters - Are your air filters clean and properly lubricated?; 6) Engines - Are your injectors, turbos, fuel pumps all working properly (have tuned if questionable)?

Once you know your vessel is tuned, clean and operating at 100%, only then can you test for proper RPM. With mechanical engines, never trust the tachometers. They are notoriously inaccurate. You must use a photo tach gun in order to get proper RPM readings. Pick a nice day with light winds and light sea conditions to run your test. Make sure to leave the engine synchronizer off during testing. The synchronizer can keep the engines from reaching full throttle and can mask if you have a problem with only one engine. Make two runs in opposite directions recording Actual RPM and GPS Speed at each RPM increment. Once you have reached your RPM target, let the speed stabilize for at least 10 seconds (check your trim and make sure you are level and optimally trimmed) and then record your findings. Do this for each RPM Level. I suggest the following for Cummins 6BTA-M3s @ 370 HP ea.

Record Fuel Level, Water Level, Number of people on board, Gear, Air Temperature, Water Temperature, Wind Direction and Speed.
600 RPM (No Trim) - let speed stabilize for 10 seconds - record speed
1000 RPM (No Trim) - stabilize for 10 seconds - record speed
1500 RPM (Little to no Trim) - Stabilize - record
2000 RPM (Moderate Trim) - Stabilize - record
2200 RPM (Moderate Trim) - Stabilize - Record -

Keep going from here in 200 RPM increments all the way to Wide Open Throttle.
Then repeat traveling in the opposite direction. Then average it out. Take this date to the prop shop and they should be able to tune your props accordingly.

Good Luck!