QSM11-M Prop pitch change

[Clark Spilker]

I have a Tiara 42 Open with QSM11-M 660 2006. 1650 hours . I just purchased the boat and we are replacing the manifolds and servicing the after coolers now. I have run an RPM WOT through all of the RPM’s 100. The boat is over propped and burning too much fuel in the cruse range 1900-2100. My prop man Willmington’s in LA says that 1inch of pitch reduction will bring about 90 RPM per inch. I told him I need to reduce the fuel burn by 4 gallons per hour at 2000 rpm and how much pitch will bring that result. He said he had no idea how to determine how many GPH would go down at any given RPM with a pitch change.

So I am asking if there is anyone who has pitched down the QSM11 on their boat and can you tell me what the effect was on your boat in GPH and how many inches of pitch you took out?

I understand all the boats are different I just need some idea so I don’t end up removing the props more than once 1400 per wheel plus diver fees.

I also wondered if it would be as simple as just moving down the power curve to the GPH I need and just adding up the GPH change. Example: If 2000 rpm is 25gph and 1900 rpm is 22gph This says 100 rpm = 3 gph. So 33 rpm = 1 gallon. Hence 4 gallons means 132 rpm. So if I reduced pitch by 2 inches = 180rpm I would have a 48 rpm cushion getting more than 4gph

Props are Nibrel 27x44x2.5 HX400 #7cup

Topic

[Tony Athens]

“And more oil to the underside of the pistons and throughout the entire engine”..

It only took them a little over 2 decades to figure that out.

Tony

[Larry Backman]

Never mind, I get it

Just went to the Cummins site and studied the 715 curve. So they add 200 RPMs and effectively deprop at the higher torque 1500-1900 RPMs. If you want 20 GPH/400 HP you get it at 2100 RPM, pushing that much more cooling air and water through the engine . This matches what you have said, deprop down even further to get well below the prop curve and run at higher RPMS to get the magic 400 HP needed.

[Larry Backman]

Tony – huh?

I’ve probably studied the QSM Curves enough that I ‘now them by heart. That 705 HP curve you showed doesn’t match the Cummins data unless they changed it recently.

Here is what I have for my 670 curve.. it shows 20.5 GPH at 2000 RPM, I have photos 2 weeks old showing me burning 21 GPH at 2000 RPMs , ie good enough.

I know you know a lot more than me but where is that chart from? It does not match my 2 year old QSM 670 curves and unless I’m missing something obvious, the 715 should be slightly above the 670 G each spot on the curve?

[Clark Spilker]

Thanks so much Tony. After I make the pitch change I will send you the results.

[Tony Athens]

Clark, this is real simple.. You need about about 430-440 HP put into your current props to get your “happy vessel speed”…Problem is you need to get the get the engine RPM up to about 2150 to get the loading righ.

How to? If you have really high end props and have a 1st class shop doing the work, then that’s about 2.5” of pitch /and or some cup..Basically you need to move the existing load up 250 RPM.

Tony

[Clark Spilker]

Yes I did tell him that and he just keeps saying the only thing he can tell me is 1 inch will equal 90 rpm

[Clark Spilker]

Clark says thanks

thanks that is very helpful 1 to 1.5 gph per inch.

[Clark Spilker]

Test results from Clark

#2 Happy cruise 1900 rpm 22.1 GPH

#3 2000 rpm 25.2 GPH 2100 rpm 29 GPH

#4 WOT is not important at all. WOT 3 different tests with different conditions 2315 rpm 2248 rpm and 2191 rpm All tests where with 4 people and 400 gallons of fuel and 80 gallons of water light gear. The tests were done in and out of San Diego Bay entrance and I’m sure the 2315 we were running with the tide using tabs

My Cummings mechanic has said that the gph will improve some after the repairs. i.e. new air filters restricted by soot from exhaust leaks and after cooler bench serviced Air side and RW side.

Thanks,

Clark

[Tony Athens]

Clark ( and Larry),

Study the QSM11 715 factory graph and comment on the note at the bottom..

I’d like your thoughts as to why want want the CRUISE ), loads in the 2000-2100 RPM range so low…

Tony

[Rob Schepis]

Did you ever tell him to prop for 580 BHP at 2,360 RPM ?

[Larry Backman]

Pitching down

36’ single engine DOWNEAST boat. 21-22K light, 24-25K with a canyon load.

QSM11-670 with a ZF310A 1.5:1 gear.

When I bought the boat it came with a 26 x 26 prop. That year I cruised at 20 knots, 1830-40 RPM burning 20-21 GPH. I had an exhaust leak as a result of a6 hr+ 16-17 knot ride home, way over the fuel curve.

That winter after replacing the manifold and much other work I reduced to 26. 25 and then made 19.5 knots at 1830-40 RPMs burning just under 19 GPH with a light load.

Th next winter I took another inch of prop out, now made 19 knots at the same 1830-40 RPMs and burned just under 17 GPH.

I should comment that I changed my throttle at this point forward to run at 1870-1880 RPM, to make 20 knots of speed burning just under 18 GPH.

I should also mention that under canyon load, 1000# of Ice , 4 extra guys and a lot of tackle I leave the dock at 1870-1880 RPM, barely making 19 knots and burning 19.5 -20 HPH GPH. After I have burned off 100 gallons of fuel I get my 20 knot speed back , fuel burn drops to just over 19 GPH. On the trip home, having burned off 200 gallons of fuel I am blaming along at 20+ knots, fuel burn down just below 19 GPH.

Bottom line – each inch of prop removal for me was about 1 to 1.5 GPH improvement , however I still have a 1 GPH variation between a light boat and a heavy one with 10% more weight.

[Tony Athens]

Here may be a better way to understand propping on your boat not any other boat..

#1———-It’s basically impossible to under prop your particular engine.. You can only over prop it.. That is what just about 100% of all prop shops cannot understand.. Your particular engine could have been shipped as a 300 HP engine / 1800 RPM in 2002 ( 15 GPH) — since then there have been close to ZERO internal changes to the engine.

2# ———–What is your happy current vessel cruise speed? What RPM is that at? What is the GPH at that cruise peed & RPM?

#3————–What is you current GPH at 2000 & 2100 RPM?

#4–How important is your vessel speed at WOT? And that current WOT RPM is ??

Let’s start with that and then we’ll discuss the rest..

Tony

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