Overpropped...But Why?

[MYTraveler]

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?

Mechanically governed engines can fail to make rated rpm if the governor is set wrong. If the engine is able to turn rated rpm (plus) without load, then its failure to produce rated rpm under load rules out a misadjusted governor and suggests an overloaded engine (although it could also be a fuel restriction).

 

[BandB]

Mechanically governed engines can fail to make rated rpm if the governor is set wrong. If the engine is able to turn rated rpm (plus) without load, then its failure to produce rated rpm under load rules out a misadjusted governor and suggests an overloaded engine (although it could also be a fuel restriction).

I just don't know where he got his 20 RPM number and the 2900 surprises me that he did that. I just don't understand this approach at trying to figure it out.

 

[SteveK]

Mechanically governed engines can fail to make rated rpm if the governor is set wrong. If the engine is able to turn rated rpm (plus) without load, then its failure to produce rated rpm under load rules out a misadjusted governor and suggests an overloaded engine (although it could also be a fuel restriction).

Yes you need the baseline. According to engine specs 2850 no load and 2600 under load for 135
The engine specs will tell you what it should be governed with no load and then max rpm under load with right prop

I just don't know where he got his 20 RPM number and the 2900 surprises me that he did that. I just don't understand this approach at trying to figure it out.

he compared his tach to a photo tach as was suggested early on. So the engine is running 20 rpm less than the tach reads.
At 2900 it is within specs without load, could get it adjusted to 2850

 

[BandB]

Yes you need the baseline. According to engine specs 2850 no load and 2600 under load for 135
The engine specs will tell you what it should be governed with no load and then max rpm under load with right prop


he compared his tach to a photo tach as was suggested early on. So the engine is running 20 rpm less than the tach reads.
At 2900 it is within specs without load, could get it adjusted to 2850

Thanks for the translation.

 

[Sidclark]

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?"

Thanks for the translation.

"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.

Sorry for the confusion there BandB. As suggested here just trying to rule out the simple stuff.



The "7 knots at 2000 rpm" came from when I brought the boat down from Deltaville, Va to Myrtle Beach, SC. That was a seven day trip down the ICW, so that included rivers, sounds and canals. On the trip down I decided to run at 7 kts because I thought it would be an easy speed for the boat to maintain without to much stress. Remember, this boat is new to me, so I wasn't sure where she was "happy" at. On the way down speeds varied from 4.5 to 9.5 knots depending on wind and current. that was at 2000 rpm. But for the most part the boat ran 7 kts at that rpm. That's where I got that number from.



Now, the river I'm on is the Waccamaw River in South Carolina. This portion of the river connects to the manmade portion of the ICW. A little history. This section of the ICW from Little River Inlet in North Carolina to Enterprise Landing in South Carolina is the longest manmade section of the ICW. It was started in 1930 and finished in 1936. The ICW connects to the Waccamaw River just south of Enterprise landing. From there the Waccamaw runs to Bucksport and then on to Georgetown and out through Winyah Inlet. Here's the cool thing about this part of the Waccamaw. It runs in both directions. It's a tidal river with a reversing current. Waccamaw translated from an Indian dialect means "coming and going or to and fro". Having said all that, I'll run the portion from Enterprise landing to Bucksport back and forth a few times and that should give me an accurate number for speed/rpm. I'll calibrate the speedometer before I do this to reflect the photo tach results.

 

[SteveZ]

Failure to Reach WOT

This article might clear up some of the issues.

 

[sunchaser]

This article might clear up some of the issues.

Thanks for sharing and for being here.

 

[mikecambrai]

Overpropped - but why

Way back in the old days before even Lehmans, the rule of thumb with engines was to go to the maximum speed and then back off by 10%. This would reduce the load on the engine such that it was not labouring and not producing black smoke.
I still carry out the same process today and my 2 Fords will go up to 2500rpm, but hell they are getting on a bit now so I would never take them over 2000rpm unless there was an emergency, and normally like to run them sweetly around 1600.
Fuel consumption on a displacement hull is assymptopic as it approaches displacement maximum and there is little to be gained by going above 1.1 root L except to hear the fuel start to gush down the pipes.
Treat them gently and they will look after you.

