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[ron f]

Hi all I just purchased a 1998 37 FT Maxum SCR and was wondering can anyone tell me what the factory Prop sizes were, I have twin 7.4 MPI Mercruisers 330 hp and want to make sure the props I am running are correct as I have seen some issues relating to throttle and RPM that are not making sense, my driver is going to get me the prop info later this week but wanted to know the stock size for this size of boat.

Thank you

[shrew]

What is your Max RPM at wide open throttle (WOT)? Do you know the pitch of the props on there now?

[ron f]

do not know pitch, 4000 rpm on stb and cannot get more than 3300 on port

[shrew]

4400-4800 is the range for Max RPM. I can see why you're concerned. Unless you have two different props, which is unlikely, you have some other variable at play here.

Is this an Inboard or an IO?
Gas or Diesel?

[ron f]

Originally Posted by shrew

4400-4800 is the range for Max RPM. I can see why you're concerned. Unless you have two different props, which is unlikely, you have some other variable at play here.

Is this an Inboard or an IO?
Gas or Diesel?

ok i just had my diver chk yesterday and i have 21R20 4 blade props motors are 7.4 330hp mercruiser mpi gas inboard and my max rpm range is 4000/4400 couple of other folks i have spoke with are running 21x23 or 21x24

[yahoo82]

If you have IO, and assuming tach’s are correct, you may want to check for a spun hub. If you start throttling up to get on plane and one engine feels like it “breaks free” while the other labors, then the spun gun would be on the higher RPM engine.

I had a similar experience when I purchased my boat. Unfortunately the survey was done lightly loaded and it didn’t slip until the boat was normally loaded for our first weekend. The slipping engine would wind up to normal RPMs and the good prop would labor and the boat would barely get on plane.


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[yahoo82]

Sorry. I missed your latest post where you specified inboards. Disregard.


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