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Old 04-14-2014, 12:43 PM   #41
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Rich- See post 5. With phototach check gear ratio. Cheap and easy.
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Old 04-14-2014, 12:59 PM   #42
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Old 04-14-2014, 01:16 PM   #43
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Rich- See post 5. With phototach check gear ratio. Cheap and easy.
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.
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Old 04-14-2014, 01:24 PM   #44
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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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Old 04-14-2014, 01:26 PM   #45
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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.
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Old 04-14-2014, 01:30 PM   #46
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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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Old 04-14-2014, 02:08 PM   #47
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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...
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Old 04-14-2014, 04:31 PM   #48
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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.

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Old 04-14-2014, 07:45 PM   #49
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Originally Posted by Ski in NC View Post
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.
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.
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Old 04-14-2014, 08:45 PM   #50
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Fantastic post Andy!
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Old 04-14-2014, 09:34 PM   #51
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Quote:
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..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...
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.
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Old 04-15-2014, 07:53 AM   #52
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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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Old 04-15-2014, 08:17 AM   #53
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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!

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Old 04-15-2014, 11:08 AM   #54
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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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Old 04-16-2014, 12:34 PM   #55
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Originally Posted by Ski in NC View Post
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.
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....
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Old 04-16-2014, 12:44 PM   #56
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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.
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.

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Old 04-16-2014, 12:46 PM   #57
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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.
What is your speed at cruising and WOT?
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Old 04-16-2014, 12:53 PM   #58
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Thanks Andy, enlightening and funny!
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Old 04-16-2014, 01:24 PM   #59
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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.
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Old 04-16-2014, 02:12 PM   #60
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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.
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