QCS 8.3 540hp prop

[Scott Deardorff]

Hello,<br />We have a Duffy 37 finished by Farrin in 2010 running a Cummins QSC 8.3 540 hp. Per the engine plate max rpm is 2600 governed at 2675.  This is turning a ZF transmission with a 1.962 gear ratio.<br />Fully loaded with 300 gal fuel, 200 gal water and 5 people plus gear WOT is 2520 rpm. Speed is 23.6 and GPH is 27.4. Cruising at 2100 rpm we get to 16.1 knots and burn 18.1 gph.<br />Dry weight is 22,000 lbs… fully loaded I would say it’s closer 26,000 lbs.

Our current prop is LH 28×27 bronze DQX with a 2 1/4″ shaft.

Our prop guy is recommending an Acme 27×27 in NiBrAl.

With a smaller diameter and the same pitch is this going to fix the over-prop situation? I was thinking the same diameter with less pitch like 28×25?

<br />Also…If we’re successful in dialing in the prop to get to WOT what can I expect an improvement in performance and/or efficiency?  More importantly, I’m sure it’s better for engine life and that’s probably reason enough to make a change.

Thanks,

Scott

Topic

[Scott Deardorff]

Thanks Steve.  After reading your excellent write-up and Tony’s comments, I looked at and compared by data to the Cummins data and concluded that I’m over-propped and need about 1″ to 1.5″ less pitch.  (slightly updated spreadsheet attached) Seems that 1″ of less pitch will get me there but with the prop slip you experienced and to get to Tony’s recommended WOT of 2685 I might need 1.5″ to 2″ of less pitch.  Our prop guy is recommending the same pitch with 1″ less diameter (New:27×27 vs Current 28×27). 

I don’t know how the less diameter equates to less pitch or does it?  Also how less diameter affects speed and/or efficiency.  

Now with better data, and great insight from this conversation, I’ll have another conversation with the prop guy to get his thoughts on all this.  

Thanks for sharing … Really appreciate it!

 

[Steve Lewis]

Scott,

Nice job on the data acquisition and spreadsheet.  I wrote a document that consolidates a lot of the guidance that Tony and Rob have made over the years.  It has to do with prop tuning.  I also included my own experience with pitch modifications and the resulting performance changes in that write up.  It was instructive and I learned quite a bit.  You seem open to digging in and learning so give the attached document a read and then compare your data to the Cummins ideal data for your engine and then you should be able to form some conclusions.  Once you have those conclusions, post them here and see how Tony, Rob and the forum in general guide you on those conclusions.  I did that and was able to learn quite a bit.

 

[Scott Deardorff]

More Data

Thanks for the replies and info.  Having a great time learning about all of this.

Attached is data from our trip out to Santa Rosa this weekend.

I hope this provides further insight.

Thanks!

[Rob Schepis]

The most economical thing now would be like Scott said and pitch down his current prop.  But the ACME will be an overall upgrade in performance for the big picture; so more money now but there is an ROI and then you have a good spare prop if/when needed.  At least she’s a single screw so it’s not to heavy a hit on the wallet..

[Luke Nelson]

Wouldn’t  it more economic to take the current prop down to 27  and re balance them?

[Tony Athens]

Not sure  how deep you want to get into propping your engine right.  I’ve attached a few things to help you understand your QSC 540.

1.  2600 RPM in the minimum rated RPM you should reach with the vessel fully loaded. Your governor is set at 2685RPM, and IMO, you should be able to reach that under any condition

2. IMO,  a properly sized ACME prop will improve your overall performance. That has been my experience 100 % of the time when switching from what you have now.  To me,  it’s $$ well spent if performance per GPH used is a goal……………… It certainly is to me.

3) Plot your current GPH per RPM in 100 RPM increments and post it.

 

 

 

 

[Author]

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