Motors sound like TAMD41s and the 3900 is correct. I have TMD41s (150 hp) in my 36' nova sundeck (25K Lb?) with 21 x 14 4 blade props. i can get 12 knt at 3600. I have not checked high idel to see if it is my govenor or the load that is limiting the max RPM. I never cruse above 2800 @ 8knt so i am not risking overloading. If you cannot get to the 3900 RPM then running near your actual maximum RPM can be a dangerous overloading situation
Get all the data on your boat ( HP, waterline length, transmissin gear ratio, estimated weight, prop size, hull type probably semi planing) and go to the Boatdiesel.com and use their prop calculator. Just pay the $20 or whatever is costs to sign up at boat diesel, it is well worth it.
It tells me that i am under propped (recomends 21 x 17), that is why i need to check the max RPM in neutral (high idle) and see if possibly my govenor is holding me back from full RPMs or mabey my boat has added 10K lb over the years.
Got to the seaboard marine site and read Tony Athens article "Propellers move boats engines only turn them" It explaines all the facts and myths about propellers, pitch and engines ratings.