Small Motor Tachometer

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gogittum
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Small Motor Tachometer

Post by gogittum »

I'm going to be shopping soon for a spare prop for my old Johnson 15 hp and would like to buy the best choice. If it's over or under propped now, the new prop will become #1 and the present prop will be the spare. I do realize it's only a 15 hp, but proper equipment is still important.

I'll be running a lot in the Crystal River to Cedar Key areas which are notorious for very shallow rocky flats. A spare prop will almost certainly be a necessity.

With that in mind, I recently bought a cheap digital tach that has a pickup that wraps around a spark plug wire and counts impulses.

It does work, sorta, but rpm readings jump around a lot and it cuts out frequently with no reading at all for just a blink, then comes back on. I've got 5 wraps around plug lead and wrapped them tightly with electrical tape to give the best possible contact, but no joy.

Actually, I think the present prop is close enuf to "right," since motor sounds good at full throttle, not lugging or screaming, and pushes a basic, open 16 ft Starcraft tin boat to a 20.5 mph GPS reading. Still.....?? Also, in not too distant future I'll be putting a bigger motor on it - 25 to 35 hp and propping will be even more important.

Thoughts or ideas would be welcome. Thanks.

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DaleH
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Small Motor Tachometer

Post by DaleH »

Try this here: https://forum.tinboats.net/viewtopic.php?f=4&t=37433 and be honest with yourself in regards to the weight of the typical gear carried. This has worked PHENOMENALLY for the dozens of hulls I've checked or rigged whilst using it.

If your hull isn't listed, find a comparable hull, i.e., they didn't list my specific 14' ProKraft hull, but the specs on a mid-90s to later Smoker Craft 14' V-hull were close enough to fit.

gogittum
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Small Motor Tachometer

Post by gogittum »

OK, thanks. I went thru the procedure and they recommend a 9x9 prop. Should be an email waiting from them, too, but haven't checked yet.

Original question still remains, tho' - how do I get this little tach to read properly ?? I've got it....I'd like it to work.

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DaleH
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Small Motor Tachometer

Post by DaleH »

Try less wraps, you could be overloading the signal, i.e., impedence, etc.

gogittum
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Small Motor Tachometer

Post by gogittum »

DaleH wrote: 22 Jul 2021, 07:25 Try less wraps, you could be overloading the signal, i.e., impedence, etc.
Haha....OK, thanks. Yesterday I added a length and tied it all down - after giving it 7 wraps. Figured more is better. If it doesn't do it, I'll try 3 wraps.

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DaleH
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Small Motor Tachometer

Post by DaleH »

gogittum wrote: 22 Jul 2021, 09:35
DaleH wrote: 22 Jul 2021, 07:25 Try less wraps, you could be overloading the signal, i.e., impedence, etc.
Haha....OK, thanks. Yesterday I added a length and tied it all down - after giving it 7 wraps. Figured more is better. If it doesn't do it, I'll try 3 wraps.
Me thinks MORE is WORSE actually …

gogittum
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Small Motor Tachometer

Post by gogittum »

DaleH wrote: 22 Jul 2021, 19:35
gogittum wrote: 22 Jul 2021, 09:35
DaleH wrote: 22 Jul 2021, 07:25 Try less wraps, you could be overloading the signal, i.e., impedence, etc.
Haha....OK, thanks. Yesterday I added a length and tied it all down - after giving it 7 wraps. Figured more is better. If it doesn't do it, I'll try 3 wraps.
Me thinks MORE is WORSE actually …
Planning a run on the Withlacoochee in the morning, if'n it don't rain as it did this morning. We'll find out for sure then. Removing a couple of wraps isn't a big deal.

gogittum
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Small Motor Tachometer

Post by gogittum »

Now I'm really having fits. I test drove the boat 2 weeks ago for a couple of miles in a local canal, then took it onto the Withlacoochee River and ran a total of about 5 miles. This with the 6.5 gallon tank that it came with and all was well. The motor was smooth and strong and idled well - no problems. At speed it "sounded" good but that's not a very accurate rpm measurement.

I figured I may need more capacity at times and also plan on a 25 - 35 hp motor in near future, so cleaned out the 12 gallon tank that also came with it. (it had over a gallon of water under 1/2 gallon of gas and dirt) Couldn't keep the motor running. Found that the pickup tube was sucking air where it plugs into the fitting on top of the tank. Fixed that with T-88/cabosil, then checked out the squeeze bulb and fittings that plug onto tank and motor. Replaced one. Put in 5 gallons of fresh fuel and dosed it with Yamaha fuel treatment.

Everything seems tight and the motor runs "better" but is still ragged and doesn't idle well. I'm going to put fresh gas in the 6.5 gal. tank today and start over from square 1. What a pain - I just wanna go play.

