Correct pitch

I have a twin engine 1979 Marine Trader, I have Hung Shen props 24 inch dia. The port prop has two damaged blades due to a run-in with a lobster trap and needs to be replaced or repaired. Does anyone know where to find the ID numbers on the prop, or what the pitch on these props were. These are the original props. The engines are 120 Lehmans

It may take some cleaning to find it but it should be stamped on the hub. L24x??

The diameter and pitch should be stamped on the hub of the props. My 40' Ocean has 24X18 4-blade props driven by 120 Lehmans and probably has a very similar hull and weight.

Some boats did'nt come w the right pitch on their props. You do'nt need to know what the original pitch was. Just pitch both props for 2500rpm at WOT in gear ....or slightly less ..or even slightly more. I assume your Lehman's are rated at 2500 rpm. Marin has twin Lehman's and recently re-pitched. If you have the same gear ratio .................

captherb said:

Does anyone know where to find the ID numbers on the prop, or what the pitch on these props were. These are the original props. The engines are 120 Lehmans

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As a single data point for you, our 1973 GB36 with twin FL120s was factory-fitted with 24" props with a left hand, 17" pitch prop on the port shaft and a right hand, 18" pitch prop on the starboard shaft. The 1" pitch difference was to compensate for the difference in transmission output ratio due to the counter-rotation gear in the starboard transmission. These were three-bladed props.

The boat is currently fitted with four-bladed props of the same diameter and pitches but a few years ago we had them completely reworked so they are now 23" in diameter and the pitch of both of them is 16." The prop shop we use said that four-bladed props should normally be pitched 1" less than three-bladed props of the same diameter and power input.

Thanks for all the input. I cleaned the damage prop down to bare metal and the only number I could find on it was 7130 on the hub facing outboard. Anyone know what this refers to

No idea on the numbers, but if you take the prop to a propeller repair shop, they can identify the pitch, diameter, repair and balance your wheel for you. They will stamp the correct info on the hub and you will be good to go until the next time.
Larry B

GB 36 120 Lehman..correct prop???

New owner, old boater....
American Marine says 2250 RPM should be available at full power, cruise at 1800.
Max I can get is 2075??? Mech tachs are not that good but no way 2250.
Engine in good shape, bottom slick..
Anyone know or have tried different props???
Looking for advice on where to start..
As an "experienced" boat nut...I find the info available on these sites is awesome!
Would appreciate any help-idea's...
Thanks

Cap - ... read Edelweiss

Gun - My 120 Lehman turns a 4 blade 23 x 18 to rated rpm ... to tweak props to rpm the formula is usually 1 inch pitch = 300 rpm / 1 inch diameter = 150 rpm . Cupping can help some if you dont have room for more diameter.

gunfighter said:

New owner, old boater....
American Marine says 2250 RPM should be available at full power, cruise at 1800.
Max I can get is 2075??? Mech tachs are not that good but no way 2250.
Engine in good shape, bottom slick..
Anyone know or have tried different props???
Looking for advice on where to start..
As an "experienced" boat nut...I find the info available on these sites is awesome!
Would appreciate any help-idea's...
Thanks

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Generally, if the max RPM per the engine rating is within 10% of the measured RPMs you can get, the prop pitch is acceptable. Ideally they match. In reality, they frequently do not.

captherb said:

I have a twin engine 1979 Marine Trader, I have Hung Shen props 24 inch dia. The port prop has two damaged blades due to a run-in with a lobster trap and needs to be replaced or repaired. Does anyone know where to find the ID numbers on the prop, or what the pitch on these props were. These are the original props. The engines are 120 Lehmans

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Consider taking both props to a good prop shop. If you've had good performance in the past, they can match the damaged one to the undamaged one. They can also clean and "tune" the undamaged prop.

If need be, they can change both for better performance on your boat.

gunfighter said:

New owner, old boater....
American Marine says 2250 RPM should be available at full power, cruise at 1800.
Max I can get is 2075??? Mech tachs are not that good but no way 2250.
Engine in good shape, bottom slick..
Anyone know or have tried different props???
Looking for advice on where to start..
As an "experienced" boat nut...I find the info available on these sites is awesome!
Would appreciate any help-idea's...
Thanks

Click to expand...

