Fuel burn says I’m over loading the motors..

[Joe Monaco]

CAT 3126 420 HP 2800 WOT. Purchased new 3 blade ACME props 2 seasons ago and it made a big performance difference and the boat is running great. But… here’s the rest of the story.

Finally got fuel flow info.

RPM CAT spec. GPH My burn

2850. 23.0 23.1

2500. 15.0 15.5

2400. 13.2. 13.9

2300. 12.0. 13.6

2200. 10.6. 13.0

So t’s counter intuitive that 2200 is much more overloaded than 2400 and looks like WOT is right at CAT spec. Running @ 2200 rpm gets me about 20 knots which i thought was a good cruise speed for me and the motors. I obviously was wrong. At 2400 is my speed is about 24 knots which sometimes is too fast for the sea states. I can run at 2350 about 22 to 23 knots and that’s what I’ll try to do going forward till I can fix this knowing the fuel flow numbers.

In the world of overloaded motors how bad is this? Can I live with it another season or 2 until I need to haul for bottom paint or does this need to be fixed right away.

I emailed the ACME rep that sized the props this info and I’m waiting to see what he thinks the proper adjustment to the props needs to be.

Topic

[Gene Fuller]

The standard power requirement curve says that the power (and fuel burn) needed is directly related to the RPM raised to some exponential factor. Cummins uses 2.7 for the exponential factor.

I analyzed the Cat specs in your first post, and it is clear in that case the exponential factor is 3.0.

So, yes, the factory specs are different. The Cat spec is a bit more restrictive at mid-range RPMs.

However, that is irrelevant for either Cummins or Cat.

Specs are specs. The engine could be designed for almost anything, but it was actually designed to meet the published spec. Either stay within or you are over-propped.

Whether that actually causes a problem is not something I can address.

[Joe Monaco]

Learned today that CAT uses “displacement hulls” for prop demand curves. That probably is what’s causing the discrepancy for some heavy planning hull boats. Figured I’d pass that along. Ties in with what everything that’s been said here.

[Joe Monaco]

Hi Rob, What I find concerning is in my case if the fuel flow is accurate then to burn 10.6 gallons a side at @ 2200 rpm I’d need to take over 2 inches of pitch out. I’m not sure what the speed would be then but not the current 21 knots. Seems to drastic a measure. At that point it would be turning around 3100 rpm WOT. I’m not sure that’s ok. Whats needed is a variable pitch prop like what’s on some airplanes. I’m guessing the engineering and expense of that makes it impractical.

[Rob Schepis]

You have a fairly heavy cup if removing 30 thousands of cupping leaves you with 45 thousands cupping. 45 thou is considered a medium cup in my experience.

That is interesting. A tad old school for sure. Can’t say I agree with ignoring cruise fuel burn prop load on a vessel where you are fortunate enough to get instantaneous fuel burn data.

So consider to remove some cup next time the boat is on the hard and the wheels are off…

[Joe Monaco]

Spoke with the ACME propping specialist. He has never heard of anyone propping a boat for cruise fuel burn. As a boat propping “expert” he’s never heard of anyone trying to do this, which I thought interesting. He says he props 10 boats a day and if did that customers in my situation would complain boats are being under propped. As in my boat I’d be running about 3150rpm (2800 rpm motor) at WOT to drop the fuel flow closer to spec at my cruise. He said It’s only WOT rpm thats targeted in propping calculations he does.

He is suggesting if I must do something remove some cupping not pitch. He said removing even 1 inch of pitch would be too much with my 1.48 to 1 transmission. He suggested removing 30 thousands of cupping, leaves me with 45 thousands cupping in the blades. He said that will increase WOT rpm by 50 rpm putting me at 2900 rpm @WOT, but it looks like he and Tony are in agreement in that i should probably just leave it be. Always interesting discussions here, really appreciate this forum. Thanks to all!

[willyok]

The props were too big for his boat as well, Propeller of SD and ACME took all the data and recommended an increase in diameter of 1″ and dropping 3″ of pitch. The 420HP QSB’s now reach 3050RPM fully loaded and are within the fuel burn spec at cruising speeds. Not really certain as to the “how” of the diver doing the prop work underwater but all i can tell you is he does it often and we haven’t run into any issues.

