Overpropped...But Why?

[Sidclark]

Yep, some homework is in order. I'll get a photo tach and make sure the onboard tach is accurate. As soon as I can get to the boat, (we have flooding on the Waccamaw here in the Sockastee area of Myrtle Beach and the fixed part of the dock is under water), I'll check the linkage. Also, I'll check size and pitch of the spare prop. The boat won't be pulled for a couple of months, so this will give me a chance to get the preliminaries out of the way.

 

[Al]

Controls are normally set up to stop 100rpm short of max-WOT-under-load.


"The mechanic said it was crazy, but folks did that to cruise at lower RPMs and get a quieter ride.

"


Exactly!! For that same purpose on our boat. I am over propped by purpose. The Perkins 4-236 max RPM is 2800 RPM. I am able to obtain 2400 RPM. I NEVER run anywhere close to that setting. In fact, when my injector pump failed (rated for 2800 RPM), I obtained a Massy Ferguson tractor unit for under $500 +- new. over a OE Marine Perkins at $2,000. The difference is the Massy Ferguson pump is restricted to 2200 RPM. That proved to be true, as now my engine is topped out at 2150/2200 RPM. Here is the kicker. I NEVER approach 2200 RPM. My max running RPM is 1600 in a pinch and normal cruise RPM is 1450-1500 RPM. This is due to hull speed factor. At that cruise RPM we make between 6.5 and 7 knots. Hull speed by formula on our hull is 6.8 knots,
Point of all this is I am running quite at 1500 RPM (70 decibels in the saloon), We are at or near hull speed, The engine is not straining, it purrs for hours at 1.25 +- gallons per hour.


I mentioned the Massy Ferguson connection for the point of reflecting the vast price difference between marine and farm applications for the same engine

 

[Al]

To get your gear ratio use your photo tachometer to measure engine RPM while in gear at a set RPM. Say 1,000 RPM. Without touching the throttle / changing RPM measure the rotation on the prop shaft using the photo tachometer. Divide the two together to get your gear ratio.

You need to know all this info before going anywhere near the prop.

WOT RPM no load
Max Rpm obtainable under load
Gear ratio

My opinion FWIW, is that as long as you are running under hull speed and under 2,000 RPM continuous (135 Lehman) being a little over propped won’t hurt.

Why not just look at the plate on the gear????
Lest it is missing, then caculations are in order, of that no doubt. Lot of fuss if the plate is on the gear housing.

 

[MYTraveler]

Every boat is different, but in general, the faster you want to go, the more power PER speed is required. In other words, at any given speed, a 5% increase in speed requires more than a 5% increase in power. As such, max power is required at max rpms, and therefore, if you are able to make max rpms (even if it takes 99% or 100% of the engine's max power output to do so), you should be able to run at 85% - 90% (depending on make of engine) of max rpm without imposing more than a 90% load. (And it is important to understand that in this context load means power actually being produced / max power that the engine can produce AT THAT RPM; an engine can be overloaded at 50% of max rpm even though it isn't coming close to the engine's absolute max power -- the only thing that matters is the max power at that particular rpm).

In an ideal world, props would be variable pitch, and the pitch would be adjusted to keep the load at 85% - 90% (depending on engine) of max load at all times. In the real world, boat props are fixed and the engine is capable of producing much more power at lower rpms than is required. So, even if the boat is over propped, it won't matter except at higher rpms.

When my boat is fully loaded, and the bottom dirty, it is slightly overloaded. I talked to Tony Athens at SBAR Marine about this. He agreed that when my engines are running at 1600 rpm or below, where the max load is about 70% (modern engines can tell you that), the over propping is of absolutely no consequence. In fact, if I had variable pitch props, I could safely and efficiently add pitch to keep the load at 85% - 90% with no adverse consequence. Moreover, since 95% of my engine hours are at 1200 or below, reducing my props' pitch is absolutely unnecessary.

 

[SteveK]

Yep, some homework is in order. I'll get a photo tach and make sure the onboard tach is accurate. As soon as I can get to the boat, (we have flooding on the Waccamaw here in the Sockastee area of Myrtle Beach and the fixed part of the dock is under water), I'll check the linkage. Also, I'll check size and pitch of the spare prop. The boat won't be pulled for a couple of months, so this will give me a chance to get the preliminaries out of the way.