 

[ssteakley]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

But make sure your tachometer is properly calibrated. As stated before yes a handheld laser instrument at engine, and adjust the tach instrument at the helm, to match, usually a small screw on the back. Measure at several different RPM’s .

 

[Sangraal]

Talking to Brian at American Diesel he advised that the target RPM for the Lehman 135 is between 1750 and 1850 for my boat ( heavier and longer). Perhaps you check in with him before setting out to run at the higher RPM with different props. I know fuel burn increases significantly with my set up for very little return at 1775.

Paul
KK 54
Ursa Major

 

[Ric in Richmond]

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

With the boat off fully advance the throttle at the control then go down and disconnect the cable at the throttle arm and fully advance the throttle arm by hand. Does it have a little or a lot more movement?

Or, if you can leave someone else with the helm underway and are comfortable, you can do the same thing with the boat running. Cruise with full throttle at the controls, then disconnect the cable at the arm and fully advance the arm. How many more RPM do you get?

If the nominal number is 2500 and you are seeing 2300 from the controls, you are only 100 RPM off -- and it could be at the controls.

Maybe you are off an inch of pitch. Maybe. But, I'd check simple stuff first.

My last boat (1977 Californian 42') had way too much pitch. It should have been getting tob2400 or 2500 WOT-Load andbonly hit 2000. My idle in a trawler was about 7kn!

2" of pitch later, I was at 2400 WOT-Load and cruised at 2100. Idle was nixe and slow. Cruise was almost 10kn

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

Reminds me of an MG race car I bought. Guy was disappointed his engine he built wasn't making big HP. I was fine with it since I wasn't going to race it but just have fun.

One day had someone sitting in drivers seat and push throttle down......weber side draft carbs only opened about 75%.

Made a new linkage...now they opened to full throttle...damn that thing was fast!!!!!


Great idea to check your linkages. Stuff moves, bends, binds, slips...worth a look.

 

[rgano]

Regarding those two props you have

More than likely you have a four-bladed prop on there because the three-blade "spare" was original and caused unwanted vibration being behind the deadwood. I spent several years on a battleship which had three-bladed props on the shafts while the inboards, which were behind two big skegs, were four-bladed with the stated design purpose to reduce vibration.

 

[Alisske]

The first thing I would do is make sure the tachometer is accurate by using a hand held tachometer on the engine.
Then you have accurate data.

+1. Spot on.

 

[Sidclark]

More than likely you have a four-bladed prop on there because the three-blade "spare" was original and caused unwanted vibration being behind the deadwood. I spent several years on a battleship which had three-bladed props on the shafts while the inboards, which were behind two big skegs, were four-bladed with the stated design purpose to reduce vibration.

I do have a four blade prop mounted right now and it has been on there for the last 10 years according to the PO. Don't know the specs on it. The spare is a three blade that measures 23x16...

+1. Spot on.

Have checked the tach and now calibrated for accuracy. Was only off 20 rpm..

 

[BruceK]

Reminds me of an MG race car I bought. Guy was disappointed his engine he built wasn't making big HP. I was fine with it since I wasn't going to race it but just have fun.

One day had someone sitting in drivers seat and push throttle down......weber side draft carbs only opened about 75%.

Made a new linkage...now they opened to full throttle...damn that thing was fast!!!!!


Great idea to check your linkages. Stuff moves, bends, binds, slips...worth a look.

We once had a 45mm SD Weber on the bench with fuel in it. Operating the throttle lever, it squirted fuel about 5 feet. If we had fuel injection and then Weber invented his carburetor it might have been seen as an advance

 

[Peter Pan]

I agree with you. They are talking about WOT which is not the proper RPM to run any diesel engine for more than 5 minutes every hour. Also, being a displacement vessel with only 36' he will not get over 7 knots, no matter what he does.

