Tach pegs out at WOT

[sturgis]

I have a 1972 Newman 15' Tri-Haul. It has a very strong running 1974, 85 HP Evinrude V4. At WOT the Tach is pegged above 6000, and I am getting 40 MPH at WOT, when I back it off to around 4800 I am getting 30 MPH. This boat has an unbelievable hole shot and will turn on a dime with no list and not loose any RPMS doing it. The prop is a 13 1/4 X 17, I need to loose 600 to 800 RPMs to get to in the 5500 range. What Pitch should I be running.

[guy74]

I don't think your tach is right, what pole setting did you use? My V-4 motors with a 17 pitch prop turn around 5000 at 40+ so I don't think something is right here.
I think you are supposed to have it set for 12 poles, sounds like you are set on 10.

[SparkieBoat]

if all is good you should go with a 21P to lose 800 RPMs, but I would try a 19 first, should drop 400 RPMs.

[sturgis]

I don't think your tach is right, what pole setting did you use? My V-4 motors with a 17 pitch prop turn around 5000 at 40+ so I don't think something is right here.
I think you are supposed to have it set for 12 poles, sounds like you are set on 10.

The only thing I've done recently is install a new stator/ignition power coil assembly, the number of coils are identical (eight)to the original. This should give two unrectified sinewaves per revolution, which would produce 4 rectified pulses per revolution.The engine wiring has a factory provided output that comes off of the rectifier output (rectified AC). The tack is also original and has settings 1,2,3,4. It is set on the #4 setting, it seems that if it is counting rectified AC pulses that a lower setting would give an even higher reading (3 would = 8000?). If your reading 5000 at 12 you would be reading 6000 at 10? I've had mine for 15 years and it has always been set the same. Are you running a 60 HP? If so you are absolutly right something doesn't add up, 60HP w/17p = 40+ @ 5000RPM vs. 85HP w/17p = 40@ 6000? I can verify my speed with a GPS, the only sure way to verify my Tach setup is to measure the pulses on an "O" scope. Let me do some checking. Thanks for your thoughts.

[guy74]

The 40mph @ 5000 was on my cousin's boat with a 90hp Evinrude, I have never ran a 17 pitch prop on my 100hp that is on my boat in the pic on my post. The number of pulses is determined by the number of magnets on the flywheel, not the coils on the stator. All of my 3 and 4 cylinder engines all have to be set at 12, but I haven't worked on one as old as your's. Might be different, but the instuctions that I read on Faria's web site seem to say they are all 12. A V4 motor @ 6000 is really buzzing.
One thing I wonder, all the tachs I find for outboards go up to 7000 rpm, the ones for I/O's and inboards (automotive type engines) just go to 6000

[SparkieBoat]

nice point guy, he may have a auto tach.

[sturgis]

The magnets produce the lines of flux that cross through the stator coil windings which creates a current flow from the stator windings which goes to the rectifier which is where the tach output is. The number of coils on the "stator" and the number of magnets produce the pulses. From may hot rod days I remember the red line beginning at 7000 and going to around 9000, just looked at my MG and it goes to 7000. It could be an automotive type not sure what difference that makes, they all run off pulses. 6000 RPM is with WOT only 400 RPMS above the engines WOT spec. I am changing the prop and will look further into the Tach issue.

[guy74]

I'm sure you are right with that, though it is just a bit past my knowlege, LOL! Anyway, it will be interesting to see what a different prop does for your performance. It seems that Faria makes a 6000rpm tach that can be used with outboards. Maybe that is what you have, here is the info on it http://faria-instruments.com/site_manuals/IS0012u.pdf

[sturgis]

I'm sure you are right with that, though it is just a bit past my knowlege, LOL! Anyway, it will be interesting to see what a different prop does for your performance. It seems that Faria makes a 6000rpm tach that can be used with outboards. Maybe that is what you have, here is the info on it http://faria-instruments.com/site_manuals/IS0012u.pdf

I climbed under the dash and found that both the speedo and tach are Medallion, it has a simple two wire set up. Signal and ground. From what I am seeing todays tachs are capable of working on outboard (alternator output pulse) or inboard (auto type using the coil input pulse). I'll let yopu know how the test works out. Thanks for your interest and comments. By the way do they have lakes in Kansas?

[furyman]

don't wanna steer you in another direction. But i have the same dia and pitch prop on my 120 force. I noticed the same thing happens to me if i dont have the trim down right. I would play with trim again and see if you still have the same issue.

[guy74]

Sometimes props don't perform exactly like they should. Case in point, I have 2 OMC factory props, and the 14X17 one turns more rpms at the same speed than my 13 3/4X15, both run the same day right after each other.
If your figures are correct, 6000rpm @ 40mph with a 17 pitch and 2:1 gears in the LU gives you about 17% slip.
It has been my experience that most boats arond here set up like yours, run 17 or 19 pitch with WOT rpms of around 5300. Seems like most cross-flow V4 Johny-rudes I've been around get their best top speed with a WOT around 52-5300rpm. The crossflows don't run out at high rpms like the loopers do.
BTW, yes we do have lakes here in KS, just not alot of big ones here in the Southeast corner. I go down to Grand lake in OK, or over to Truman lake in MO sometimes, but most of my boating is on Lake Crawford which is just 12 miles away.

[sturgis]

Wow You are right! I checked out the tach using an induction timing light on my son' Celica, (4 stroke= 1pulse/2rev, or 2 rev per OB rev) at 800/1500/2000 RPM on the OB, double on the car, could actually go 100 RPM in both direction on the car and see the the OB flywheel change directions with the strobe. Pretty cool. This was after I played with the settings on the tach. Turns out the plastic arrow wasn't engaging the switch inside the tach, so no effect until I removed it and moved the switch directly. Now for the test with the 13 1/4 X 17, 5300 RPM at 40mph, 4600 RPM at 34mph. I move the trim rod out one hole away from hull and increased Speed about two or three mph. With the 13 X 19 it ran about 4900 RPM and 37mph as RPMs dropped speed droppped drasticly. I also ran an old 14 X 15 it's now about 13 1/2 x 15 after dressing it up, 5800 RPM at 35 slow but could probably pull the dock with it. I am convinced now that the 17P is right for this boat/motor are there any other issues to consider ie, 13, 13 1/4, 13 1/2, 14 inch, 4 blade vs three, stainless vs aluminum. I know stainless is better but three times the cost, not sure could justify for 85 HP on an old tub. Every thing I've tried has been 3 blade. Thanks Guy an everyone else for all you help.