I don't disagree with ensuring your tachs are correct.
However they do not have to be accurate for this discussion. IF your speed over land is 7KN and your hull speed is 7.5 then something is not right besides what the tachs are saying. The 135 should push you at hull speed.
Further your tachs can still be used to reference max under load compared to 75-80% at normal usage.
IMO your rpm at 7 kn was too high for an overpropped condition.
I am ready to be corrected.

 

[Cigatoo]

Why not just look at the plate on the gear????
Lest it is missing, then caculations are in order, of that no doubt. Lot of fuss if the plate is on the gear housing.

I did exactly that. Nothing added up?? Finally called the people that built the boat. They changed the ratio to get better performance. Marked the new ratio with a die stamp on the housing. The plate was never changed and reflected the original ratio. Only takes a few minutes with a photo tachometer. The numbers don’t lie.

 

[markpierce]

I consider my boat/prop/engine combination well-balanced. (NA John Deere 4045)

Dead-slow, 800 RPM, 3.3 knots
No-wake, 1000 RPM, 4.2 knots
Slow, 1400 RPM, 5.7 knots
Standard cruise, 1800 RPM, 6.3 knots, 43% load, 1.7 GPM
Full speed, 2200 RPM, 7.3 knots (hull-speed), 72% load, 2.9 GPM
Wide-open-throttle, 2400 RPM (engine rating), insignificant speed increase, 4 GPM

Meant GPH rather than GPM.

 

[SteveK]

Meant GPH rather than GPM.

Oh! I thought you had gassers

 

[Island Cessna]

Yep, some homework is in order. I'll get a photo tach and make sure the onboard tach is accurate.........

Search for tachometer in the App Store for your phone. If your smartphone has an LED flash you will find a free strobe tachometer app for it.

 

[Al]

I did exactly that. Nothing added up?? Finally called the people that built the boat. They changed the ratio to get better performance. Marked the new ratio with a die stamp on the housing. The plate was never changed and reflected the original ratio. Only takes a few minutes with a photo tachometer. The numbers don’t lie.

Well, you did find the information, so, out of forum curiosity, what did the die stamp say???

 

[catalinajack]

Running a Ford Lehman constantly at 2,200 RPM would not be something I would do. I have never seen an owner posit they run their Lehmans at any more than 1,800 with most saying 1,600 to 1,700.

 

[Cigatoo]

Well, you did find the information, so, out of forum curiosity, what did the die stamp say???

Plate said 2.0. Stamp was 1.47. Or maybe vise versa?

 

[Al]

Plate said 2.0. Stamp was 1.47. Or maybe vise versa?

Thanks, from this branch of the wisdom tree, that is a big "Vise Versa" when posing the original question. If it is the 1:47, my guess is that the engine power is struggling with the ratio to turn faster than the wheel diameter/pitch will allow. I'd say you had a correct wheel on a wrong ratio.
The rated RPM of the engine is 2500 RPM and you are achieving only 2300 RPM, your by calculation about one inch in diameter to large, or two inches in pitch.
to obtain top RPM. or the 1:47 ratio is proving as stated, too much for the engine to turn a too large wheel.
I'd go with rslifkin: quote:

"If it's easy enough for you to switch props, I'd check for any markings on the spare to indicate size, then throw it on and see if the boat behaves better with it."

 

[kchace]

Running a Ford Lehman constantly at 2,200 RPM would not be something I would do. I have never seen an owner posit they run their Lehmans at any more than 1,800 with most saying 1,600 to 1,700.

Actually, there was someone posting on here maybe a year or 2 ago who said he ran WOT ALL the time!

 

[kchace]

IMHO, what sounds off is you are only getting 7KN @ 2000 RPM with a more efficient 4 blade.
I can only compare to my GB and defer to others with same boat as yours.
With one engine down I was getting 7 knots @ 1600 with 2500 max under load.
Overpropped usually lowers the RPM to attain same speed.

I totally agree! The FIRST thing I thought when I read 2000 to get 7kts is that something isn't right. That's about 110hp (according to the HP graph) to move a 36' boat at *almost* hull speed??!! It doesn't make sense.

As a total guess, I would first check tach calibration, then check the transmission ratio, then check the prop pitch. Its possible this boat has an *underpitched* prop or had a transmission with a higher gear ratio swapped in AND the throttle stop (located INSIDE the throttle control on a FL135) does not allow actual full throttle.

Definitely need some investigation - it doesn't add up.

Edit - another correction - WOT throttle on a FL135 is 2600.