 

[rolomart3]

A four-blade prop creates les vibration than a three blade prop. "Overpropping" is done usually to increase the speed per engine RPM and thereby operating the engine at higher torque. When operating at lower RPMs these engines are severy under loaded as far as torque is concerned. Therfore, the efficiency and thereby the fuel mileage is increased at lower or cruising speed with an overpropped propeller.
The down side is that the engine will not reach FOT RPMs and should not be subject to prolonged operation at high RPMs. By the way, 7 knots at 2000 is fairly good. For a 36-footer, I would not cryise it at more than 6 Knots. I operated my 40-footer single diesel with an overpropped four four-blade at 1750 RPMs and traveled at 6 knots. My fuel economy improved approximately about 6% or about one gallon saved every 8 hours of operation consuming 16 instead 17 gal
per hour at 6 knots.

 

[Nomad Willy]

I agree with you. They are talking about WOT which is not the proper RPM to run any diesel engine for more than 5 minutes every hour. Also, being a displacement vessel with only 36' he will not get over 7 knots, no matter what he does.

It’s SD. Will probably go considerably faster like 9 - 12 knots w enough power.

 

[jleonard]

Have checked the tach and now calibrated for accuracy. Was only off 20 rpm..

That's right on the money then.

 

[yamabuki]

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.

Tried to find a tachometer ap but none seemed any good, which do you use please.

 

[yamabuki]

Comfort

We usually Cruise at 2000rpm and get about 9.7 knots which seems everything is comfortable with, when you and your boat are getting on some comfort is nice.

 

[Sidclark]

The numbers are up

Ok, finally got a chance to run the boat today and do some rpm/speed runs.
Since there is some current here, at each rpm I did a run in both directions and then averaged out the speed between both runs. I wish I could have gotten the higher rpm numbers, but today she wouldn't hold 2000 rpm. She held at 1800 just fine, but that extra 200 rpm wasn't happening. So now I've either have small air leak or a developing clog on the port side fuel system. Anyways, here are the numbers...

800 rpm 3.1 kts
1000 rpm 4.1 kts
1300 rpm 5.2 kts
1500 rpm 5.9 kts
1700 rpm 6.2 kts
1800 rpm 6.5 kts

 

[Nomad Willy]

I’d take an inch or two of pitch out.
Has the prop been rep-pitched in the past?
When you get that boat pitched right you’ll think it’s a new boat.

You got more rpm in post #1. So yup there could be an engine problem.

 

[rolomart3]

FOT RPMs

Replying to : "...Ok, finally got a chance to run the boat today and do some rpm/speed runs.
Since there is some current here, at each rpm I did a run in both directions and then averaged out the speed between both runs. I wish I could have gotten the higher rpm numbers, but today she wouldn't hold 2000 rpm. She held at 1800 just fine, but that extra 200 rpm wasn't happening. So now I've either have small air leak or a developing clog on the port side fuel system. Anyways, here are the numbers...


800 rpm 3.1 kts
1000 rpm 4.1 kts
1300 rpm 5.2 kts
1500 rpm 5.9 kts
1700 rpm 6.2 kts
1800 rpm 6.5 kts"


Your numbers remind me of something that happened to me. Any barnacles in the blades of the propeller can have devastating effect on speed and RPMs of the engine. In addition, if it were summer, the engine would overheat. It happened to me and by removing the barnacles, a 10 minute job, I was up and running at 100%.


My 2 cents.




Rolo

 

[SteveK]

Sidclark
In post 1 you said bottom was clean, was it?
How did you confirm it clean. Is it clean now?
I am leaning towards growth on bottom must be eliminated.
I guess I do not want you to have engine problems causing this.

 

[Al]

I’d take an inch or two of pitch out.
Has the prop been rep-pitched in the past?
When you get that boat pitched right you’ll think it’s a new boat.

You got more rpm in post #1. So yup there could be an engine problem.

Tend to agree with Eric.to a degree. Reducing the pitch will allow increased RPM, however, I am doutful it will increase your SOG.

