A series duoprops for diesel

[dugggy]

Hi Guys.
Just after some opinions regarding a set of props for a 290dp behind a Cummins 8.3L N/A diesel.Drive is 1.65 ratio.Total weight is around 6300lbs. Fitted to a custom solid glass 26 footer flopped of an F233, slightly extended transom and increased free board.
Drive was originally a 290sp 1.61 and had no prop.I have just swapped on a DP lower and need to get a set of props as a baseline to get it into the water and work from there.
My thoughts are a set of A4's to A6's

Thanks in advance,
Doug

 

[Maclin]

Re: A series duoprops for diesel

What is the rpm range of that engine?

Is the 1.65 ratio the net ratio after the DP swap in?

 

[dugggy]

Re: A series duoprops for diesel

197hp @ 3000rpm light duty
210-215 hp @ 3300rpm max
Yes 1.65 is correct.
Cheers

 

[Maclin]

Re: A series duoprops for diesel

Cheers back-at-ya Duggy. A6 might be a little steep, I am thinking A3-A4 maybe. I think those would get 35mph at the 3300rpm. This is just speculation, I would like to hear more experienced diesel duo-propper's stories first.

My guesses are based on an A5 being about a 23 pitch, again this is all speculation because VP does not seem to provide the pitch anywhere, I just used real world numbers from mine and other's setups to back into the pitch. So, with a 19 pitch at 3300rpm and 1.65 ratio that would be 35mph. I used 35mph as theoretical/attainable/desirable top speed for your boat, and a simple prop calc tool.

I was also trying to factor in the torque available rather than the HP thinking the diesel powerband is different than the gas. A typical gas ratio in duoprops is 1.95. The diesel will have 500 foot-lbs available at the 3000rpm where typical gas would still be building torque and may be 300 at that point.

HP is HP, but torque allows for the lower ratio which is needed for the lower rpm diesel powerband, to keep the boat speed up.

 

[dugggy]

Re: A series duoprops for diesel

Yeah the torque factor is where I am stuck at as well.With the heavy diesel it will never be a speed demon but it is a beast of an engine for a trailer boat (Heavy too!)and I am keen to have it propped as efficient as possible for both economy and speed.The problem I have encountered with the prop calculators is none i have seen cater for dual props .Closest I have found is a guy in Hawaii with a Formula 233 with a Volvo Penta AqAd41A 200hp @ 3800rpm.He is using A4's, however from the specs I can find the Volvo while turboed is only 3.6L and way down on torque compared with the Cummins.
Doug

 

[Maclin]

Re: A series duoprops for diesel

It would be nice to know what pitch the single prop was at and what numbers it gave!

Maybe others with experience will chime in soon. I would probably go with A4 if I had to take a stab at it, mostly based on the heaviness of the boat.

 

[Scott Danforth]

Re: A series duoprops for diesel

your 8.3 will not turn big RPM's 2600 RPM max being N/A, there wont be much power either from the 8.3. just curious on the engine choice over a 5.9 or 6.5 TA engine.

 

[dugggy]

Re: A series duoprops for diesel

The motor was already installed when purchased.Boat was custom build for Government dept,install is well sorted.3 foot jack shaft to drive with raw water pumps ect run off that with easy access.Transom is clear of engine box which is only 12 inches high anyway.Motor is way over square and likes to rev,they don't like lugging at all.Only has 400hrs on it always trailered so engine is like new.PO did mention with the SP 1.61 ratio drive and whatever prop it had on it it would max revs way too easily.he had been advised to re-gear it,not sure VP make a taller set though.I am hoping the Duoprop will be of benefit,has taken a while to find a diesel lower with the 1.65 ratio instead of the more common 1.95.He was hoping to get 50knots with a re-gear and prop which is laughable,ill be happy with half that.
Doug

 

[Scott Danforth]

Re: A series duoprops for diesel

the 8.3 is a torque monster, if its reving freely, you do not have enough prop, and you may not be able to with the existing drive. the factory never coupled the 8.3 with an outdrive, only marine gears and ZF pods. Even the 5.9/6.7 only got coupled with the Bravo X, XR, and NXT1

 

[dugggy]

Re: A series duoprops for diesel

Yeah I figured it was a torquey beast.On the one had despite the max rpm differences I am thinking 200hp Volvo diesel in the same hull turns A4's so 200HP Cummins will turn A4's.But with the extra 100lbs of torque surely it would handle more pitch.
Doug

 

[dugggy]

Re: A series duoprops for diesel

OK so I finally have got hold of the torque/rpm figures.
Torque Ft lbs 408@1900 337@3300
Torque Nm 554@1900 457@3300
. . .
Fuel Consumption l/hour 26 43
. . .
Max Toque Ft lbs 408 @ 1900 RPM
Max Torque Nm 554 @ 1900 RPM

Which gives 212hp at 3300 RPM