Semi-displacement hull w/single 120hp Lehman. What Prop Size?

Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine.

Each engine has a long term proper rating at most rpm below full tilt .

Most eng mfg provide a graph.

Operating at 40 hp on our 50 hp at that rpm rated engine INCREASES engine life , and lowers fuel burn too.

The best place to look for acceptable low rpm loading is at the graph for the engine when selected for use as a generator.

Usually 1200, 1500 and 1800 will have suggested loading that the eng mfg finds safe 24/7 .

Operating at 1200 or 1500 at rated loading is far kinder on the engine than proping for never used WOT and pulling back to minor hp at high rpm..

FF: I'm not doubting what you said. But Bob Smith, who was responsible for marinizing the Lehman engine encourages owners to run their vessel at WOT regularly. If it won't go there there is something wrong...either improper propellor sizing or a perhaps a larger problem with the engine. Could be injectors, or could be something else. This is I why suggested he contact Bob Smith. Bob said that Lehman engines are industrial engines (not originally designed for boats) and are designed to be run wide open for extended periods. The issue isn't whether he can make do with the incorrect prop at a lower RPM. It's whether he has he proper prop in the first place. If he has the proper prop, he should get things looked at if he can't get to WOT. If he doesn't have the correct prop, he should change it.

Jim,
Sent from my iPad using Trawler

Running an overpropped boat isn't for someone who doesn't mostly understand what's going on with their engines for the full performance curve.

If you don't understand...probably you shouldn't overprop even if you feel there are benefits.

For those that want to anyway...learn the ups and downs of doing it.

For those that do understand...hopefully you know a lot posted truly doesn't affect those that do who take appropriate measures and understand.

Hopefully those who overprop leave the proper info with the next purchaser for them to decide.

JDCAVE,
Absolutely correct. ALL OF IT.

But ..... FF is also correct when he says;
"Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine." But he should have said "the skipper is overloading the engine".

Bob Smith is not an engineer but what the guys say here about what he says seems good IMO. And when Mr Smith says "run their vessel at WOT regularly" I'm sure he was referring only to engines propped at rated rpm.

manyboats said:

JDCAVE, Absolutely correct. ALL OF IT. But ..... FF is also correct when he says; "Over propped ,a prop not chosen to spin at peak rated RPM , does NOT mean the vessel is overloading the engine." But he should have said "the skipper is overloading the engine". Bob Smith is not an engineer but what the guys say here about what he says seems good IMO. And when Mr Smith says "run their vessel at WOT regularly" I'm sure he was referring only to engines propped at rated rpm.

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...and for sure he's not an engineer. But he did spend considerable time with the Lehmann guys, during the day and also worked with the vessel manufacturers on the sizing of the props for their boat. And Fred certainly knows more than I do on any of this. If it this boat were mine, I'd be looking into the issue further to determine next steps.

Jim, Sent from my iPad using Trawler

psneeld said:

Hopefully those who overprop leave the proper info with the next purchaser for them to decide.

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Here is the note:

"Dear new prospective owner, I ignored the advice and warranty stipulations as required by Cat, Cummins, JD, Yanmar, Volvo, AD, Perkins Sabre, Lugger etc and propped my vessel so it would not achieve fully rated RPM. I did this because I'm smarter than their hundreds of engine design engineers who are backed by nearly a full century of field experience. I promise you I always operated my vessel at a very low RPM and never damaged the engine in any way.

And that rusty turbo, drip from the RW pump and moldy smell you ask - just normal stuff you'd expect from any Bristol condition yacht!"

Sorry, PS, I couldn't resist it.

sunchaser said:

Here is the note:

"Dear new prospective owner, I ignored the advice and warranty stipulations as required by Cat, Cummins, JD, Yanmar, Volvo, AD, Perkins Sabre, Lugger etc and propped my vessel so it would not achieve fully rated RPM. I did this because I'm smarter than their hundreds of engine design engineers who are backed by nearly a full century of field experience. I promise you I always operated my vessel at a very low RPM and never damaged the engine in any way.

And that rusty turbo, drip from the RW pump and moldy smell you ask - just normal stuff you'd expect from any Bristol condition yacht!"

Sorry, PS, I couldn't resist it.

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Can I use that except for the part about the turbo...he'll think I removed it...

