Are TinyTachs accurate?

[LORDY611]

I'm only getting 4450 RPM at WOT on my 16' Lund, 40hp Merc while doing 35mph on GPS. Doesnt seem like there is another 500-1000 RPM left, but according to Mercury I should top out at 5000-5500.
Has anyone come to find that their TinyTach was not correct? I'd rather not spend the cost of a smaller pitch prop only to find that I'm really already at the top of the power curve.
Thanks for your thoughts.

 

[codertimt]

Re: Are TinyTachs accurate?

I intended to respond to your original post, but never managed. I'm not sure of the accuracy of tiny tachs in general, but yours is definitely wrong. Are you sure it's the correct model for the application? TT226R-2XLR?

But the numbers you've provided calculate to a -17% slip which is obviously quite impossible. The gps speed should be accurate, so given the 2:1 gear ratio your motor has and a 14" prop, the calculator I use says you're anywhere between 5800 and 6200 rpm. So definitely don't drop any more pitch until you doubly verify those rpms...

 

[Rancherlee]

Re: Are TinyTachs accurate?

I'm only getting 4450 RPM at WOT on my 16' Lund, 40hp Merc while doing 35mph on GPS. Doesnt seem like there is another 500-1000 RPM left, but according to Mercury I should top out at 5000-5500.
Has anyone come to find that their TinyTach was not correct? I'd rather not spend the cost of a smaller pitch prop only to find that I'm really already at the top of the power curve.
Thanks for your thoughts.

What model lund you you have? 35mph GPS is pretty dang fast for a 40hp on a Lund, even a bare bones SV-16 tiller model. My 16' Mr.Pike only runs 38mph with a 70 hp and loafed along at 26mph with a 40hp.

 

[steelespike]

Re: Are TinyTachs accurate?

I do recall some discussion some time ago about careful installation and routing of the hook up on a tiny tach.And 35 does seem optimistic to me.
I believe a 40 is rated at max of about 33 on a typical setup.

 

[Silvertip]

Re: Are TinyTachs accurate?

Depending on the year, your Merc can have a 1.86:1 gear ratio so your boat may be seriously over propped. Check the prop for part number and or pitch. If that motor is the three cylinder 2-stroke, it would likely have a 13P prop. Using your numbers (4450 RPM, 1.86 gear ratio, 13P prop, and 13% slip) your speed would be 25.6 MPH which is a very real number. To advance this, 5500RPM, 1.86 gear ratio, 13P prop, and 13% slip would yield 31.7 MPH which from experience would be very correct. The result of this is that your Tiny Tach is very wrong and you are very likely running between 5500 and 6000 rpm. This set of numbers very typical for a Merc 40/50 HP triple non a 16 - 17 foot boat. One thing you can check is to see what the idle rpm is. Check that and let us know. Since it appears you have about a 35% rpm error, the idle rpm should also be 35% low.

 

[craze1cars]

Re: Are TinyTachs accurate?

There's at least 15 different models of Tiny Tachs available...each for a different type of motor (2 stroke vs 4 stroke, odd number of cylinders vs even number of cylinders, how many times does the plug fire per rev, points or non-points ignition, etc.)

You sure you have the CORRECT Tiny Tach for your particular motor? And it was installed correctly?

 

[LORDY611]

Re: Are TinyTachs accurate?

16' Lund model S-16-20-DLX, which has wooden seats, carpeted wood floor riveted to the stringers (guessing).
1996 Mercury 40hp 4cylinder 2 stroke serial no. 0G084106 with very few hours
The prop stamp is 48-73138-14, which was determined by someone here on iBoats to be 10-3/8x14p, aluminum (probably OEM due to what I learned from the previous owner of the motor).
When purchasing the TinyTach, I contacted the company (Design Technology, Inc.) that sells them and told them what motor I have. They told me which model TinyTach to get: TT226R-2XL which includes an hours meter. I installed it per manufacturer's instructions.
The idle speed is approx 800RPM, and WOT is 4450 with me alone, about 5 gals of gas in a plastic can, and an anchor that might weigh 10 lbs.
My speed is according to a handheld Garmin etrex GPS which consistently shows in excess of 35 mph.
Short of trying a different tach, I'm at a bit of a loss. Then, even if I do try another, what says that one is correct? Theoretically, I should try a third to see which it is closer to, then go with the averages.

 

[Silvertip]

Re: Are TinyTachs accurate?

You have a two stroke engine. It doesn't matter whether the engine has one cylinder or 16. The cylinder that feeds the tach fires once every rev. That's the correct tach. But something is obviously wrong.

 

[AltheaToldMe]

Re: Are TinyTachs accurate?

I read something here about the tiny tach being wrong because he was using resistive type plugs. Not really sure, I just remembered the tip and thought I would pass it along.

 

[LORDY611]

Re: Are TinyTachs accurate?

I read something here about the tiny tach being wrong because he was using resistive type plugs. Not really sure, I just remembered the tip and thought I would pass it along.

That topic never came up in my discussion with the manufacturer to decide which model I should purchase, nor does it show up anywhere on the instruction sheet. I suppose it could be true, but my gut says no. I never saw the thread you refer to, but if you happen upon it, please forward the link. I would appreciate it.
My time on the water is about done for this year. I will pull the boat tomorrow AM and winterize, then leave here until I return next summer. I guess that I will keep my eye out for a conventional tach to verify what the TinyTach tells me, and then either I do nothing due to proper RPM and being happy with the speed, or replace the prop to achieve another 600 to 800 RPM. Another option is to replace the blade to acheive a better hole shot with a skier. That is the most likely scenario.