I/O Tach

[Tahoe 55]

Why are boat tachs supposedly so inaccurate???
Why are they any different than an automotive tach?

 

[QC]

Re: I/O Tach

Who says they are inaccurate? If they are setup wrong they can be and they often are . . . Simple switch in the back for 4, 6, 8 cyl and some OB settings too. Just has to be set right and it should be dead on.

 

[Bondo]

Re: I/O Tach

Why are boat tachs supposedly so inaccurate???

Ayuh,....

Are you confusing Tachs with pitot powered Speedometers,..??

The pitot powered Speedometers are so notoriously inaccurate that they're known as Wishometers,.......

 

[JustJason]

Re: I/O Tach

hehe.... wishometers.... LOVE IT!!!

Seriously though....
Take a new tach, and known good wiring... and that tach should be dead on to within a couple of percent.

So where did you hear "supposedly so innaccurate" from Tahoe?

 

[Tahoe 55]

Re: I/O Tach

I got that idea from reading posts on here....
The "wishometers" part makes sense, but I've read more than a few posts that recommend using a "shop tach" when taking RPM readings for prop selection(and that's what I'm really interested in doing here).... Guess that's over-kill??

 

[JustJason]

Re: I/O Tach

mechanics use shop tachs because they are using "known good" tachs for customers boats.
if, as a mechanic, you used the boats tach to set things up/troubleshoot, you'd be wrong half the time.

 

[Tahoe 55]

Re: I/O Tach

That makes sense.
Thanks.

 

[daydreamer1252]

Re: I/O Tach

mechanics use shop tachs because they are using "known good" tachs for customers boats.
if, as a mechanic, you used the boats tach to set things up/troubleshoot, you'd be wrong half the time.

But doesn't that go right along with the "supposedly so innaccurate" statement if using the tach would be wrong half the time? That doesn't make me have a lot of confidence in their accuracy............

I am not in disagreement about your figures only that I would typify something that is wrong half the time as not dependably accurate.

As to why. Pounding and vibration that your vehicle (short of a vehicle running the Baja) will not see. Add to that the moisture always present and you have a recipe for something going "off" of any calibration it may have had.

My thoughts.

 

[QC]

Re: I/O Tach

I am probably part of this confusion as I often mention the tach when people are trying to determine prop selection etc. The reason being is that the tach can be wrong if they are set wrong or the tach is just plain f'ed up. When they are right, they are absolutely right, and the only thing that makes them less dependable than automotive tachs is that they are generic and can be used for a variety of engines and boats, and they can be adjusted. Not percentage adjusted, but "switched" for different applications.

Sooooooo . . . if you are trying to make decisions on engine repairs, prop selection, performance stuff etc. etc. Then you have to confirm all your numbers are right: prop pitch, GPS speed, gear ratio, and . . . RPM. If those numbers are all known to be correct then the data can be used for making decisions. If even one of those numbers is wrong then they might as well all be wrong . . .

 

[Tahoe 55]

Re: I/O Tach

Delete