Tom,
I love your sense of humor.

But that brings this whole issue into sharp focus!!!!

Well said

sunchaser said:

Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.

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I think you are right. The engine is running too hot as well.

Alright guys, here's the latest. Again this is with the 3 blade, Michigan, 24x16. The boat weight is ~23,000lbs. She is semi displacement and 36'. This weekend we went out to test the cruising speed and WOT speed. WOT RMP is only 1,900!! This was confirmed with a mechanical SW tach on the flywheel. The boat ran at ~8.3 knots, with some black smoke and steam from the exhaust and the engine temp spiked to 220 degrees F. Way too hot I believe for a Lehman. The sweet spot ranged from 1,600 RPM to 1,650RPM, no black smoke, 7.5 knots cruise speed and the temp dropped to 200 degrees F. The high temp worried me so I went back to the slip and changed out the impeller and wear plate. Temp didn't change but by about 5 degrees F. There are NOT mechanical gauges so I DO worry about the accuracy of the electrical meters for the water temp. I want to install some mechanical gauges for oil pres and water temp in the engine room as a good back up.

Utimately I wanted to know what you guys are running on your Lehmans and how big your boats are. My general understanding is that this prop doesn't seem too large for similar applications elsewhere and other guys get 2,400 RPM at WOT. For some reason I cannot reach the RPM and I can't understand why. Maybe it is an injector issue although the mechanic said not.

I definitely want to contact Bob at American Marine and get his opinion. After I do, I will post what he had to say for everyone info.

Rich- See post 5. With phototach check gear ratio. Cheap and easy.

Photos

Ski in NC said:

Rich- See post 5. With phototach check gear ratio. Cheap and easy.

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The BW transmission is a 2:1 according to the serial tag (I am finding this is typical for this set-up on this boat). A phototach would be good. I will try to find one this week and give it a shot one evening this week and post the findings here.

You are right that I should confirm this, especially before committing to having any prop resizing done.

The reason I focused on this is that the reduction unit bolts on to the back of the basic BW tranny, which is 1:1. And reductions fail. So some mech finds a good reduction unit and bolts it on. Ratio could be who knows what on the replacement unit. The BW clutch part (fwd unit) still will have the 2:1 tag.

Lower gears may be more suitable than a prop for the 2-1 drive but if you don't need more power the prop change should be a good solution.

That 24x16 is a pretty flat wheel already. I'm kind of surprised he's missing 500-600turns from top.

What props and ratios are you other Lehman 120 guys running? Seems like a 3:1 and a near square wheel would be more appropriate. I have not really run the numbers, though.

I run a 4 blade 22x17 guess I'll have to check my upper rpm again but I thought it was OK last time...

Not sure what my tranny reduction is...

WRT to the temperature gauge. These may be faulty, if they are the AC Delco gauges. Bob Smith recommended switching these out. In other words, the temp problem may be unrelated. Best to get an engine temp directly, bypassing the helm.

Jim, Sent from my iPad using Trawler

Ski in NC said:

That 24x16 is a pretty flat wheel already. I'm kind of surprised he's missing 500-600turns from top.

What props and ratios are you other Lehman 120 guys running? Seems like a 3:1 and a near square wheel would be more appropriate. I have not really run the numbers, though.

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My 36' is pretty much identical to Rich F's boat,except we have twins Lehman's and an aft cabin model.

According to the IG owners manual for 36' boats the standard issue for Lehman 120's on IG's is a gear box of 2:1 with 3 blade 24" props with a 16" Pitch. This is what my boat runs.

My boat seems to top out at about 2500 rpm.

At this wide open speed, you are enveloped in clouds of blue smoke, with a prop wash that makes Niagara Falls seem like a milk pond. Unburnt diesel chokes the life out of your nearest and dearest, your lungs turn to cinder The engine sounds like 1,000 Wasps have been poured into the cylinders, the wife is thinking of her options, and crew are crying and wanting to go home. You are a pariah.

Whereas at 1600 rpm, the soft calls of the Whippet birds float down, the light softly shimmers through the branches of the water gums, the fine spicy Shiraz awaits. The wife lovingly glances in your direction, the children smile and laugh, and all is good with the world.You are the captain of all you survey.

The difference, one Knot.

