Small Motor Tachometer

Boating related questions, small projects, custom work and talk go here.
gogittum
Posts: 98
Joined: 27 Aug 2020, 16:35

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.

User avatar
DaleH
Donor
Posts: 3005
Joined: 12 Dec 2014, 14:32
6
Location: Eastern Mass

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
Posts: 98
Joined: 27 Aug 2020, 16:35

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.

User avatar
DaleH
Donor
Posts: 3005
Joined: 12 Dec 2014, 14:32
6
Location: Eastern Mass

Small Motor Tachometer

Post by DaleH »

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

gogittum
Posts: 98
Joined: 27 Aug 2020, 16:35

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.

User avatar
DaleH
Donor
Posts: 3005
Joined: 12 Dec 2014, 14:32
6
Location: Eastern Mass

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
Posts: 98
Joined: 27 Aug 2020, 16:35

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.