Determining small outboard RPM's ?

[Fish Chris]

The reason that I need to know the RPM's of my brand new Yamaha 25hp, is so that I will be able to prop it for maximum performance.

But how can one determine the RPM's without an RPM gauge ? Is there some way to temporarily connect an RPM gauge to a small outboard, so you can take it out for a spin, see what it's turning, then use that info to decide the optimum pitch and diameter for your prop ?

My new Yamaha 25hp (which should be here shortly Yeeehaaa ! is the max HP for my relatively small, light, 14ft G3. I'm assuming that it will probably come with a "standard, middle of the road" type of prop, which could probably handle a heavier boat / load if it had too. (In fact, my buddy has the exact same motor {and probably the same standard prop}, on a MUCH heavier river style boat, and it does surprisingly well).
So I'm thinking, I could probably go up a pitch or even two, and maybe a half inch in diameter also, while still keeping my RPM's up, because of the relatively light load.

Any propping specialists here ? Advice please. Am I on the right track ?

Thanks in advance,
Fish Chris

[mac (Doyle McEwen)]

No engine/prop expert, but I think you are on the right track..There should be a way of doing it, although I don't really know what it is..Perhaps something connected to the ignition system to count how many times per minute the plugs fire..

Also something else to keep in mind, once you change the prop, there is a good chance your max RPM's will also change..No doubt one of these mechanically inclined "nutcases" will come along and give you a more solid answer..

mac

[NoCAL]

Fish Chris,

I have an hour meter hooked up to my 115. It has a coil of wire that loops around one of the spark plug wires. When the motor is on, it shows RPMs. When the motor is off, it shows hours of use. It cost me around $50 at Gone Fishin Marine.

NoCAL

[Fish Chris]

That sounds like a great little item that I will have to get and put right on from day 1 ! I have always wondered whether I was changing the oil in my little Honda way more often that I needed to, or in some cases, not often enough.

My motor use (in hours of actual running time) is very inconsistent throughout the year. I might have it on the water 12 days in one month, when I run the motor for 25 hours. The next month, depending on where I am fishing, I might have it on the water the same 12 days, and run the motor 5 hours...... or some times, the boat might be on the water 12 days, while the motor sits in the garage

But anyway, pertaining to my original reason for wanting to know my RPM's, this little device sounds like it will tell me just what I need to know....... and then I can just relay that info to the prop specialists. BTW NoCal, do you recall the name, or brand of this little device ? Easy to hook up ? Small enough to fit under the compact hood of a 25 hp ?

Thanks very much,
Peace,
Fish

[fishnrich]

Chris, most manufactors have a prop calculator that you can use online. You enter the data, hp, boat type and length, and altitude. It will give you a very close prop pitch #. If your motor manufactor doesnt have one, you can try prop manufactors. Good luck.

[ryan pearson]

Call Tony at Phils Props. 1-800-462-3917 He can help you.

[Andy Giannini]

There is also a tach you can put right on the end of the flywheel. It is driven by a little rubber knob that you press against the shaft to be measured. It measures in either direction, and works well, but it is probably the most redneck/dangerous ways of doing it.

The engine cowl must be removed, and you are going to be standing up in the boat in order to read it.

I think Northern tool sells a non contact tach as well.

A.G.

[Fish Chris]

I have heard good things about Phil's Props. I will certainly keep them in mind.

Oh, and one more interesting thing (to a propping dummy like myself anyway)
My buddy who has the 25hp on the big, heavy 17 1/2 ft riverboat, was surprised that this boat, went darn near as fast as his much lighter 16 ft boat before it......
I could be wrong, but my guess is, with his new, heavier boat, the more modest "standard pitch prop" is probably just about all the motor can handle. While with his old, lighter boat, he probably could have went up a pitch or two, and got quite a bit more whole shot and top end speed, than he does with his new boat. I believe that the reason he didn't see much difference is because he didn't change the propping. Agree ?

Peace,
Fish

[Whoopbass]

I don't think your hole shot will improve with a higher pitched prop but your top end might improve slightly.
You might get a 1-3 mph gain if your lucky (just a guess).

I have put my 25 hp Johnson on different size boats and the results are always the same speed. Only one prop style is made for my outboard so I can't test different pitches.
The river bottom has changed the pitch on my props many times and the boat always runs the same speed.

I just don't think you can do much to a smaller outboard to get a noticable increase in performance.

You will eventually need another prop so buying one now with a different pitch won't hurt.

My 15' Gregor with front and rear decks runs about 20 mph. Your open boat with a new 25 should run 25-30 mph.

You'll probably get better results experimenting with motor heigth and gear distribution.

Just my .02 and you'll probably owe me change.

[NoCAL]

FC,

I can't remember the brand meter I got. I have the manual that came with it but it's out in the boat right now under the cover in the rain! If you call Gone Fishin Marine and ask for Roy or Mike in the service dept. either will be able to tell you about the unit. Hook up couldn't be simpler. Just wrap the wire that comes out of the unit around a spark plug wire the number of times is recommends for your size engine. They provide enough wire that I routed mine from under the cowl into the bilge area where I taped it with duct tape! It is about 2"X1.5X.75" in size. Should fit just about anywhere you want to put it. Let me know if you need more info and I can go grab it out of the boat.

NoCAL