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Run DuoProp Rig with Only 1 Propeller

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Solarity

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I have a 1998 Volvo Penta 5.0 Gi (PBYCCE) engine with a DuoProp (DP-SM) outdrive in a Monterey 242 mini-cruiser. Although VP specs say it should get up to 4800 RPM, it maxes out at about 3700 and top speed of about 40MPH. Its running with F7 propellers, and I've been advised to try a smaller prop (F6 or F5). One friend suggested I test by running without the back prop at all and see if RPM improves. Another lent me an F5 back prop, to try with the F7 front prop.

Are either of these ideas worth trying? Or could doing so have negative long-term impact on the outdrive?
 
Or could doing so have negative long-term impact on the outdrive?
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I've learned the hard way that using equipment as it's designed to be used really reduces the need to repair unintended damage from not doing so. Conventional wisdom says put matched F4s on and see what the RPMs do ...
 
To further this discussion:

Solarity said:
Another lent me an F5 back prop, to try with the F7 front prop.

The Duo Prop propellers are designated FWD and AFT.
The larger diameter and less aggressive propeller is the FWD prop.... the prop closest to the transom.
The smaller diameter and more aggressive propeller is the AFT prop..... the prop furthest from the transom.


Are either of these ideas worth trying? Or could doing so have negative long-term impact on the outdrive?

The Volvo Penta engineers designed the Duo Prop unit with several things in mind..... the foremost being increased hydro propulsion.

With the Volvo Penta DP, you'll notice two different diameter and two different pitch props being used.
This is what gives the Volvo Penta Duo Prop an advantage over that of the BIII, for example.

Inside of the lower gear unit you'll see one "drive" gear and two "driven" gears.
Each of the two "driven" gears must share an equal torque load.
By design, the unique and matched Volvo Penta DP propellers offer this!

If you were to run an F5 (in either an AFT or FWD prop) paired with an F7 (in either an AFT or FWD prop), you'll not only upset the propulsion balance, you will upset the gear load.
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Another suggestion would be to verify the accuracy of your tachometer!
If the tach is off, it may explain why your WOT RPM appears to be so low, when it may not be.




.
 
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.... and if you try to run with only one prop, it likely won't move the boat at all.... Think a car's non posi differential/tires with one wheel on the ice.
 
Did it [ever] run to 4800 rpm? YES (kind-of: 4500), in fresh water.
I am currently running in salt water, and its higher density may be the problem.
Before switching to a smaller prop, I plan to test-out the current rig in a local lake.
 
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Solarity said:
Did it [ever] run to 4800 rpm? YES (kind-of: 4500), in fresh water.
I am currently running in salt water, and its higher density may be the problem.
Before switching to a smaller prop, I plan to test-out the current rig in a local lake.
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FYI:
If your engine cannot reach the OEM specified WOT RPM, it will be over-burdened throughout the entire RPM range.
This may damage your engine!

I would encourage you to:

.... look for marine growth on the hull bottom.
.... look for propeller damage.
.... look for anything else that may be holding it back.
.... drop down in prop size and test.
 
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