One idea would be to check the back pressure on the exhaust.
Look at the easy things before changing the prop.
Excess back pressure can reduce RPM'S

PassageMaker did an article on just this thing a few years back.

SD

gunfighter said:

Looking for advice on where to start..

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An accurate tachometer is the only starting point.

Buy, rent, or borrow and optical tachometer and find out exactly what rpm the engine is turning under load and at high idle.

what rpm can you get out of gear? My 120 would only turn 2150 in netural

motion30 said:

what rpm can you get out of gear? My 120 would only turn 2150 in netural

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I don't believe you're supposed to run an engine at WOT without a load on it. I never would.

motion30 said:

what rpm can you get out of gear? My 120 would only turn 2150 in netural

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There is a max governed speed under load and not under load.

Different models have different numbers so I can't help there.. you may have to call someone like American Diesel with your serial or arrangement number for that number.

Usually no load max is 100-200 rpm over loaded.

Running a diesel at max rpm NOT in gear is NO big deal...it is usually only for short periods of time for maintenance and without doing it...you may never be able to keep it properly running.

RickB is correct that without an accurate tach...everything is a guess unless you are looking for a ballpark and a direction to head...but sometimes that can be a wild goose chase if you start with info that is too far off.

rwidman said:

I don't believe you're supposed to run an engine at WOT without a load on it. I never would.

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What, you've never run a generator?

RickB said:

What, you've never run a generator?

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No, I have not. But, doesn't a generator have a governor to maintain the 60 HZ frequency?

rwidman said:

No, I have not. But, doesn't a generator have a governor to maintain the 60 HZ frequency?

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The governor on a diesel powered generator is set to maintain the rpm required to produce the desired frequency. The amount of fuel required depends on load.

The "throttle" lever on a propulsion diesel sets a governor to maintain whatever rpm you want within the fixed upper and lower limits of rpm and fuel quantity. Don't equate the "throttle" on a diesel with the gas pedal on a gasoline powered car.

You are not directly adding fuel when you move the "throttle" forward, you are simply changing the tension on a spring inside the governor to establish a new speed setpoint.

Diesels are speed/rpm governed through the injection pump.So they are very hard to over rev.Gasoline engines are not usually rev limited,especially carb engines.Some newer designs have rev limiters.Those are a good thing in boats that have a habit of popping the props out of the water in rough weather.There still should be no issue running a gas engine at full rpm unloaded.

Most diesel experts suggest running at WOT every 4 to 6 hours of continues operation.
5 or 10 minutes should do the trick.
Or so I have been told.

SD

ben2go said:

Diesels are speed/rpm governed through the injection pump.So they are very hard to over rev..

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Give yours a snoot full of lube oil and let us know what happens.

Captherb,
120 Lehmans should run 2500rpm at WOT in gear.
Anything else is under propping or over propping.
One usually uses pitch, diameter and trimming blade area to achieve rated rpm in gear.
I recently mounted a new prop and am down 100rpm. So I'm 100 rpm over propped. The boat has less power, runs to fast at idle and the engine seems to not run as freely as it did before.
I think American Marine is telling people what they think they want to hear. 2250rpm is well over propped and telling people to "cruise" at 1800 is excluding many very important variables. From all I've heard about AM I think one should seek advice elsewhere. Boat Diesel.com is a good place to look but of course there will be opinions there too. I've said for some time here to seek technical advice from engineers and manufacturers and seek service from mechanics.

"If need be, they can change both for better performance on your boat."

In every case "BETTER " will need to be defined.

Higher top speed?

More bollard pull?

Lower fuel burn at cruise?

Longer engine life?

Lower noise level?

Less shaft noise and vibration?

Are all beginning consideration for the eventual compromise..

As I said....The info here is great!
Optical tach??? Will research..
Exhaust clear..but is long on a 36 GB,,
Dia-Pitch vs rpm great info..
I am burning 2.85GPH at 1950 GPS sez 8-8.3 KTS still water. Very happy with that.
Will ck prop size on next haul in fall. Till then happy-just trying to sqeeze a little more ground speed.. As an old (very) F8Crusader driver it;s hard to get used to the difference in speed!Ha Ha...

Harbor Freight for the optical tach. Until you have calibrated your tachs you are just guessing.