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Old 04-22-2013, 12:41 PM   #1
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Checking engine RPM

I need to get new props but before I do I need to get accurate rpm and speed data. I believe my tach's are somewhat accurate but what is the cheapest way to get a good accurate engine rpm reading? I don't want anything permanent just something I can use once.
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Old 04-22-2013, 12:50 PM   #2
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I bought one of these:

http://www.amazon.com/Neiko-20713A-P.../dp/B000I5LDVC

Works well the couple times Ive used it. Not well made for heavy use, but ok to calibrate the tachs.
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Old 04-22-2013, 01:24 PM   #3
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Originally Posted by timjet View Post
I need to get new props but before I do I need to get accurate rpm and speed data. I believe my tach's are somewhat accurate but what is the cheapest way to get a good accurate engine rpm reading? I don't want anything permanent just something I can use once.
Never thought of that. That's a good idea. Thanks.
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Old 04-22-2013, 02:55 PM   #4
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Originally Posted by Ebaugh View Post
I bought one of these:

http://www.amazon.com/Neiko-20713A-P.../dp/B000I5LDVC

Works well the couple times Ive used it. Not well made for heavy use, but ok to calibrate the tachs.
I have the identical same item. Money well spent. Not the sort of instrument you will use every day, so remove battery after use.
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Old 04-22-2013, 03:15 PM   #5
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When we thought we needed to get new props I asked the prop shop what kind of information they would need to determine the best props for our boat. They said they wanted to know:

The type of engine(s) we had. (FL120s)
The horsepower rating of the engine(s) (120 @ 2500 rpm)
The gear ratio of the marine gear attached to each engine (1.91:1 on one, 2.1:1 on the other).
The rpm of the engine(s) in gear at full throttle (2200 on one engine, 2300 on the other)
The diameter and pitch of the current props (24"x 18" on one, 24" x 17" on the other)

Actually, they wanted us to bring the props in so they could see and check them for themselves.

I asked how accurate the rpm at WOT had to be since the boat's original SW tachs are definitely in the ballpark but are not dead-on accurate. They said as long as the tachs were "close" that would be sufficient.

Not long after we got the boat we had the diesel shop check the actual rpm of each engine against the tachs. They used a "gun" that shot a light (I assume) at a piece of reflective tape they put on the drive pulley on the front of each engine. I have no idea what the device is called or the brand or what they cost but it seemed a simple enough thing.

But I suspect that if your tachs are reasonably accurate, that will do for the prop shop just fine and there is no need to get an exact figure.

(Turns out we didn't need new props, the prop shop said they could rework the ones we had just fine. So now both props are 23" x 16".)
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Old 04-22-2013, 03:39 PM   #6
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I have the identical same item. Money well spent. Not the sort of instrument you will use every day, so remove battery after use.
Me 3. Used it to cal tachs and get performance data.
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Old 04-22-2013, 04:11 PM   #7
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I use a lasar tach to check the engines every time I move the throttle. I need to sink the engines and the difference in 50rpm over a 14 hour day in fuel burn is huge.
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Old 04-22-2013, 05:39 PM   #8
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The hand held tachs are generically called photo tachs. They are very accurate because they use a crystal based digital reference to measure speed at the flywheel with a piece of tape that they detect light pulses that is bounced of if it.

It probably isn't critical to set the wot rpms of the old Lehmans and Perkins that many older trawlers use. But it is very important to get the maximum wot rpm right for modern, high output diesels.

The Faria tachs on my 2006 Mainship 34T with a Yanmar 370 engine were off 200 rpm. What looked like 3,600 rpm was really 3,400 rpm. 3,400 rpm was fine as the engine is rated to produce 370 hp at 3,300 rpm, but it might not have been ok and I could have overloaded the engine with too heavy a prop if I had relied on the Faria tachs and reworked the props to reach an erroneous 3,400 rpm.

So check your wot rpm with a photo tach, especially on high output diesels.

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Old 04-23-2013, 07:07 AM   #9
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So check your wot rpm with a photo tach, especially on high output diesels.

David
My engines are high output diesels and from the research I've done the tachs need to be accurate if I want properly pitched props. The engines have been overloaded for years so I've got to change this. Accurate tachs is the first step.

I'm going to check my local NAPA dealer and see if they have something similar to Neiko 20713A Professional Digital Laser Photo Non-Contact Tachometer.

Now I have to get rid of two pair of props that I can't use. 23x26 and 23x27, any ideas?
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Old 04-23-2013, 07:11 AM   #10
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Old 04-23-2013, 09:20 AM   #11
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Bought mine at Harbor Freight. Looked at a device called, I believe, Tiny Tach a number of years ago. Uses a pressure sensor clamped to an injection line to determine RPM. Seems like it was relatively cheap at the time.

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Old 04-24-2013, 12:41 AM   #12
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Get the digital laser pointer phototach. It's usefull in more applications once you have it than just checking this engine.

I bought one 7-8 years ago and it has helped a lot, along with a few fellows on the dock.
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