I would go with double checking thottle cable to the injector pump for full throttle, adjustment. What happens when you draw from the Starboard fuel tanks in terms of max RPM?
Unrelated yet of interest. I replaced a failed injector pump rated at 2800 RPM on the plate, with a new Injector puimp that was rated at 2200 RPM.on the plate.
This new pump was designed for a Massy Fergerson farm tractor application.
Sure enough, the max RPM I can obtain is near 2200 RPM with the same throttle linkage unchanged from one to the new. Just a thought that the injector pump may be the culprit
Al-Ketchikan

 

[Sidclark]

Sidclark
In post 1 you said bottom was clean, was it?
How did you confirm it clean. Is it clean now?
I am leaning towards growth on bottom must be eliminated.
I guess I do not want you to have engine problems causing this.

The bottom is clean. Boat was on the hard when I bought it. Dropped it in the water in February and brought it from the Cheasapeake to Myrtle Beach. So it's only been in the water a few weeks and in fresh water now. I'm pretty sure it's a fuel problem and not an engine problem.

Tend to agree with Eric.to a degree. Reducing the pitch will allow increased RPM, however, I am doutful it will increase your SOG.

I would go with double checking thottle cable to the injector pump for full throttle, adjustment. What happens when you draw from the Starboard fuel tanks in terms of max RPM?
Unrelated yet of interest. I replaced a failed injector pump rated at 2800 RPM on the plate, with a new Injector puimp that was rated at 2200 RPM.on the plate.
This new pump was designed for a Massy Fergerson farm tractor application.
Sure enough, the max RPM I can obtain is near 2200 RPM with the same throttle linkage unchanged from one to the new. Just a thought that the injector pump may be the culprit
Al-Ketchikan

I've checked the throttle cable and linkage and I'm getting full throw. I can't draw from the starboard tank because it has a clog somewhere in the line. Now, with the engine wanting to die at 2000 rpm, I'm betting that the port side is building up a clog also. The boat sat on the hard for a couple of years so there's sure to be sludge in the tanks. Who knows, maybe that's why WOT was only 2300 at survey?? I've got to go through the fuel system and make it pristine. I don't mind doing it, except there can be no residual diesel smell when I'm done. There's a "wife acceptance factor" involved. So, I figure I'll diaper every thing where fuel may drip or run to and take my time. Once I have clean fuel I'll update the speed/rpm numbers...

 

[SteveK]

The bottom is clean. Boat was on the hard when I bought it. Dropped it in the water in February and brought it from the Cheasapeake to Myrtle Beach. So it's only been in the water a few weeks and in fresh water now. I'm pretty sure it's a fuel problem and not an engine problem.

I've checked the throttle cable and linkage and I'm getting full throw. I can't draw from the starboard tank because it has a clog somewhere in the line. Now, with the engine wanting to die at 2000 rpm, I'm betting that the port side is building up a clog also. The boat sat on the hard for a couple of years so there's sure to be sludge in the tanks. Who knows, maybe that's why WOT was only 2300 at survey?? I've got to go through the fuel system and make it pristine. I don't mind doing it, except there can be no residual diesel smell when I'm done. There's a "wife acceptance factor" involved. So, I figure I'll diaper every thing where fuel may drip or run to and take my time. Once I have clean fuel I'll update the speed/rpm numbers...

I just read your "Marine Trader fuel tank" thread and at the same time see your comment here about knowing you have a fuel flow problem. That explains it then, fix the fuel flow issue before worrying about prop pitch. You probably will gain SOG with the same RPM's.

 

[FF]

"Once I have clean fuel I'll update the speed/rpm numbers..."#88

Would be lots quicker to just borrow an outboard tank and hook it directly to the injector inlet .

Gain a few hundred RPM underway , the cleaning will be worth the effort.

 

[OldDan1943]

Ah yup, the benefits of an installed fuel polishing system. You can use a 2 micron filter to capture everything of importance.
What are the condition of your on-line and type of fuel filters?
How is the fuel supply to your generator? Can you successfully load up the generator?
Let's start small. Cycle all fuel isolation valves a number of times to break up any crap at the valves. Next, remove the hose between the tank and the the secondary fuel filter (for now we shall deem them Racors) A bit of air pressure reverse blow .... collect this fuel into a bucket for further inspection. Next, fuel polishing and tank inspection, cleaning the tanks as necessary. Okay, that's the easy stuff.