[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]

Another forum said possible leaking injectors or injector O rings?  Anyone seen this type of symptom before?

[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.

[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!

[Joe Monaco]

Spoke with the owner today. He said they will fix it. So I’m gonna hope for the best and see what happens next. Hard to understand how this got missed but it did.

[Joe Monaco]

This is the company who built the part.

[Joe Monaco]

Hi Tony,

The system was designed by CABO and installed in 2002 when the boat was new. All I needed was a replacement part. The original exhaust company is still in business so I contacted them and sent them the part asked them to just build it again. Somehow they got the diameter wrong. Now they want me to live with it. I want the part built correctly as per original design.

I want to be able to make the case as to why it needs to be built as originally designed. Im suppose to speak to the owner today. I was told he wants me to do back pressure tests to see if it’s acceptable as wrongly built. I think that’s ridiculous, who’s going to pay for that and who decides what’s acceptable? I was hoping someone here knows what diameter pipe is minimum for 420HP to be exhausted into to confirm the chart info I quickly found on the Internet.

This system worked fine for 20 years. If I can get the same part built correctly it should work fine again.

[Joe Monaco]

Motors are 420 HP according to a quickly searched chart it needs 3 1/2 inch diameter for that much HP not 3. The new risers smaller diameter wasn’t noticed by me until after installed. Also mech had trouble using the pictured flange bolt brackets. They are we’re not getting the pipe flange tight enough to the turbo. Any ideas why this is happening? Mech wants to grind them down so they can draw flange tighter. Pipe manufacturer wants to send me a 1/8 vs currently used 1/16 gasket. Should I have them make the flange thicker if they need to remake the pipe the correct diameter anyway? Right now mech flipped them over just to make it work which obviously isn’t right. I will need another riser built but I was going to wait for winter. This one cracked. Manufacturer said they last 12 to 15 years boats almost 20. I had no idea they needed to be replaced thought SS lasted forever. Anyway So much for a simple R&R…

Thanks for any insights on what’s best to do next.

[Joe Monaco]

I didn’t notice the size difference until after it was installed of course.

[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.

[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.

[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?

[Joe Monaco]

This is what I discovered with fuel flow. My boat is propped correctly at WOT per the factory fuel flow spec but at cruising speeds it’s a bit over propped. Right now I think I’m going to live with it and plan to increase my normal cruising speed a few knots of sea states allow. The overload is less the faster the hull goes.

Just checking rpm may not be enough but obviously if you can’t reach WOT rated RPM with a clean hull and normal load then you’ve got too much prop for the motors or a mechanical problem with the motor.

[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.

[Joe Monaco]

Yikes! I’ll wait and see. I’m Curious what the cold temperatures relationship to this problem might be? Are Injectors more prone to leaking when it’s cold?

[Joe Monaco]

Still pondering this. Anyone figure out why Tony responded to the OP that He is not overloading his engines when the data he posted shows him burning .5 to 1 gph above the OEM rpm/fuel burn curve.

[Joe Monaco]

Thanks Steve,

So you’re thinking because the data point is farther left on the curve that it’s not as critical? So slightly overloading < 1GPH on say a MAX HP @ 2800 RPM motor at 2000 RPM is not as bad as overloading 1GPH at 2500 RPM for the same motor because it’s farther away from its MAX HP rpm? The Motors under less stress overall at the lower RPM making calling for more than rated HP at that lower RPM less problematic? Could be..

[Joe Monaco]

Is the consensus if you are burning .5 to 1 GPH more than the manufacturers fuel burn curve you still are not overloading? I thought below the curve was the goal or are these numbers close enough?

Is there a GPH over spec that is the maximum allowed?

[Joe Monaco]

I worked with Jim @ ACME props. I was looking for better performance and range and thought my 2002 boat’s stock props were not performing based on what I was reading about similar boat’s with less HP motors running ACME props. The boat has 420HP 3126 MUI motors. You’re right I need to get fuel flow numbers and see where I am on the HP curve at my cruise power setting.

I picked up about 50 NM range on a 350 gal tank, which is significant and the boat can easily cruise fully loaded loaded at 20.5 – 21 KTS at 2100-2150 RPM (which is normal for me) but I’m wondering about how much loading “smoke” is normal while getting on to plane. I may be obsessing.

I posted the speed RPM numbers again here but need to install a fuel flow device to really understand what’s really happening load wise at cruise. Just been procrastinating.

[Author]

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