anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

[fisherguy123]

My 1982 merc 50 doesn`t have a tach and I dont Know if the prop pitch is correct....seems like it is underpropped a bit. My friend is an auto tech and he says I should be able to check the RPM at WOT with his hand held DIGITAL PHOTO TACH. The thing is ,I have to run it full out without the engine cowling . I have a few questions for anyone who has used one : How close to the flywheel must the instrument be held?.......Will it work in sunlight ?
Any tips would be greatly appreciated as this will be a first for me .
thanks

 

[eavega]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

Ive used an optical tach (I am assuming this is the same thing) to verify RPM on my engine as reported by my dash tach. I have found that in broad daylight, its not terribly reliable, but in even half light its fine. I had a piece of reflective tape on the flywheel, and had to hold the unit about 6-8 inches away to get consistent readings.
I've found that its near impossible on a moving boat going WOT unless the water is glass-smooth. You have to hold it on target for at least 5 seconds to get an accurate reading.

Rgds

 

[fisherguy123]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

yes Eric .....this is an optical tach...i just used the exact name from the instruction booklet...my friend used it on auto engines in a garage where there is no sunlight and the vehicle is stationary.....hence my concern about using it on a boat outside at 30 mph. I will try it first in the driveway with watermuffs at idle to see how close it must be then on an overcast evening when the bay is flat calm...oh yes with an experienced driver of course so i can concentrate on doing the reading !
thanx

 

[steelespike]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

Perhaps you could rest it or your hand on the motor to steady the unit.
Probably need a cloudy day or dawn or dusk.Likely to be calmer at dawn or dusk anyway.
Needless to say keep hair and clothes under control and avoid the flywheel.Let us know how you make out.

 

[fisherguy123]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

I painted the flywheel black and stuck the tape on then tried the tach in the driveway at idle . The tach picked the signal up to about 12 inches away very easily, so that gives me a bit of comfort knowing that I dont have to be too close to the flywheel at full speed. Should be able to try it within next couple of days. By the way my idle was 1160 to 1165 rpm in neutral......anybody know the specs for a 1980..merc 50 hp ?

 

[steelespike]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

Can't speak directly to the Merc but 2 stroke outboards are typically set around 700 rpm in gear. There may be a single idle speed adjust turn it down to about 750 in gear
it should be strong and steady if its unsteady and weak your probably trying to set it too slow.

 

[eavega]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

Can't speak directly to the Merc but 2 stroke outboards are typically set around 700 rpm in gear.

That would be in gear in the water, right? On muffs with no backpressure on the prop, you should be getting 1000 (I think)

Rgds

 

[steelespike]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

That would be on the water.Not sure about on muffs.Sounds about right.

 

[fisherguy123]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

well I tried the optical tach on the water and my motor runs at 90,000 RPM....LMAO......obviously a waste of time and effort. I think the tach picks up reflections of the water or such......I even put it up to within 5 inches of the flywheel mark and readings were all over the map.....as they say live and learn. In conclusion I did find something out about the prop I am using though.....I had 4 adults in the boat and another on the tube and had no problem getting on plane and towing with the 11" pitch prop on it. I use the boat mostly for scooting around with only 2 or 3 people and no tube ,so I think a 12" pitch should be right for general usage.

 

[Bob_VT]

Re: anybody ever used a DIGITAL PHOTO TACH to measure RPM at WOT?

Why not try a tiny tach? http://www.tinytach.com/tinytach/index.php