[Joe Monaco]

T

[Joe Monaco]

Thanks for the info. I’ll think about it. The new props are Acme 3 blade 22×23. Old ones where stock 20×25 4 blades, huge increase in performance I posted the numbers in another thread here. I’m very happy with them. I didn’t have fuel burn info with the old props so no idea what cruise burn was with them but both sets of props reach rated RPM at WOT. I’m very reluctant to take 2 or 3 inches of pitch off them to drop 3GPH at my happy cruising speed 20 knots for comfort. I’ve been going a bit faster now 22-23 knots because the burn is closer to Cat spec the faster I go.Maybe I’ll compromise and reduce pitch 1.5 inches sometime down the road. Honestly I’d be nervous having the props reinstalled while the boats in the water, not sure how they can get them tight.

What horse power is on that 32? Has to be more than my 420 hp, Those props have way too much pitch for my boat and I think my shafts are 1.75.

[willyok]

Justin at Aqua Force in SD can remove and install your props in the water. He will even deliver and pick up from Ed at propellers of SD. We did this for my buddies Cabo 32. Ed is also a great guy to use hear in San Diego and listens and understands the need to meet rated RPM and fuel burn.

I didnt see you current prop or shaft sizes but he still has his original four blade 22×31 props for 2″ shafts if you wanted to give those a try.

[Joe Monaco]

Thanks Steve what you’re saying makes perfect sense. Ugh… I really didn’t want to drop 2 inches of pitch on brand new props supposedly “perfect” for my boat/motors. What a pain in the ass. Maybe next time I haul the boat I’ll have it done. Right now I’m just gonna go fishing and hope the motors can hang with me.

[Steve Lewis]

Joe,

You and I are in a similar situation. Our boat makes 2680rpm with current props. So we are making the governor rating. However, at 2200RPM(@23mph) we are burning 30gph total. That is about 3 gallons per hour more than we should. So I need to move that 30gph from 2200 to 2300. So we will be taking 1 inch of pitch out here very soon. We are having heads replaced on our 480CE’s so while she is out of the water we will have the props worked on.

I am going to ignore WOT as we will never run these engines up there. I know, never say never but we will be cruising our boat in the 2200 – 2250 range as we get reasonable speed at 2150 now. I say dial in your boat for your cruise speed as that is where you will run your boat and ignore the fact that rated RPM will come a whole lot easier with less pitch.

Prop load determines fuel burn. To decrease fuel burn you decrease prop load. To reduce prop load you can remove pitch, or as Tony indicated previously, remove weight from your boat. I am guessing the easiest thing to do is remove pitch. We loaded our boat to cruising weight for our fuel burn runs. We have everything on board that we would have with us on our long trips minus some food stuff that is negligible weight.

Our only practical way to reduce fuel burn is reduce pitch. We are going to hand over our boat to our diesel mechanic in a week or so and he will do his thing. Once all of the work is completed we will re-run all of these tests and will report back on our results. Based on guidance from Tony and other 460 Dancer owner experience, we should have a nicely dialed in cruising platform.

Looking forward to Cruising the Great Lakes. I hope you are able to dial yours in to your liking.

Cheers!

[Joe Monaco]

I did some me more “flight testing” off shore in moderate sea conditions monitoring the fuel flow and looks to me like with every 100 rpm increase while on the step starting at 2200 rpm I burn about 1 gal/hr more. So if I’m 2 gal/hour over spec I’d have to remove at least 2 inches of pitch to drop 2 gal/hour at that cruise rpm. But Then my motors would be turning about 3050 rpm at WOT vs 2850 now ( rated 420 HP @2800 rpm)

I’m planning on living with the props as is but I’m wondering is this the only way to get to the fuel burn numbers that CAT advertises? Have the motor turn 200+ rpm over rated WOT Rpm so they are closer at my cruise numbers? Does reaching rated 2850 rpm not really matter?

[Tony Athens]

Your vessel is on the heavy side in relation to its planing surface/design and HP available, that it is not as efficient at the lower speeds..

You are far from alone.. There is nothing to explain .. Take 2500 lbs off the boat and the numbers will change……………………………….Add 2500 lbs more, and you may not even get up on plane.. Its all about weight from what I see, assuming your bottom is clean and the props are good. ..

Gospel #2 says it all……

Propellers Move Boats, Engines Just Turn Them

[Joe Monaco]

Thanks Tony,

I’ll post what the ACME prop rep says once he responds here in case anyone is curious. I’ll try running at 2350 rpm this season and see how it goes. I’m guessing as the boat goes faster it’s lifting out of the water more and has less drag and that’s what’s dropping the fuel flow at higher hull speeds. That’s all I can come up with to explained it.

[Tony Athens]

IMO, You are probably fine.. But understand you are asking everything this engine has to offer and are leaving nothing in reserve…

[Author]

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