new prop... low numbers need help

[countvlad]

My old prop was an old SS prop with 17 pitch.... i used to get 42mph@5800 rpms.... since the prop was painted and started to rust i purchased an Stiletto SS 17 pitch prop for my johnson 120 HP 1989....

well the problem is that i dropped 1000 RPMS at WOT, right now 38@4600WOT... im just confuzed by this huge change, i definetly didnt think that just by getting an different prop my rpms would drop so drastic....

in order for me to bring my rpms up with the same prop i would need to get a prop with a pitch of 13 and i cant imagine a motor of that size with a 13 pitch... help me out please.... is the stiletto the wrong prop for me??? if thats the case which prop should i get.... thank you

1. Year, make and model of boat
1976 Renken CC
2. Length, width and base weight of boat, look for boat decal on back of boat
18?8" approx 1600 lbs
Around 2000 lbs loaded with fuel, equipment, and standard crew.
2a.What is the maximum recommended HP for your boat
150
3. Number of people and gallons of gas normally on boat
2, 20 gals
4. What do you use the boat for
Fishing
5. Is it a Deep Vee and if so how many degrees of Deadrise
Yes no idea
6. Year, manufacturer and model of motor
1989 Johnson 120
7. HP and gear ratio of motor IMPORTANT
120 HP 2-1
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
5000-6000

 

[Bifflefan]

Re: new prop... low numbers need help

Was the old prop beat up? Or possible it had a bad hub and was slipping.

What else has changed?

 

[countvlad]

Re: new prop... low numbers need help

Was the old prop beat up? Or possible it had a bad hub and was slipping.

What else has changed?

thats it.... slipping? not sure... acording to the slip calculator the slip on the old prop was 10% which i guess its pretty good

a year ago i had an aluminum prop 19 pitch that used to do 43@5200 WOT ... so i assume the old prop didnt slip....

what i might do is mark the prop and hub and see if its slipping? how can i tell if the hub is bad?

 

[steelespike]

Re: new prop... low numbers need help

That sure seems like a huge difference. A 13 would be in pontoon range.
I would suspect a motor problem.missing a cylinder?Perhaps the throttle adjustment has gotten changed. Something is screwy.

 

[countvlad]

Re: new prop... low numbers need help

That sure seems like a huge difference. A 13 would be in pontoon range.
I would suspect a motor problem.missing a cylinder?Perhaps the throttle adjustment has gotten changed. Something is screwy.

trust me i know what you mean.... i would definetly take a look make sure i didnt change break something... i thought about puting back the old prop and see how it does....

when i had the new prop the motor seemed to work well.... i had it before when only 3 cylinders were working and you could definetely tell a difference...

 

[1kruzer1]

Re: new prop... low numbers need help

trust me i know what you mean.... i would definetly take a look make sure i didnt change break something... i thought about puting back the old prop and see how it does....

when i had the new prop the motor seemed to work well.... i had it before when only 3 cylinders were working and you could definetely tell a difference...

I would agree, put the old prop back on and run it again and confirm your old numbers come back. If everything's back to as it was I'd suspect the new prop is not matching specs. Look at the new prop and find the diameter and pitch stampings, don't trust the labeling on the box. If the prop is stamped as 17 pitch I'd suspect a manufacturing error. If your motor returns to normal RPM and speed on the old prop, all the symptoms of the new prop say you have way too much pitch lugging the motor down. Exact pitch and performance will vary a bit between propeller manufacturers, and a well designed and cupped prop will act like a slightly higher pitch than a poorer performing model, but your results are way too different between the two props for that to be the issue. I suspect you have an incorrectly labeled or mis-manufactured new prop.

 

[countvlad]

Re: new prop... low numbers need help

Was the old prop beat up? Or possible it had a bad hub and was slipping.

What else has changed?

forgot to mention that i also installed a Faria fuel management system... i really didnt think that could be the problem..... but now that i think about it, it's possible that the motor is not getting enough fuel..... the FMS says that at WOT i was burning about 12.9GPH which sounds about right.... i think putting back the old prop will tell whats going on

i looked at the two props and the old one says 13" something, couldnt tell, and 17 pitch... the new one says 811317 which means it is a 17 pitch prop....

 

[countvlad]

Re: new prop... low numbers need help

so bypassed the fuel filter and the fuel management system and gained 200 rpms at WOT with motor trimmed in/down... but once i started to trim up the rpms jumped to 5400 .... while thats decent im still concerned about the low rpms while trimmed down/in.... what doo you guys think?