Fantastic post Andy!

Andy G said:

..According to the IG owners manual for 36' boats the standard issue for Lehman 120's on IG's is a gear box of 2:1 with 3 blade 24" props with a 16" Pitch. This is what my boat runs...My boat seems to top out at about 2500 rpm...

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Great descriptive post Andy.
The Manual for my 1981 IG36 with twin Lehman 120s, lists the props as "Michigan Federal 3 blade 24" Diameter,17" Pitch ( note pitch variation).
At purchase survey, from memory, engines ran to around 2400 or 2500rpm. The survey mechanic was happy, it was very noisy,the wake near flipped the owners towed 10ft aluminum dinghy.

I am running a 20 x 16 4 blade thru a 2:1 tranny.
I get 2500 at WOT with just a trace of smoke, temp stays at just under 200F.

Hello RichF

If that gear box is a real 2:1, you should be spinning a 24 x 13.5 or 24 x 14. The best indication for my statement is that you are making too much speed at 1650 RPM. According with 3 different propeller calculators. your arrangement must spin a 3 blade 24 x 13.5

...My 2 cents!

Portuguese

A prop closer to square like but perhaps not as close as 22X20 may be preferable. There is an ideal relationship for power applied, blade area, diameter and pitch among other lesser things that should be taken into consideration. See your prop man or manufacturer. You can call Michigan. Many trawlers seem to stray into too much dia and too little pitch. Contrary to popular opinion diameter isn't everything.

Ski in NC said:

The reason I focused on this is that the reduction unit bolts on to the back of the basic BW tranny, which is 1:1. And reductions fail. So some mech finds a good reduction unit and bolts it on. Ratio could be who knows what on the replacement unit. The BW clutch part (fwd unit) still will have the 2:1 tag.

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Interesting, didn't know that. Based on the prop sizes from other members seen here, the engine should run up to 2,400 no problem with that size prop. I wonder about the reduction gear now....

manyboats said:

A prop closer to square like but perhaps not as close as 22X20 may be preferable. There is an ideal relationship for power applied, blade area, diameter and pitch among other lesser things that should be taken into consideration. See your prop man or manufacturer. You can call Michigan. Many trawlers seem to stray into too much dia and too little pitch. Contrary to popular opinion diameter isn't everything.

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You are right. This is another boat my family has, the "Portola". It has the original 600 RPM @ WOT Winton diesel with no gear box. She is 85 tons and cruises at 9 knots, check out the prop, insane pitch and hardly any diameter. The feathering prop on the right is just a come-home.

jleonard said:

I am running a 20 x 16 4 blade thru a 2:1 tranny.
I get 2500 at WOT with just a trace of smoke, temp stays at just under 200F.

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What is your speed at cruising and WOT?

Thanks Andy, enlightening and funny!

RichF,
Yes the drag (parasitic is the right word I think) of the water over the surface of the prop is power lost that could be used with pitch to push more water aft for more thrust.

In addition to having too much blade area you can have too much pitch. When the pitch it so great that water tumbling around the tips of the propeller blades becomes lost thrust too as too much water is thrust in a radial direction instead of aft.

My Michigan MP prop is very similar to that of Portola's but w less pitch. 18X14. Square (or there-a-bouts) I think is good for fast boats where there is less time for water at the blade tips to wash off the tips of the blades. For slow boats like most of ours less pitch and more area is best. But not much less. The rounded Elephant Ear style props like mine w the MP are scoffed at by many as not efficient but Portola's prop says otherwise. I think a skewed blade IS more efficient but only very slightly. I prefer the MP type blade and they have excellent reverse thrust. Something a heavy low powered boat can use.

This is an "MP" Michigan prop. NOT an MP-3. The MP-3 is a different prop that is not symmetrical ... that is .. skewed. And "3" has nothing to do w the number of blades.

Rich- You MUST post photos of that Winton engine. Drooling heavily...

Also, on your boat the hp required vs. speed curve probably takes a sharp bend upward at near full power (See Andy's fabulous post!!). This definitely does not fit the classic hull speed 2.7exp load curve the engine builders publish and use. That uptick at the end is not part of the hull speed curve. And there is a good argument to ignore that area when selecting prop, which also means don't run there!!

Do check the ratio, though.