Right prop???????

Ok my boat tops out at 5000 rpm's (assuming the tach is correct) and runs 50.3 by GPS. It has a 3 blade stainless prop that has S22 stamped on it. Other than being a smokey old two stroke it runs fine and seams to plane out decently enough, about 4-5 seconds. Does this sound right for my boat:confused2:. Motor is a 1983 Mercury 150 (Mercury dealer said it was an XR2 by serial number???) on a 1986 ranger 372V hull.

If the tach is right you should be turning 5-600 more rpm. The engine is lugging at 5000. I don’t know what the S22 means but if you remove the prop nut and washer there may be some numbers stamped on the hub that will tell the pitch and diameter of the prop. Then you will know what you have and go from there.

Ok will look at that this afternoon. The tach may have been wrong as it started bouncing and then quit working yesterday. Prop has precision performance 14" 24 etched on it so 24 pitch. So if I understand what I've read I need to go down in pitch to get the rpm's where they need to be

http://www.mercuryracing.com/prop-slip-calculator/

I don't know your gear ratio but your looking for 10% or less slip.

This will give you an idea of where to start.

Post results.

My gear ratio is 2:1. 11% is what the calculator gave me plus did it with the equation that I found on another sight. So what does this tell me? Sorry newbe to this.

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Originally Posted by .44mag

Ok will look at that this afternoon. The tach may have been wrong as it started bouncing and then quit working yesterday. Prop has precision performance 14" 24 etched on it so 24 pitch. So if I understand what I've read I need to go down in pitch to get the rpm's where they need to be

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If your tach was reading 5k and jumping around and finally stopped working, your voltage regulator/rectifier is bad. That's what sends the signal to the tach. Most bass boats have about 12 -15% slip. Not likely you are getting 11%. Did you put in 5k rpms? Also, you must add 1" of pitch for a cupped prop. You should put in a 25p. 2-1 gears, 25p prop, 5000 rpms = 59 theoretical MPH, actual speed is 50 = 15% slip.

My guess is that you are turning more like 5500 rpms = 65 theoretical MPH, 50 MPH = 23% slip. Get your tach fixed before you start worrying about the prop.

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Originally Posted by Fatshaft Merc

If your tach was reading 5k and jumping around and finally stopped working, your voltage regulator/rectifier is bad. That's what sends the signal to the tach. Most bass boats have about 12 -15% slip. Not likely you are getting 11%. Did you put in 5k rpms? Also, you must add 1" of pitch for a cupped prop. You should put in a 25p. 2-1 gears, 25p prop, 5000 rpms = 59 theoretical MPH, actual speed is 50 = 15% slip.

My guess is that you are turning more like 5500 rpms = 65 theoretical MPH, 50 MPH = 23% slip. Get your tach fixed before you start worrying about the prop.

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Yes I put in 5000 rpm's, no I did not add 1 inch for the cup. My goof. I am going Wednesday to get a new rectifier to put on. First post/question about this was before the tach started acting up, although like stated it may not have been reading right to begin with before it quit.......

Ok Changed out the rectifier and took the tach out of the console and turned the dial in the back, no change. Guess the gauge has given up the ghost.

23p prop is what you need, I agree that at 5000 your lugging a bit that motor should turn 5500-5800 all day. With a 3 blade 23p yo should get mid 50's if you can turn it at 5500 or better.

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Originally Posted by Teadow1972

23p prop is what you need, I agree that at 5000 your lugging a bit that motor should turn 5500-5800 all day. With a 3 blade 23p yo should get mid 50's if you can turn it at 5500 or better.

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Thanks. Will try that as soon as I get the quirks worked out. May take a week or two but will let yall know what happens. Again thanks for all the suggestions/help guys.

The older Merc/Mariners did not appreciate having the starting battery charged at over 2 amps, or so I was told. Doing so will burn out some of the electronic stuff that runs the tachometer. Been there. Don't do that! ALWAYS disconnect the starting battery if you need to charge it. However, other than the tach not working, my XR-4 didn't seem to care. Once you get it setup, or know it is setup well, you don't really need the tach IMO. However, until the setup has been altered or confirmed, you do need the tach to work.

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Originally Posted by Walkabout7781

The older Merc/Mariners did not appreciate having the starting battery charged at over 2 amps, or so I was told. Doing so will burn out some of the electronic stuff that runs the tachometer. Been there. Don't do that! ALWAYS disconnect the starting battery if you need to charge it. However, other than the tach not working, my XR-4 didn't seem to care. Once you get it setup, or know it is setup well, you don't really need the tach IMO. However, until the setup has been altered or confirmed, you do need the tach to work.

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Yeah I disconnect at the least when I charge the batteries (when they need it). I some times take them all the way out of the boat. The only charger I have is a 2 amp charger. Learned a long time ago that its better to put a "slow" trickle charge on a battery than those fast ones when possible.

Just an update. My boat will hit 56 mph according to my gps, 56 to 56.2 reading. That's with just me half of my fishing gear, one tank empty and the other full (1/2 fuel load). With my son and I, all our gear, and about the same amount of fuel it will run 50 to 51. So I think I got it worked out. Borrowed a known working tach and had the prop that came with the motor "tuned" up. So I think this will do till its time to upgrade to a new or newer boat.

that sounds logical for that rig...…..good luck and enjoy