 

[steelespike]

Re: new prop... low numbers need help

I've kind of lost track of your rpm.Are you saying it now is at 4,800 untill you trim and it then jumps to 5,400?Is that jump accompanied with proper additional speed?
or does it seem like slip?
It certainly possible that when trimmed down you could lose signifigant rpm,
different from your old prop.Put the old prop back on; at least it may prove the change is prop oriented or not.

 

[countvlad]

Re: new prop... low numbers need help

I've kind of lost track of your rpm.Are you saying it now is at 4,800 untill you trim and it then jumps to 5,400?Is that jump accompanied with proper additional speed?
or does it seem like slip?
It certainly possible that when trimmed down you could lose signifigant rpm,
different from your old prop.Put the old prop back on; at least it may prove the change is prop oriented or not.

yes... now its 40mph@4800 until i trim the motor out and the rpms and speed jumps to 45mph@5400 rpms... no, no slip...

yes, very different from my old prop... i always got to 40mph@ 5800 RPMS but never gained any rpms by trimming out just speed 43mph...

the other difference between the two props is that the old prop i could cruise at 28mph@4200 rpms and with the new 33mph@4100rpms....

if not mistaken i should be able to achive manuf. recomended WOT with the motor down(trim down) right? as of right now the only way i can get that is by trimming the motor up which if im not mistaken is not the correct way.. right?

i took a few pics of the anti-cav plate compared to the hull and plate is slightly higher than the bottom of the hull... i will post tomorrow

 

[trendsetter240]

Re: new prop... low numbers need help

yes... now its 40mph@4800 until i trim the motor out and the rpms and speed jumps to 45mph@5400 rpms... no, no slip...

yes, very different from my old prop... i always got to 40mph@ 5800 RPMS but never gained any rpms by trimming out just speed 43mph...

the other difference between the two props is that the old prop i could cruise at 28mph@4200 rpms and with the new 33mph@4100rpms....

if not mistaken i should be able to achive manuf. recomended WOT with the motor down(trim down) right? as of right now the only way i can get that is by trimming the motor up which if im not mistaken is not the correct way.. right?

i took a few pics of the anti-cav plate compared to the hull and plate is slightly higher than the bottom of the hull... i will post tomorrow

Different prop blade designs yield different results. Just because two props are 17" pitch does NOT mean they will perform the same. RPM, Speed and handling can change drastically.

WOT numbers should be taken when fully trimmed out, not when trimmed in. So it sounds like you are still about 600RPM shy of where you want to be at WOT.

The same stiletto prop in a 15p under the same conditions should allow you to just touch on 6000RPM fully trimmed out. I would expect you will see about the same top end speed, maybe 1 or 2 MPH difference. The important thing is that your motor will not be overloaded and will have a longer useful life.

If you have a prop shop in your area then you can likely get that one adjusted to remove 2" of pitch.

Also if the prop isn't ventilating easily right now then you could try raising the motor another hole and pick up a few hundred RPM just from that.

 

[1kruzer1]

Re: new prop... low numbers need help

Different prop blade designs yield different results. Just because two props are 17" pitch does NOT mean they will perform the same. RPM, Speed and handling can change drastically.

WOT numbers should be taken when fully trimmed out, not when trimmed in. So it sounds like you are still about 600RPM shy of where you want to be at WOT.

The same stiletto prop in a 15p under the same conditions should allow you to just touch on 6000RPM fully trimmed out. I would expect you will see about the same top end speed, maybe 1 or 2 MPH difference. The important thing is that your motor will not be overloaded and will have a longer useful life.

If you have a prop shop in your area then you can likely get that one adjusted to remove 2" of pitch.

Also if the prop isn't ventilating easily right now then you could try raising the motor another hole and pick up a few hundred RPM just from that.

With all of the information pieces added bit by bit, this now all adds up. You changed from a less efficient, higher dynamic slippage prop to a highly efficient, low slippage prop. Changing to a 15 pitch Stiletto prop as Trendsetter suggests should set you up just about perfectly.

 

[countvlad]

Re: new prop... low numbers need help

Different prop blade designs yield different results. Just because two props are 17" pitch does NOT mean they will perform the same. RPM, Speed and handling can change drastically.

WOT numbers should be taken when fully trimmed out, not when trimmed in. So it sounds like you are still about 600RPM shy of where you want to be at WOT.

The same stiletto prop in a 15p under the same conditions should allow you to just touch on 6000RPM fully trimmed out. I would expect you will see about the same top end speed, maybe 1 or 2 MPH difference. The important thing is that your motor will not be overloaded and will have a longer useful life.

If you have a prop shop in your area then you can likely get that one adjusted to remove 2" of pitch.

Also if the prop isn't ventilating easily right now then you could try raising the motor another hole and pick up a few hundred RPM just from that.

sounds good... it makes sense...

i'll probably try to raise the motor a hole and go from there...