Propping Opinions On QSB 5.9 380 HO

[Brian noody]

I’m running a 30ft Henriques Express, displaces 16,500lbs, engines have about 950hrs. I just had the props rebalanced and took an inch of pitch out. It corrected a load imbalance and I’m now reaching max RPM of 3,040. The trial runs today were with 3 men onboard, full fuel tank and a full livewell with boat fully rigged. Only thing I was shy on from weight perspective was the fish boxes which were empty. Air temps were around 50 degrees with a light swell on the ocean. Overall I was very happy with the performance, but it looks like I’m burning more fuel than the performance curve indicates I should. Below are the numbers I achieved, my cruise speed at 2700 RPM showing 78% load was 27knts. At full throttle I was running at 30.5 knts.

Port Star
RPM GPH RPM GPH
2030 9.5 2030 9.5
2210 11.5 2210 11.5
2290 12.4 2300 12.1
2400 12.7 2400 12.4
2600 14 2600 14
2650 14.7 2650 14.5
2700 15.2 2690 15.3
2770 16.1 2770 16.1
3030 20.2 3030 20.2
3040 20.2 3040 20.2

I want to be certain I’m not over working the engine and with the rpm’s reached I believe I’m where I should be, but they are a little thirsty compared to the performance curves. looking for educated opinions regarding current performance and if I should by changing props further or investigating other factors that may impact fuel burn.

Topic

[David Amble]

Duty Cycle Monitor

I presume the QSB has similar diagnostic capabilities as QSC and if so I would get a Cummins Engineer down to the boat and have him plug in his laptop and bring up the Duty Cycle Monitor. This will show exactly where your engine is running with respect to torque % over the full RPM range. This information in addition to the theoretical (and corrected) fuel curve will help in your decision on whether to keep adjusting propeller pitch.

[Fireisland1]

Some data

QSB fuel burn/ load

This was a thread I started on this topic. There is some very good explanation in it.

[Brian noody]

My objective is to keep the engines happy and healthy while also keeping them economical. I’m all about driving that fuel burn rate down and will happily sacrifice a knot or two of cruising speed to get there. Those Cummins engines call the fish in anyway, getting there 15 minutes latter wont matter! They proved it on the grounds over and over all summer.

Your explanation actually helped me better understand what another inch of pitch will deliver and it looks like another inch is exactly what she needs.

Thanks for taking the time to offer up your advice.

[Steve Lewis]

Some theory to consider

Brian,

If you spend time reading the similar threads to yours, and there are a lot of them, the main idea is to keep your fuel burn below the curve in the cruise rpm range. The Cruise RPM Range….. WOT rpm is important but not the only measure. The Cruise RPM range is where most people run their engines and that is where the most “Damage” can be caused if you are overloading the engines in that range.

If you dig in to the Cummins report for your engine a little more you will find sections that are below the chart of Prop Demand fuel burn. The first section goes into the Full Throttle Requirements. It is the next section that has some important stuff buried in it. The High Output definition. “Reduced power must be at or below 300rpm of the rated rpm.” At or Below……..

That means the MAXIMUM sustained cruise RPM as recommended by Cummins is 3000 – 300 = 2700. So if we look at the chart and use linear interpolation to get a fuel burn for 2700, I come up with 14.35gph. Your numbers are 1 gallon per hour over that mark and your boat still is not fully loaded. As you throttle back a bit your overage becomes greater. At 2600 Cummins says 12.7 and yours are 14 and at 2400 Cummins says 10.4 and you are 12.4/12.7. You are still over the mark and your sea trail was done in nice conditions. As mother nature throws adverse conditions at you and your loading is greater you will be even farther over the mark.

Generally speaking 1 inch of pitch removed equals a shift of ~150RPM. So in theory if you remove 1 inch of pitch then the fuel burn at 2700RPM will be moved up to 2850. The fuel burn you have now at 2400 will be moved up to 2550 etc etc……… So you do as Rob recommends and create a chart that has the Cummins information in it for your engine and then you run a sea trail Fully Loaded and plot your numbers for the SAME rpm values as in the chart. You take that completed chart to your prop shop and have a discussion with them about trying to dial in your setup to get just under the curve Fully Loaded. That is the Ideal goal.

so the ultimate question is “What balance are you trying to strike?” Do you want to squeeze the absolute maximum out of your engines and even a little more than that at the expense of longevity or do you want to slow down a touch by removing more pitch and extending the life of the engines. That is the balance and only you can answer that for yourself.

The ethos of those that run this site is to dial it back so that the engines are running under the recommended fuel burn curve AND below the maximum recommendations. That leads to longer life and lower ongoing costs to keep your engines operating.

Hope that helps provide some perspective on the recommendations from SBMAR and why they are made that way. Good Luck and let us know.

[Brian noody]

thanks Rob

I will collect the prop sheet from the marina this weekend when I finish winterizing the boat to see exactly where the props were and where they took them. I know this is just speculative before reviewing the sheet at best, but being as close the the max RPM as I am, I suspect an inch of pitch would be too much, should I be talking to the prop guys up the cupping in the prop? Hopefully the prop shop will have a good idea what to do.

[Rob Schepis]

She’s not hitting the governor and is over the burn curve in the cruise range. Make a chart with your numbers against the attached curve so you can see side by side. Seems to me further tweaking is warranted this off season. Especially that it appears your use of the boat is fast cruise speed runs to the offshore fishing grounds, not wine-n-cheese harbor cruises and short inshore cruises for gunkholing. Fill the fish boxes and get in a 75 mile run with rolling seas and you’ll be working the engines even harder than what you’ve just recorded.

[Author]

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