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Old 02-27-2020, 10:09 PM   #61
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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).
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Old 02-27-2020, 10:39 PM   #62
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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.
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Old 02-27-2020, 11:14 PM   #63
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Originally Posted by MYTraveler View Post
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

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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
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Old 02-27-2020, 11:36 PM   #64
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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.
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Old 02-28-2020, 09:37 AM   #65
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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?"

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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.
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Old 02-28-2020, 01:33 PM   #66
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Failure to Reach WOT

This article might clear up some of the issues.
Attached Files
File Type: pdf Wide Open Throttle.pdf (768.7 KB, 56 views)
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Old 02-28-2020, 01:52 PM   #67
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This article might clear up some of the issues.
Thanks for sharing and for being here.
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Old 02-28-2020, 02:34 PM   #68
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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.
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Old 02-28-2020, 02:38 PM   #69
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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 .
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Old 02-28-2020, 03:25 PM   #70
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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.

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Old 02-28-2020, 03:30 PM   #71
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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.
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Old 02-28-2020, 04:09 PM   #72
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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.
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Old 02-28-2020, 04:46 PM   #73
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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.
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Old 02-28-2020, 06:13 PM   #74
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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...

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+1. Spot on.

Have checked the tach and now calibrated for accuracy. Was only off 20 rpm..
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Old 02-28-2020, 06:24 PM   #75
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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
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Old 02-28-2020, 07:36 PM   #76
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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.
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Old 02-28-2020, 07:37 PM   #77
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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.
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Old 02-28-2020, 07:58 PM   #78
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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.
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Old 02-29-2020, 07:43 AM   #79
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Have checked the tach and now calibrated for accuracy. Was only off 20 rpm..
That's right on the money then.
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Old 03-01-2020, 07:52 PM   #80
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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.
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