Ken

 

[Sidclark]

Search for tachometer in the App Store for your phone. If your smartphone has an LED flash you will find a free strobe tachometer app for it.

Now ya tell me.. I went and bought a Extech Photo tach 461895.

Went down to the dock today and the Waccamaw is still up. Maybe a couple more days until I can get down to the boat.

 

[Nomad Willy]

I keep hearing copy reading like this in the last post. “I read 2000 to get 7kts”.

Sounds so much like the rpm produces 7 knots. So often people are implying that some rpm (engine speed) is the motive reason a boat achieves a certian speed. It’s the power that one would “get” 7 knots from. Nobodies propped the same. And what rpm varies a lot just w that one variable.

 

[Sidclark]

I totally agree! The FIRST thing I thought when I read 2000 to get 7kts is that something isn't right. That's about 110hp (according to the HP graph) to move a 36' boat at *almost* hull speed??!! It doesn't make sense.

As a total guess, I would first check tach calibration, then check the transmission ratio, then check the prop pitch. Its possible this boat has an *underpitched* prop or had a transmission with a higher gear ratio swapped in AND the throttle stop (located INSIDE the throttle control on a FL135) does not allow actual full throttle.

Definitely need some investigation - it doesn't add up.

Edit - another correction - WOT throttle on a FL135 is 2600.

Ken

I put in an email to the PO to see if he has any info to shed some light here. I'm not real sure he used the boat that much. Most of his ownership seemed to be renovating the whole boat to liveaboard and cruise. I do know the boat has done the Loop at least once. In what configuration I don't know. It's getting more and more curious as I read all the posts here.

As soon as I get the tach and take the boat up and down the river I'll fill in the blanks....

 

[Al]

I put in an email to the PO to see if he has any info to shed some light here. I'm not real sure he used the boat that much. Most of his ownership seemed to be renovating the whole boat to liveaboard and cruise. I do know the boat has done the Loop at least once. In what configuration I don't know. It's getting more and more curious as I read all the posts here.

As soon as I get the tach and take the boat up and down the river I'll fill in the blanks....

Thanks Sidclark- There will be several of us "Over Propped" guys who will be really interested in the end result of your situation.

 

[Sidclark]

Ok, back with a few more pieces of information. According to the photo tach I'm running 20rpm low. I checked at 600, 1000 and 2000rpm. No load. The no load rpm reached 2900. I just bumped it up there for a couple of seconds. I couldn't bear to hold it there any longer. In my mind this can't be good for the engine. Tell me if I'm wrong. I didn't get a chance to check actual shaft rpm, but the gear ratio stamped on the transmission is 2.10-1.

I heard back form the PO and he sent me a chart on his fuel burn and rpm. His summary is:
"Average Consumption:2.19 GPH at 1800/1850rpm or 6.5-7.5 kts".
This includes generator use which he says wasn't much. This is based on using 956 gallons of fuel.

I'm beginning to think some current had to be in play with my 7 kts at 2000rpm. Sometime next week I'm going to run up and down the river to get more useful rpm/kts numbers. I'll photo tach the shaft rpm and recheck WOT also. I do question his statement that he went from 6.5 kts to 7.5 kts with only an increase of 50 rpm. Thoughts on that?

 

[STB]

I think the way I'd read the PO's statement a little differently. My read is that he typically cruised at 1800-1850 rpms and, depending upon the seas, he'd typically see 6.5-7kts in that RPM range.

We also don't know how that speed was observed, e.g. relative to water? Relative to ground? By GPS? Relative to course? By paddle wheel? Etc. So, there are a lot of variables in play beyond tach calibration.

"20rpms low" isn't low at all. That is totally well within tolerances.

The something in play could have been sea conditions. Or the mechanism of observing speed. Or the type of speed measured. Or, maybe tolerances changed over time and the tachs need, or needed, an adjustment, etc.

 

[SteveK]

measured by GPS I have seen large drops and increase in speed over ground.
20 rpm off is negligible.2850 no load and 2600 under load for 135.
The 7KN at 2000 may have had opposing current.
look forward to your up/down average report

 

[Cigatoo]

“1800/1850rpm or 6.5-7.5 kts”
That is what my GB 36 with a 135 turns. With slack current I am at about 7.5 knots at 1800 RPM. Which is just a touch below hull speed. I consider that about right.

