RPM are at 7000, need help. Already Been Playing with Different Pitches

[FishBlitz]

I have a 15ft fiberglass hightide with a 1997 Johnson 50hp on the back. On my 19in pitch prop, my rpm are 6100 at WOT but it takes 9 seconds to get up on plane with a light load and only me in the boat. I bought a 15 in pitch prop that puts me on plane in 3 seconds but the rpm max out at 7000 at WOT. The owners manual says WOT should be no more then 5500rpm. What do I do? If I buy a 17in prop, my rpm at WOT will be around 6500rpm which is too high.

 

[jestor68]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

Well, according to the prop selector, which estimates you should be turning about 5000 rpm with a 19 pitch @ about 36 mph; it seems that your tachometer is way off.

Another possibility is your set up. The motor may be mounted too high, causing high prop slippage, which can cause the rpm to run high.

I would verify the tach before doing anything else.

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

It's a brand new tach that I just had installed a couple days ago.

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

An at WOT I'm going 32mph with the 19in prop

 

[steelespike]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

Very likely your tach is set wrong. At 6100 you would be doing about 39 or 40 mph.
At 32 mph the motor is turning about 4900.
Usually there is a selector switch on the back of the tach Not sure what your setting would be
but select another one and try it. I have a feeling that you need less prop for a 15 ft glass boat.
We can tell better when you get your tach setup.
By the way is your speed by gps? The only reasonable dependable speed measure.
get us your lightly loaded wot rpm and gps speed.

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

Lightly loaded gps speed is 32mph. That's with nothing in the boat except my battery and 6gallon fuel tank. I'm almost certain that I need less prop for the boat. That's why I bought a 15in prop too, because with the 19in it takes too long to get on plane. I'll check for a little switch or dial in the back. Is it something I need to adjust with a screw driver?

 

[jestor68]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

There should be a small "dial" with a slot for turning it.

There will be a bunch of numbers spaced around the dial. Try setting it on " 5P " for your 2 cylinder Johnson 50 with a ten pole alternator.

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

It's already set on 5p, should I try 6p? What does the p mean?

 

[jestor68]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

It's already set on 5p, should I try 6p? What does the p mean?

The P stands for pulses of the alternator. You set it on 1/2 the number of poles on the alternator.

According to the instructions for a Faria tach, Johnson motors over 50 hp have 12 pole alternators(6P). You can try that setting.

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

I just want the tach to me accurate so I don't blow my motor up. I searched around to try and find a way to physically count the poles but I'm not exactly sure what im looking for. They're supposed to be inside the flywheel behind some hub. What exactly do they look like?

 

[FishBlitz]

Re: RPM are at 7000, need help. Already Been Playing with Different Pitches

Fixed the problem, it has 6 pulses, not 5