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Old 06-09-2020, 01:39 PM   #1
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Work/Heat/Overprop Discussion

I didn't know what to title this but thought this would be a fun mental exercise.

I took my boat in for prop work as well as shaft seals, cutlass bearings and other stuff. I had a spare set of props. They were 24x24. We had no clue what the props were that were currently on the boat. But we did know that the boat was correctly propped as it achieved rated RPM+75-100. The boat is a Meridian 411 with Cummins 6CTA at 450hp. It turns out the props that were taken off were 22x24. So with the 24x24 we were most assuredly overpropped. And that was confirmed once the boat went into the water. It wasn't terrible....about 2500-2550 on a 2600rpm engine. I am very well aware of the dangers of overpropping.

I was gonna get those other props but in the meantime still wanted to run the boat. Running it slightly overpropped for a few hours did not concern me as I could run it easily. The yard guy was like...."Just run the boat at whatever speed you normally run it. I mean....20 knots on the exact same boat requires the exact amount of work regardless of what props are on it....right???".

Like I said above...being overpropped for a few hours wasn't bothering me. I was going to get the correct props back on(and I already have). But I did have to chew on what the yard guy said....20 knots is the same amount of work being done regardless of prop.

WIth the correct props, 20 knots came at 2200 RPM. With the bigger props 20 knots came around 1900Rpms....maybe 2000.

So my question to y'all is....do you agree with the yard guy?? Why or why not??? And is there any difference in wear and tear in these two scenarios???...that is the real question. I think we can assume the same amount of work is being done. But is wear and tear increased with the overpropped boat even though it is doing the same amount of work???

I have my opinion/theory....but don't want to "lead" the answers(witness).
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Old 06-09-2020, 01:59 PM   #2
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It's the same amount of work, but the engines may or may not be comfortable doing that work at a lower RPM. Personally, I'd have an inch of pitch taken out of the spare set, then try them to see where you fall for WOT RPM. Then compare the 2 sets to see which feels better on the boat.
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Old 06-09-2020, 02:04 PM   #3
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It's the same amount of work, but the engines may or may not be comfortable doing that work at a lower RPM. Personally, I'd have an inch of pitch taken out of the spare set, then try them to see where you fall for WOT RPM. Then compare the 2 sets to see which feels better on the boat.
The “old” ones are back on and the boat is quite happy. If I were to repurchase the spares, I’d probably take 2 inches of pitch out.

Back to your answer though. You say the engine may not be comfortable doing the same work at a lower RPM....why?
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Old 06-09-2020, 02:09 PM   #4
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I agree with above. Engines flow more air when turning faster that keeps valves etc. cooler.. That is one reason it is possible to be over loaded at any RPM
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Old 06-09-2020, 02:14 PM   #5
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I agree with above. Engines flow more air when turning faster that keeps valves etc. cooler.. That is one reason it is possible to be over loaded at any RPM
Exactly. Exhaust temps are typically higher when working hard at lower RPM. Depending on the engine design, etc. a slight overprop may not hurt anything or it could provide a shorter lifespan.
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Old 06-09-2020, 02:45 PM   #6
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I'l bet $10 that if you took 100 engines and ran them in one condition until they failed, and another 100 and ran them in the other condition, the difference would be statistically null. A 2600 rpm boat engine running at 2000 is at something like 50% available torque. Changing that to 2200 will be 45% torque. A tiny bit less stress and a tiny bit more piston and bearing travel.
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Old 06-09-2020, 02:52 PM   #7
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I'l bet $10 that if you took 100 engines and ran them in one condition until they failed, and another 100 and ran them in the other condition, the difference would be statistically null. A 2600 rpm boat engine running at 2000 is at something like 50% available torque. Changing that to 2200 will be 45% torque. A tiny bit less stress and a tiny bit more piston and bearing travel.
For some engines that's absolutely the case. But for others, either valves become an issue with higher exhaust temps, or the higher cylinder pressure causes faster ring or bearing wear. Generally the harder the basic design is pushed, the more sensitive it'll be to high loads at lower RPM. And most won't protect themselves from it, as they need to briefly operate at high load / low RPM while accelerating, they're just not expected to stay there.
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Old 06-09-2020, 02:58 PM   #8
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I'l bet $10 that if you took 100 engines and ran them in one condition until they failed, and another 100 and ran them in the other condition, the difference would be statistically null. A 2600 rpm boat engine running at 2000 is at something like 50% available torque. Changing that to 2200 will be 45% torque. A tiny bit less stress and a tiny bit more piston and bearing travel.
So you're saying you would run the overpropped boat and not worry about it???

