19 pitch prop 24mph what do i need to increase?

[spacerust]

i have an 18' flats boat with a 90hp johnson. the prop i have on it is a 19 pitch and i clocked it a 24mph at WOT the other day. i don't have a tach installed yet but was wondering if i bought a lower pitch (possibly 15 pitch) would it increase to recommended rpms. i used to have a mercury 150 on it with 15 pitch and it would go at recommended rpms (had tach then). not looking for speed but just wondering if this will do any damage if i leave it with the 19 pitch. i just got the 19 pitch so would it hurt to keep as a spare?

 

[Barnacle_Bill]

Re: 19 pitch prop 24mph what do i need to increase?

How are you going to know if its at the recommended RPMs without a tach?

 

[spacerust]

Re: 19 pitch prop 24mph what do i need to increase?

i ordered a tach harness so i should be getting sometime next week. but i guess i'm asking would this do any harm going out this way? is there a way to know relation between mph and rpms? i kinda like the way it was cruising, real smooth and i'm fine with that. i just don't want to harm the motor.

 

[jdsgrog]

Re: 19 pitch prop 24mph what do i need to increase?

In all honest, I am no expert in this, but from reading and studying manual, formums, etc, I'll try to give my take.

There is a recommended rpm range of the motor by the manufacturer at WOT. From my understanding, if you are unable to reach that RPM at WOT, then you are placing a much larger load on the motor. I can't say what that means for the motor exactly, and I'm sure some of the experts on the forum could tell you more.

Also, there is a relation to speed and rpm, and there are calculators that will give the top end speed based upon the proper rpm range, gear ratio, prop pitch, and estimated slip. The goal is always to prop the motor properly to get the best performance out of the motor for the particular load and usage. If the motor is working as it is supposed to, but is not getting to the proper rpm, then it is overloading the motor (to my understanding), which probably means you are not propped correctly. Likely, you can possibly throw a rod or something even worse. If you are going beyond the rpm range, then you have the possibility of overheating the motor to say the least.

Please, anyone, correct me if I am wrong on any of this.

 

[jdsgrog]

Re: 19 pitch prop 24mph what do i need to increase?

In all honest, I am no expert in this, but from reading and studying manual, formums, etc, I'll try to give my take.

There is a recommended rpm range of the motor by the manufacturer at WOT. From my understanding, if you are unable to reach that RPM at WOT, then you are placing a much larger load on the motor. I can't say what that means for the motor exactly, and I'm sure some of the experts on the forum could tell you more.

Also, there is a relation to speed and rpm, and there are calculators that will give the top end speed based upon the proper rpm range, gear ratio, prop pitch, and estimated slip. But in all honesty, speed is not the main concern and it's only a byproduct of the motor. If you are not getting the speed you are looking for (which doesn't seem to be your concern), and the motor is propped correctly, then you/anyone should get a larger motor.

The goal is always to prop the motor properly to get the best performance out of the motor for the particular load and usage. If the motor is working as it is supposed to, but is not getting to the proper rpm, then it is overloading the motor (to my understanding), which probably means you are not propped correctly. Likely, you can possibly throw a rod or something even worse. If you are going beyond the rpm range, then you have the possibility of overheating the motor and locking up the motor to say the least.

Please, anyone, correct me if I am wrong on any of this.

 

[hwsiii]

Re: 19 pitch prop 24mph what do i need to increase?

Spacerust, I can guarantee you that prop has way too much pitch for your motor and is making it lug tremendously. All the johnson 90 motors I found had a 2:1 gear ratio, and at that ratio and a 10% slip factor you should be doing 45MPH. I find that you are turning that prop at about 3,000 RPM if the slip factor is at 10%, but a lot of flats boats can't reach that low a slip especially if it has a tunnel.
It appears to me you are running a motor that is way too small for the boat it is pushing, tell us everything about the boat and motor the year of each, the weight, if it has a tunnel and exactly what make and model of prop you are using on it as well as the diameter and if there is any cup in it.

H

 

[Dhadley]

Re: 19 pitch prop 24mph what do i need to increase?

While we're waiting on rpm numbers, fill us in on the boat and set up. What brand / model boat? Do you have a jackplate? What brand / style prop is it?

What year is the 90? Are you sure the motor is running right? A 19 on a V4 on a typical 18' flats boat shouldn't be that far off if the motor is running right and the set up is close.

 

[spacerust]

Re: 19 pitch prop 24mph what do i need to increase?

ok, this morning i installed a tach and speedometer on it. another thing i am going to check is to see if butterfly is opening all the way at WOT. i just recently had the carbs cleaned and right before it would turn off when i put it in gear. had to adjust the idle. she is an 18' flats, not sure on weight but would say about 800 lbs. motor is actually a 73 johnson 135 hp but the guy told me he switched the carbs to a 90hp. i usually fish with me and one to two other people at most. i do have a jack plate, anti-cavitation plate, and prop is not cupped. the guy who sold me the motor told me if i ever wanted to boost the hp that i could just change the carbs. i am going to take her out when it gets a little cooler this afternoon to check rpms.

 

[Dhadley]

Re: 19 pitch prop 24mph what do i need to increase?

OK, what brand and model of flats boat? At this point we're not that worried about the weight, just need to know the hull design. And the make and model of the prop.

 

[spacerust]

Re: 19 pitch prop 24mph what do i need to increase?

well originally it was a 1978 chapparal but was modified in 1999 to its new form. it's a frankenstein. i'm not sure how to describe it but it's the boat pic of my avatar. i will try to upload a bigger one. the prop is 13 x 19 stainless steel. not sure about make. i know it's not a lot of info but that's really all i know on it.