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Old 13-06-2018, 07:01   #1
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Which tach is correct?

My single Perkins has 2 tachometers, one on the bridge and another at the pilot station in the salon. There is about 200 RPM difference in the readings. How can I determine which (if either) is correct?
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Old 13-06-2018, 07:06   #2
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Re: Which tach is correct?

Laser tachometers are $12 on EBay.
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Old 13-06-2018, 07:24   #3
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Re: Which tach is correct?

One of these.

They’re not the best piece of gear, but do what they’re supposed to do.

https://www.google.com/search?q=phot...e1SBV4QgTYIpQQ
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Old 13-06-2018, 07:56   #4
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Re: Which tach is correct?

If you have one, hold a fluorescent trouble light in front of the engine. In the USA it will stop the pulley at 1800 rpm with 60 cycle power..
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Old 13-06-2018, 08:26   #5
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Re: Which tach is correct?

Quote:
Originally Posted by Ecos View Post
If you have one, hold a fluorescent trouble light in front of the engine. In the USA it will stop the pulley at 1800 rpm with 60 cycle power..
Cool idea! Maybe put a piece of tape on the pulley so it is easier to tell if it has stopped?
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Old 13-06-2018, 08:31   #6
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Re: Which tach is correct?

Since a lot of tachs are adjustable, there's a good chance that *neither* of them is accurate. There should be an adjustment screw on the back.

I have one of these. Works well but take the batteries out of it when you're not using it.\\https://www.amazon.com/Digital-Photo...70_&dpSrc=srch
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Old 13-06-2018, 09:39   #7
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Re: Which tach is correct?

TinyTach. https://www.tinytach.com/
Connects to an injector line plus 12v +-. Additionallly it is an hourmeter. 1 boat buck.
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Old 13-06-2018, 09:45   #8
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Re: Which tach is correct?

You're completely overthinking this. Whether your WOT is 3000 or 3600, it just doesn't matter. If it does to your"senses" then adjust the tach, one of them, to read equally with the other. Issue will be that they will be out of synch differently at different rpms.
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Old 13-06-2018, 10:23   #9
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Re: Which tach is correct?

Actually it does matter. If you don't know the actual RPM's (at a fully clean bottom) and a fairly light load for the boat then you have know way of knowing if the boat/engine is propped right or if the are performance hits from bottom issues, fuel filter issues, etc.
While it may not be critical for the day to day worries is is good info to have to keep an eye on your boat.
If your engine specs call for the engine to top out at 3000 RPM and you constantly have it loaded and propped such that it only develops 2500 RPM then that can affect engine longevity and fuel usage in the meantime.

While it doesn't need to be +- 10 RPM the closer you can get to a "real" value it is better.

Like everything else on a boat - some will say "good enough" at different levels. We all have to decide how much "slop factor" we are comfortable with. But yes our ears can be a valuable tool and actually the one we often use the most - but even those need calibration from time to time.
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Old 13-06-2018, 17:08   #10
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Re: Which tach is correct?

Quote:
Originally Posted by nichollsjr View Post
My single Perkins has 2 tachometers, one on the bridge and another at the pilot station in the salon. There is about 200 RPM difference in the readings. How can I determine which (if either) is correct?

Squander, consider an alternate approach because one would expect on a known-to-him boat he would have gone through all that performance/prop/clean hull/STW stuff a long time ago. I agree completely IF this is all new.


But even so, an option is to simply KNOW that it's thre tachs that are out of whack, not what they are measuring. He knows inside is 200 rpm either lower or higher than the outside one.


Is it worth the return on invest to narrow the gap? You're right, good question.


He can only be looking at one at a time???



Your boat, your choice.


Thanks for the detailed post, fun read, well written.
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Old 13-06-2018, 17:23   #11
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Re: Which tach is correct?

Quote:
Originally Posted by nichollsjr View Post
My single Perkins has 2 tachometers, one on the bridge and another at the pilot station in the salon. There is about 200 RPM difference in the readings. How can I determine which (if either) is correct?
A good first post!

As others suggested, get a handheld laser tach or use the Ecos method for 1800 rpm if you have a 60Hz mains supply (or 1500RPM for a 50Hz mains).

Is the error constant (i.e. at idle and WOT). If so, then use you ear to "guess" which one is correct (when at idle).
If the error varies, then when are they closest is probably the correct RPM.
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Old 13-06-2018, 19:10   #12
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Re: Which tach is correct?

look up the engine maximum rpm assuming(oops we don't assume things) that's the governed rpm / make sure everything is safe / warm up your motor , bring your engine up to maximum revs(hold her flat) / compare both tacho's / you may now have 3 measurements or 1 might agree with manufacturers specs / other than that have mechanic fit a portable tacho to check your rpm and compare with your fitted products / a lot of boats cover up the tacho to lessen stress on helmsmen / the governors work well on diesels /
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