Cat 3208 435 to Cummins 425 QSB propping

[Andy Novey]

I am repowering a 38 Pheonix that had Cat 3208 435 with a pair of Cummins QSB at 425. I am keeping the ZF 301c with a 1.862 ratio and I am curious what the thoughts are on propping. The CAT´s are rated at 2800 RPM and when the engines were right (Which was rarely) it turned 2900. The Cummins, while 10 hp less, turn 3000 RPM.

My question is do you think the boat will need to be repropped or the almost 1400 lbs weight saving make up for the lost HP and higher RPM. I am running 4 blade 24r27 Nibral props. Maybe just a little more pitch?

Boat specs with the cats

2400=20-22 knots (Cruise)

2600=23-25 knots

2900=27-29 knots

Thanks in advance

Topic

[Andy Novey]

Just fitted the ZF 301C to the QSB and it fits! We even used the old bellhousing and cut off the CAT “Ear” (Cutout from the 3208 starter I had “references of others who had done this with no issue). hopefully the install goes smoothly form here . Threw a foto on here for reference.

[Tony Athens]

Yes, your QSB 5.9’s are standard SAE 3, thru & thru.. The CAT 3208 is NOT–It’s a CAT SAE#3 “screwy” standard………….. So at the very least, you will need a new SAE#3 adapter plate, and 50-50, a new torsional coupling..

But that is only part of the issue– Turbo clearance with the ZF 301 castings……………………………………….Time will tell

[Andy Novey]

Have not tried yet. I have an saw #3 bell housing on it now so I am hoping this will not be a problem but I believe it works but could be very wrong.

[Tony Athens]

Curious……………..Have you actually tried mounting the 301C on the back of a QSB 5.9? If I remember correctly, it was long ways from “Plug & Play” install..

[Andy Novey]

thanks Tony. I’ll update you after the 1st sea trial.

[Tony Athens]

QSB 5.9 420 CPL 8732

That was the ODD ball QSB 5.9…Mostly all were sold into a Military application and most were 24VDC.——————-Curve attached.. Actually this CPL was rated at 420 BHP/425 Metric at 3000.. Governor speed is 3080-ish.. Remember this if nothing else–You can never underprop an engine like this–You can only over prop it..

Seatrial your boat with the existing props.. Then, do it again “loaded for bear” and make a GPM per RPM graph–Forget load %–It’s GPH that matters at cruise RPM between 2100 at 2600 RPM that matters..

[Andy Novey]

Hey Tony,

Engines are both CPL8732 and are running takeouts from the Navy 1700 hours (They have been run, servced, dynoed by a trusted mechanic shop in the US).

Ser# 46928443 and 46928428

I guess I don’t really get the governor speed vs the rated speed. At full throttle I though I need to be at 92%-95% load,

[Tony Athens]

1) Post and engine serial # so I can ID the exact QSB you have

2) The QSB’s——History of them

3) All of the 3000 RPM QSB’s need to propped to “as if they were” 3100 RPM engines at the rated BHP number, BTW which is NOT 420HP. Also, understand rated speed vs. governor speed. Two different animals.

4) Per Cummins propping guide lines for the entire QSB product line, read the attached a few times until you understand it.

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