I would disagree. Think about it. I understand your answer and agree partially. But you are leaving out some things. The water pump is running slower. The circulation pump is running slower. So heat is not being dissipated as quickly as it should be. And most importantly, like the other posters have said, exhaust gases need to be moving....the faster they move, the less wear they cause on all of the components they come in contact with. I am not saying I am correct. I am just saying you are only factoring the work being done. We are saying there is indeed more wear and tear due to a slow down in heat dissipation....whether it be raw water...coolant...or exhause gases. NOW....if I had pyro gauges I coule provide concrete proof as in this example, if what I am saying is correct, you would see an increase in EGT...and that would tell you just how much heat/danger you are putting through the engine.

I will leave you with a somewhat rhetorical question. My last boat had 6BTAs with pyro gauges. EGT was fairly stable under all planing loads around 900 degrees. If I ran it up to WOT, EGT would DECREASE about 100 degrees....why do you think that is?
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Old 06-09-2020, 03:30 PM   #9
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In your example I don't think it matters much.

The power and prop curve explain things pretty well, at least to me. The prop curve shows that properly propped you would be using ~250 hp @ 2200 rpm. Looking at the power curve the motor can produce that power at anything above ~1600 rpm.

Did you note any fuel consumption differences between the two configurations?




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Old 06-09-2020, 03:33 PM   #10
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In your example I don't think it matters much.

The power and prop curve explain things pretty well, at least to me. The prop curve shows that properly propped you would be using ~250 hp @ 2200 rpm. Looking at the power curve the motor can produce that power at anything above ~1600 rpm.

Did you note any fuel consumption differences between the two configurations?
All that says is that the boat performance will be about the same as the engines will still put out adequate power. It doesn't say that engine durability will be the same, however.
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Old 06-09-2020, 04:03 PM   #11
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Guesses and speculation are trumped by data. Normally the answer to propping optimization requires having high end weight onboard, measurement of fuel burn, EGT, speed and accurate RPM.

If the cooling capacity is close to overtaxed, over propping can lead to an overheat. This issue though, is seldom noted until a 85% + fuel load is occurring for more than a few minutes.
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Old 06-09-2020, 04:31 PM   #12
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Baker
Guesses and speculation are trumped by data. Normally the answer to propping optimization requires having high end weight onboard, measurement of fuel burn, EGT, speed and accurate RPM.

If the cooling capacity is close to overtaxed, over propping can lead to an overheat. This issue though, is seldom noted until a 85% + fuel load is occurring for more than a few minutes.
Yep. I did run it up to 2200 which yielded almost 25 knots and temps were rising. I do wish I would have had pyro gauges.
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Old 06-09-2020, 06:42 PM   #13
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If your boat is "over propped" and cannot meet the rated rpm or match the power curve, you will be burning more fuel for the same rpm (comparing one prop to the other). For one engine I was just looking at 6BTA, at 2200 rpm if over propped you burned about 4 gallons per hour more fuel compared to correctly propped. That would equate to higher temperatures, (maybe not designed for) and possibly long term damage. How much of a factor, I can't say. But Tony Athens says "prop your boat correctly" and don't overload the engine! I trust Tony.
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Old 06-09-2020, 08:24 PM   #14
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Primarily when you overprop you’re asking fewer strokes to do the same work as more strokes. More work per stroke. More fuel, higher temps and higher forces all around. And probably more unburned fuel. And that creates it own problems.

But I think the most important takeaway here is greater pressure or forces delivered to the outboard sides of the cylinder walls. Probably the most significant engine wear and loss of engine life that engines experience.

