Hand Held Laser tachometer

bigvic

Cadet
Joined
Aug 10, 2010
Messages
11
My question is where would you point the laser to get your RPM's, i have never used one so i want to make sure it would work on my outboard motor before buy it. Thanks
 

tmcalavy

Rear Admiral
Joined
Aug 29, 2001
Messages
4,005
Re: Hand Held Laser tachometer

Probably the top flat part of your flywheel.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: Hand Held Laser tachometer

Revolving is what the crankshaft is doing and is what you are interested in determining. The only thing you can see rotating on an outboard engine is the flywheel which just happens to be attached to the crankshaft. When the crankshaft goes around once so does the flywheel.
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: Hand Held Laser tachometer

You put a piece of reflective tape on what you want to count. The tachometer counts the times it sees the tape. It could be on a dog's wagging tail and it would work.
 

5150abf

Vice Admiral
Joined
Aug 12, 2007
Messages
5,808
Re: Hand Held Laser tachometer

I agree, the farhter from the center you get the more rpm you will get at the same speed.

A guy was working on mine and had a handheld tach and put it right on the nut.
 

imported_Skippy

Seaman Apprentice
Joined
Nov 23, 2006
Messages
40
Re: Hand Held Laser tachometer

Comment: purged for not agreeing with the elders
 
Last edited:

eavega

Lieutenant
Joined
Apr 29, 2008
Messages
1,377
Re: Hand Held Laser tachometer

I agree, the farhter from the center you get the more rpm you will get at the same speed.

Um, no. As previously mentioned, any point on the flywheel turns once for every revolution of the shaft.

-E
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Hand Held Laser tachometer

This law of physics is why cars have differentials, snowblowers have friction plates to create multiple speeds, and race engine builders like smaller main bearings to lower the bearing speed.
Please reread this as the answer is right there. Yes the speed of the surface vs. the bearing goes up but changing the size of the crankshaft journal does not change RPM . . . If it did then we would have mega crank main journals and our cars would magically go faster. The laws of physics simply prevent this, they do not support his . . . ;)

RPM is not speed so distance from center means NOTHING . . . You could tie a flippin' string out 100 ft. put the flippin' reflective tape at the end, and you would still get the same RPM as you would by placing the tape on the nut as long as it was the side of the nut. Any point on the flywheel, other than dead center and looking from the top, will read the same RPM assuming the "laser" can see the reflective tape . . .
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: Hand Held Laser tachometer

UMMMM NO!!!! RPM is revolutions per minute -- not feet per minute. Regardless where you put that reflective tape, it will only rotate past a given point once per revolution of the flywheel. Yes - the "rate" of travel will be greater the farther you get from the hub but RPM is constant. Big difference. So if you are measuring speed (as in distance traveled) then you have the wrong instrument. If it is indeed a tach, then it makes no difference where the tape is.
 

imported_Skippy

Seaman Apprentice
Joined
Nov 23, 2006
Messages
40
Re: Hand Held Laser tachometer

As a side note: On the use of a "reflector" for the laser tach to aim at (ie: the target).

Since you often have to stick the reflector tape (any common reflective white or red tape works equally well) and the tape doesn't always like to spin around fast and 'stick' to greasy engine parts ... well, I have had good luck using a dab (or thin line) of silver paint to mark a "spot" to aim the laser tach at; if the flywheel is darker and duller in comparative "shine", and your work area is not sunshine bright, the silver paint can often be 'counted' by the laser tach A-OK.
{I have even had some luck with typewriter "white-out" as a marker for the laser target, especially when I added a few glass beads from my bead-blaster to the wet paint!}

In short, the laser tach likes a defined *reflective* spot/line it can clearly (discreetly) count. BUT it can even pick up a bright shiny piece of anything nearby rotating ...so watch your target for false readings ... and hold the laser beam *safely* close to the rotating part to avoid beam scatter, while avoiding getting yourself grabbed/hit by moving engine parts. (my laser tach likes 6"-12" from target for good readings)

Finally, my cheap Chinese Laser Tach eats the life from batteries, and will give lousy readings when the batteries are running low. I keep the batteries fresher by removing them after use. Hope your new laser tach is accurate, and doesn't eat batteries too!
 
Top