 

[Jmarsh203]

If you look at a hp to rpm curve for any given engine it will tell you the maximum hp that engine will make at that given rpm, now if the engine is spinning at lets just use 1500 rpm and say the graph says it can make 55 hp at that rpm that will be the maximum hp that that engine is capable of making at given rpm. , now if you ask the engine to do something like spin a bigger propeller and that prop requires more than 55 hp to spin it you will be overloading your engine and it WILL damage your engine in the long run for lots of reasons I just don’t feel like explaining. Now the inverse is not true on a Diesel engine if you are spinning the prop at 1500 rpm and the propeller only requires say 20 hp to spin the prop at that rpm the engine will only be producing 20hp, maybe slightly more just from parasitic loss

 

[BandB]

Ok, back with a few more pieces of information. According to the photo tach I'm running 20rpm low. I checked at 600, 1000 and 2000rpm. No load. The no load rpm reached 2900. I just bumped it up there for a couple of seconds. I couldn't bear to hold it there any longer. In my mind this can't be good for the engine. Tell me if I'm wrong. I didn't get a chance to check actual shaft rpm, but the gear ratio stamped on the transmission is 2.10-1.

I heard back form the PO and he sent me a chart on his fuel burn and rpm. His summary is:
"Average Consumption:2.19 GPH at 1800/1850rpm or 6.5-7.5 kts".
This includes generator use which he says wasn't much. This is based on using 956 gallons of fuel.

I'm beginning to think some current had to be in play with my 7 kts at 2000rpm. Sometime next week I'm going to run up and down the river to get more useful rpm/kts numbers. I'll photo tach the shaft rpm and recheck WOT also. I do question his statement that he went from 6.5 kts to 7.5 kts with only an increase of 50 rpm. Thoughts on that?

You're running 20 RPM low? Are you now saying at WOT you're running 2480 RPM? You said 2300 before. So, no load, you're running in neutral? Why? What are you learning by doing that?

 

[Jmarsh203]

And it will have a fuel burn reflecting that hp figure, or at least close enough to not matter. So all you are doing when overproping a boat is taking away your ability to produce the maximum hp that that engine is capable of producing at maximum rpm, and opening up the ability of damaging your engine in the long run from overloading and lugging your engine down from too large a propeller.

 

[Jmarsh203]

There are also other advantages of using your full hp range even if it does not produce much more speed such as more control from having more flow velocity over your rudders while the inverse only takes away control.

 

[Jmarsh203]

if You go ride 2 boats that are the same spec and one guy has proper wheels on her and the other is so over propped that she can only spin to say 1500rpm out of the rated 2500 the guy spinning the proper propeller WILL have more rudder response at any equivalent speed and it will be quite noticeable.

 

[Jmarsh203]

There would be times when underpropping would be acceptable such as running High output/hp requirement hydraulic or pto driven accessories off of the engine full time. I can think of no reason that will truly stand up to scrutiny for over propping a boat since it will just damage the engine.

 

[Al]

Ok, back with a few more pieces of information. According to the photo tach I'm running 20rpm low. I checked at 600, 1000 and 2000rpm. No load. The no load rpm reached 2900. I just bumped it up there for a couple of seconds. I couldn't bear to hold it there any longer. In my mind this can't be good for the engine. Tell me if I'm wrong. I didn't get a chance to check actual shaft rpm, but the gear ratio stamped on the transmission is 2.10-1.

I heard back form the PO and he sent me a chart on his fuel burn and rpm. His summary is:
"Average Consumption:2.19 GPH at 1800/1850rpm or 6.5-7.5 kts".
This includes generator use which he says wasn't much. This is based on using 956 gallons of fuel.

I'm beginning to think some current had to be in play with my 7 kts at 2000rpm. Sometime next week I'm going to run up and down the river to get more useful rpm/kts numbers. I'll photo tach the shaft rpm and recheck WOT also. I do question his statement that he went from 6.5 kts to 7.5 kts with only an increase of 50 rpm. Thoughts on that?

Quote BandB:"You're running 20 RPM low? Are you now saying at WOT you're running 2480 RPM? You said 2300 before. So, no load, you're running in neutral? Why? What are you learning by doing that?"
Exactly!!!


"River??" Nobody said River all throughout this thread til now. Surely currents have a large part in your predicament. Even running 'up and down' will be affected by the daily difference in 'river conditions', more so than tidal actions where tidal conditions are more predictable. Not sure even this thought can be interpreted as informative as I am not a 'river' student, just seems right to assume.
Corrections to the conclusion are welcome.