Checking/varifying tachometer readings

[tmh]

I'm about to do a bunch of prop testing on my 10 year old boat (new to me) but I want to make wure I'm not wasting time. As WOT RPMs is a big part of the prop decision, what is my easiest/cheapest way to varify my tachometer readings are accurate?

Do I borrow some gadget to do this or hire a mechanic to do it? Is it best done in the water or can it be done on the trailer with a hose hook-up? About how much should I pay to have it done and how long might it take?

Thanks.

 

[tiller7104]

Re: Checking/varifying tachometer readings

1. Do not do it on the hose, you have no back pressure and your readings will be a few hundred RPMs off.
2. I use an inexpensive dwell tach meter....it is not exactly but for marine purpose and I end up duck taping it onto the sea well, my wife yells the reading up to me but it gets you real close. These other guys may have a better idea but that is how I did mine. (Little redneck but hey)

As for how much it would cost or how long it would take? I would invest in a dwell tach meter and just do it your self, owning a boat is a constant learning experience and it will save you a bunch of loot!

 

[WriterCJ]

Re: Checking/varifying tachometer readings

I'm currently playing with an iPhone app called Engine RPM that delivers RPM readings by analysing an engine's sound frequency spectrum.

I have a little way to go before I trust it completely, but initial results are very promising. You have to read the instructions (PDF download) to understand how it works and to set it up properly, but once it locks on to the sound signature of your engine the reading seems very accurate. I've tested it on a couple of cars and the readings match the tach reading amazingly well.

In fact I reckon the minor variances could reflect inaccuracies in the vehicle tachometers rather than errors in the software's calculated RPM.

You can also download a free version for the PC -- although it looks less user friendly than the iPhone app.

I got it as an interesting experiment prior to buying either an optical/laser tach or and inductive-pickup based tach that I think work by measuring electrical pulses in the HT leads and calculating RPM from that.

You can also get multimeters with built in RPM function and inductive pickup attachments. Never used one, but may be worth a shot -- and could be used for testing lots of other aspects of your boat engine/instruments/electrics too.

 

[EricJRW]

Re: Checking/varifying tachometer readings

Good post calvinj!

I've been wondering the same thing as tmh, and knew of the optical and inductive options, but can't decide which is right for me (and least expensive). The software option certainly is the right price. I just may have to experiment with it (PC version, no iPhone for me).

 

[WriterCJ]

Re: Checking/varifying tachometer readings

I was sceptical, but playing around with it I'm fairly convinced it works. I've been doing a bit of reading around the subject too, and it seems it's a pretty well documented technique.

Apparently before they restricted RPM on Formula 1 racing cars teams used a similar technique to analyse competitors engine performance from the pit lane. Amazing what you can learn on the internet .