(I fooled myself, too. Thought the little spare tank was a 3.5 gal. Too small for an all day outing on the salt. Looked at it closely yesterday (very hard to read) and it turns out to be a 6.5 gal. Should be plenty for a day with a 15 hp motor. Sighhh)

As for the prop, the site recommended above told me a 9 x 9 prop would be just right. I checked with another prop mfg'er and their site came up with a recommendation of 10 x 12. Biiig difference. Then I checked my existing prop......9 x 10.25. Running in these waters, I've GOT to have a spare prop. Dunno WTH now. Haven't been able to try less wraps on tach yet - can't get the blamed motor going. Aaarrgh.

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wmk0002
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Small Motor Tachometer

Post by wmk0002 »

I have had great success with the inductive tachs on OMCs. I’ve found them to be a little finicky on yamahas though. I’ve always used the Hardline brand ones sold on Amazon.

I have an Innova timing light that shows rpm and I have used that in a pinch. Just need a 12 v power source and remember to divide the rpms displayed in half since it thinks it’s reading a four stroke. It wasn’t cheap - about $100 - but it’s been a valuable tool in my home shop.
2016 Alumacraft 1648 NCS - 2020 Yamaha 25 HP Tiller - 1992 Johnson 9.9 HP Tiller
https://www.tinboats.net/forum/viewtopi ... 21&t=40764

ktoelke54
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Re: Small Motor Tachometer

Post by ktoelke54 »

If you have an accurate speed, you can use an average slip (5% for example) and come very close to the rpm’s. I realize this is not the preferred method. But who’s to say that a cheapo Tach is not off a bit.

I’m familiar with that motor. If your breaking 20 mph with the stock 10 prop, your definitely in the ballpark rpm wise. I dropped from a 10 to a nine pitch because my boat with a heavy load was only doing about 18.5 and was lugging. The nine brought the rpms up where they should be and topped out at 19. It was a lot quicker out of the hole.


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gogittum
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Small Motor Tachometer

Post by gogittum »

ktoelke54 wrote: 02 Aug 2021, 11:44 If you have an accurate speed, you can use an average slip (5% for example) and come very close to the rpm’s. I realize this is not the preferred method. But who’s to say that a cheapo Tach is not off a bit.

I’m familiar with that motor. If your breaking 20 mph with the stock 10 prop, your definitely in the ballpark rpm wise. I dropped from a 10 to a nine pitch because my boat with a heavy load was only doing about 18.5 and was lugging. The nine brought the rpms up where they should be and topped out at 19. It was a lot quicker out of the hole.


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Speed should be right on - GPS readout. I've had a lot of experience, tho' very little recently, and to my ear, I think you're right. It simply sounds "good," so I'll go with that. Not screaming, not growling, just a good, solid hum. I like to run at about 70%.

Several things this morning; I made good on my threat to try fresh gas and it worked. Hooked up the 6.5 gal tank, fired it up and it was pretty ragged for a couple of minutes, then, before I really expected, it smoothed out quite a bit. I'd been fussing with idle mixture so got back after that and within about 5 minutes the goofy thing was purring like a kitten.

Dunno what's wrong with the gas in the 12 gal. That gas is fresh, too. There had been a LOT of water in the tank - at least a gallon - but I was "very" careful in getting it out of there, then added a bit of an overdose of Yamaha fuel conditioner - about 1/3 more than called for in 5 gallons. Maybe some residual water ?? Maybe too much conditioner ?? Maybe wasn''t biting my tongue just right ?? I dunno but I'm much relieved.

On that PIA tachometer, I fooled with it while the motor was running and, starting at the 7 wraps I put on it the other day, started unwinding one at a time. Around 1 1/2 - 2 wraps, it was still erratic but rpm readout seemed a bit smoother and more in line with what my ear was telling me - and agreeing with what DaleH said. Just for grins, I actually just laid a 1" length of pickup wire alongside the spark plug wire and taped it down. It worked but seemed to go back to less accuracy. There's 1 1/2 wraps taped down on it now and that's how she'll run on the next river trip - if the blamed t'storms will ever ease off for a while.

Very soon now, I'll be looking for an electric start tiller steer 25 - 35 hp motor for this boat, then the 15 will go up for sale.

Gotta see how the pup dog's shade umbrella works in real life, too. Looks good on the trailer....?? A local fishing guide suggested taking a heavy towel in the boat on hot days - soak it in the water, then lay it on floor of boat for my girl to lay on and be a bit cooler.

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wmk0002
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Small Motor Tachometer

Post by wmk0002 »

Come to think of it, on my 1992 Johnson 9.9/15 I did have trouble with the inductive tach and I ended up concluding that it was having interference with the lead touching a ground point on the engine block as it was coming out to the display. I found an old piece of small wiring loom or tube or something and ran it through that under the hood if I remember right.
2016 Alumacraft 1648 NCS - 2020 Yamaha 25 HP Tiller - 1992 Johnson 9.9 HP Tiller
https://www.tinboats.net/forum/viewtopi ... 21&t=40764