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Old 04-05-2012, 09:47 PM   #1
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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
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Old 04-05-2012, 10:19 PM   #2
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It may take some cleaning to find it but it should be stamped on the hub. L24x??
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Old 04-05-2012, 10:22 PM   #3
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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.
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Old 04-05-2012, 11:25 PM   #4
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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 .................
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Old 04-06-2012, 12:41 AM   #5
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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
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.
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Old 04-06-2012, 02:56 PM   #6
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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
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Old 04-06-2012, 09:49 PM   #7
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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.
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Old 06-25-2012, 07:45 PM   #8
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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
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Old 06-25-2012, 10:21 PM   #9
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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.
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Old 06-26-2012, 12:42 AM   #10
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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
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.
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Old 06-26-2012, 07:12 AM   #11
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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
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.
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Old 06-26-2012, 04:09 PM   #12
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Originally Posted by gunfighter View Post
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
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
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Old 06-26-2012, 04:41 PM   #13
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Looking for advice on where to start..
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.
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Old 06-26-2012, 06:06 PM   #14
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what rpm can you get out of gear? My 120 would only turn 2150 in netural
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Old 06-27-2012, 06:37 AM   #15
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what rpm can you get out of gear? My 120 would only turn 2150 in netural
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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Old 06-27-2012, 06:55 AM   #16
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what rpm can you get out of gear? My 120 would only turn 2150 in netural
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.
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Old 06-27-2012, 07:20 AM   #17
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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.
What, you've never run a generator?
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Old 06-27-2012, 09:15 AM   #18
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What, you've never run a generator?
No, I have not. But, doesn't a generator have a governor to maintain the 60 HZ frequency?
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Old 06-27-2012, 09:32 AM   #19
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No, I have not. But, doesn't a generator have a governor to maintain the 60 HZ frequency?

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.
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Old 06-27-2012, 02:55 PM   #20
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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.
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