I think it’s basically the whole ball of wax (whatever that is) and tells one what they need to know to minimize engine wear. Do it w the highest comfortable rpm known to not have excessive forces and the least forces that one can have and produce the desired power. That means higher rpm. Less force needed from more strokes taken. Truck drivers have known that for 100 years. They downshift and let the engine sing. That assumes, however, that I’m right and there is no other engine part experiencing greater wear that the piston and cylinder walls. And the above is re maximum continuous power. For less power minimum wear should be achieved by underpropping thinking. Buy using a little more rpm and lower forces and less temps one’s engine should experience max life. But if one props for rated rpm + 50 to 100rpm (depending on engine) one should more or less automatically avoid all overloading issues.
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Old 06-09-2020, 08:37 PM   #15
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In your example I don't think it matters much.

The power and prop curve explain things pretty well, at least to me. The prop curve shows that properly propped you would be using ~250 hp @ 2200 rpm. Looking at the power curve the motor can produce that power at anything above ~1600 rpm.
This is pretty much spot on. If I were going to run above 2,400 RPM, I might flatten the pitch. Pull 250 HP out of 450 HP engine with 100 RPM shortage isn't an issue. This is like having it properly pitched and then running the boat with a dirty bottom.

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Old 06-09-2020, 11:00 PM   #16
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If your boat is "over propped" and cannot meet the rated rpm or match the power curve, you will be burning more fuel for the same rpm (comparing one prop to the other). .....But Tony Athens says "prop your boat correctly" and don't overload the engine! I trust Tony.
My example was not the same RPM.....it was the same speed at a lower RPM for the overpropped boat. I was in no way advocating for overpropping. Just discussing and going through a mental exercise. I want to know what people think....by using their brain....not regurgitating everything they've been told.
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Old 06-09-2020, 11:25 PM   #17
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My example was not the same RPM.....it was the same speed at a lower RPM for the overpropped boat. I was in no way advocating for overpropping. Just discussing and going through a mental exercise. I want to know what people think....by using their brain....not regurgitating everything they've been told.
If I was going to run my boat at 80% WOT or close, for most of my cruising, I would agree 100% with those claiming overpropping is harmful.
I run my boat at 8 knots.
It came with 290hp. It now has 400 hp, so to do the 8 knots, when I burn 3.5 gph, it is using one quarter or less of available hp. I am overpropped. Have been for 20 yrs, with no ill effects. My mechanic, who suggested the engine swap from TMD40s to TAMD41s, also suggested putting a few inches of pitch into the props, to run at a lower rpm and thus extend engine life, get better fuel economy.
After 20 yrs, the fuel economy has proven itself, as at 8 knots I consistently use 3.5 gph, while at 7.5 knots on the old engines and props, I used 4 to 4.5 gph. The engine longevity will prove out long after I am gone.
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Old 06-10-2020, 04:59 AM   #18
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"I do wish I would have had pyro gauges."

The auto /truck aftermarket folks noe have pyrometer gauges that do not require a special factory made length of hook up.

Any wire can be used between sensor and gauge, about $ 125 each
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Old 06-10-2020, 01:37 PM   #19
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"More work per stroke. More fuel, higher temps and higher forces all around. And probably more unburned fuel. And that creates it own problems."

Actually there is more useful work per stroke , the fuel has longer to burn , the slower falling cylinder extracts more heat expansion (work) per stroke , there is less internal friction losses from fewer strokes to do the same work .

Over propping is not the correct word for a cruising prop that better matches the engine ability to the spin prop at the cruise speed desired.


Proper propping?
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Old 06-10-2020, 01:46 PM   #20
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"More work per stroke. More fuel, higher temps and higher forces all around. And probably more unburned fuel. And that creates it own problems."

Actually there is more useful work per stroke , the fuel has longer to burn , the slower falling cylinder extracts more heat expansion (work) per stroke , there is less internal friction losses from fewer strokes to do the same work .

Over propping is not the correct word for a cruising prop that better matches the engine ability to the spin prop at the cruise speed desired.


Proper propping?
It's still overpropping relative to the manufacturer specs, but depending on the engine, boat and usage patterns, it's not necessarily inappropriate.
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