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Old 09-08-2016, 09:21 AM   #1
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Prop

I have a new to me 34' CHB with a Lehman 120. I think it is under propped? The tach will run above 2600 no problem, I don't know how much over I didn't want to chance it!
It has a 23 13 LH bronze prop.
I have ordered a digital laser photo tach to check if the gauges are correct I'm sure they are not. I'm hoping they can be adjusted?
Either way from what I can find it seems as if I'm under propped, most of the info I can find suggests many a 23 18 or so?
Any ideas where I should start?
Thanks
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Old 09-08-2016, 09:52 AM   #2
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A good place to start if you haven't already been there, just answer the questions and it will give you an idea......
https://www.vicprop.com/displacement_size.phphttps://www.vicprop.com/displacement_size.php
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Old 09-08-2016, 10:00 AM   #3
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I'm a bit over propped and would like to be a bit under propped.
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Old 09-09-2016, 06:48 AM   #4
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Quote:
Originally Posted by Island15 View Post
I have a new to me 34' CHB with a Lehman 120. I think it is under propped? The tach will run above 2600 no problem, I don't know how much over I didn't want to chance it!
It has a 23 13 LH bronze prop.
I have ordered a digital laser photo tach to check if the gauges are correct I'm sure they are not. I'm hoping they can be adjusted?
Either way from what I can find it seems as if I'm under propped, most of the info I can find suggests many a 23 18 or so?
Any ideas where I should start?
Thanks
I have a GB32 (20,000 lbs with fuel and water) Factory prop was a 24" X 20". That gave a top RPM of 2,600 with a Lehman 135. How much does your boat weigh?
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Old 09-09-2016, 07:45 AM   #5
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Around 17,500 depending on fuel and water levels!
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Old 09-09-2016, 07:59 AM   #6
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What's the rated RPM on the Lehman? You should be able to hit 100 rpm or so OVER the rated RPM if you are propped correctly. That's with full tanks, full load, full everything.
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Old 09-09-2016, 08:25 AM   #7
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What's the rated RPM on the Lehman? You should be able to hit 100 rpm or so OVER the rated RPM if you are propped correctly. That's with full tanks, full load, full everything.
Two thumbs up
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Old 09-09-2016, 08:59 AM   #8
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Around 17,500 depending on fuel and water levels!
BoatDiesel's prop calculator gives the following suggestions for your boat (I used a gear ratio 2.45 to 1)

3 Blade 25" X 19" 4 blade 23.5" X 19"

So your 23" X 13" is definitely under propped. Tachs aren't always correct but most of the time they are in the ball park (+- 50 rpm) I tend to run around 1200 rpm so I don't worry about my tach being off a bit.

Good luck with getting the right prop!
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Old 09-09-2016, 09:21 AM   #9
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All of the folks here comparing props and rpm - do you all have the same transmission ratios?
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Old 09-09-2016, 12:36 PM   #10
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Quote:
Originally Posted by twistedtree View Post
What's the rated RPM on the Lehman? You should be able to hit 100 rpm or so OVER the rated RPM if you are propped correctly. That's with full tanks, full load, full everything.
2500 at WOT under load is rated
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Old 09-09-2016, 04:11 PM   #11
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13 inch pitch does sound way under propped but it also can depend on your transmission reduction ratio. The Lehman 120 is governed to 2600 no load, 2500 under load so even in neutral it shouldn't go over that. If you're seeing higher than 2600, the first thing to do is what you're doing to verify the tach. If you truly are getting more than 2600 rpms you may have an injector pump problem. Some of the older 120's had a mechanical tach. The newer ones used pulses from the alternator. Hopefully it's either a tach adjustment or somebody changed the alternator and it has a smaller pulley than standard.

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Old 09-10-2016, 08:24 AM   #12
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For folks that like to play with numbers, A Rule of thumb,

At 1000 shaft RPM an inch of pitch is one Knot of fwd travel with NO PROP SLIP.

If you have a good tach and a gps lots of rainy day fun can be had working the props actual slip in different conditions.
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Old 09-10-2016, 09:03 AM   #13
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FF with that rainy day philosophy you'd never do anything in SE Alaska.

Interesting though. I've found a 1" change in pitch gives a 200rpm change. 50% slip I guess.
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Old 09-10-2016, 09:19 AM   #14
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The transmission is a Paragon P 33 AL 2 to 1 gear ratio I believe?
I received the digital tach yesterday hope to get some numbers today.
Thanks for the help!
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Old 09-10-2016, 05:30 PM   #15
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If you do need to adjust the prop then you will NEED to know the gear ratio. No guesses.

There is nothing wrong with being g under propped. Of course how much. But 100 revs would be good as that will allow for some bottom fouling or extra weight so you do not go into over propped.

To go. CRUiSE PROP setting be careful. If that is done then you cannot ask the engine for full power. If you go over prop get an EGT guage as it will tell you fast when you are asking g too much of the engine.
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Old 09-10-2016, 06:18 PM   #16
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According to my survey I have a 22x16 LH prop with a spare of the same size. If I recall during sea trials at WOT it was about 2450 or so. I run at about 1750-1800 for about 7 Knots usually.
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Old 09-12-2016, 06:49 AM   #17
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New Prop figures

Quote:
Originally Posted by Island15 View Post
The transmission is a Paragon P 33 AL 2 to 1 gear ratio I believe?
I received the digital tach yesterday hope to get some numbers today.
Thanks for the help!
I re-ran your prop calc on Boatdiesel using 2:1 ratio and it dropped the pitch (originally I used 2.45:1) the new suggested props are;

3 blade no cupping 22 X 15.5
4 blade no cupping 20 X 15.3

Let us know how accurate your tach was and if you can what make tach you have.
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Old 09-14-2016, 06:22 AM   #18
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"To go. CRUiSE PROP setting be careful. If that is done then you cannot ask the engine for full power."

You can go to full power for that RPM , you can not go to full throttle and hope for rated power..

Many boats cruise on 2-3 GPH, having that minor power available at modest 1300-1600 RPM does wonders for the engine life and noise aboard.
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Old 09-14-2016, 11:19 AM   #19
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Thumbs up on the vicprop calculator per rochepoint. It came up with the same prop choices that my local prop shop came up with. Just make sure the numbers you use are accurate, remember, "garbage in, garbage out".

Dan
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