Will I see a speed and rpm improvement?

[Chas0218]

Currently I have a 92 Johnson 150 behind my boat with a 15.5x15p prop and pull about 5400rpm trimmed. Will I see see any improvement in my speed if I drop to a 15"x17p prop? I would like to keep my rpms about the same or a little lower? Currently I pull about 32mph but would like closer to 35-40. My motor doesn't like smaller diameter props I have tried a 14"x22p and only pulled 4000 rpms and with a 14"x 19p i pulled 4500rpms.

Thanks,
Chas

 

[Chinewalker]

Re: Will I see a speed and rpm improvement?

Going up in pitch will lower your RPM, and likely your speed as well. You actually want to aim for just above 5500 RPM....

 

[Chas0218]

Re: Will I see a speed and rpm improvement?

I just went on Turning Point website and they recommend a 14-1/4 x 17. Will this prop make much of a difference for my motor? The diameter is smaller but the pitch is about the same.

 

[Chinewalker]

Re: Will I see a speed and rpm improvement?

Losing an 1.25 inches in diameter may make some difference, but that will probably be eaten up by the 2-inches extra pitch. My gut says that one will be a push. If you could go smaller in diameter and stay the same pitch, that would probably be your best bet. You're close as it is - only 100 revs off the base mark. Going down an inch in pitch (if you can find a 14) might get you smack in the middle of where you want to be...

 

[bob johnson]

Re: Will I see a speed and rpm improvement?

so you know dont even think about running anything with more pitch than a 17!!!

if your motor is running good and you already have played with the trim to maximize performance...then there isnt much else to do other than play with styles of props around 15-17 pitch.

i think you are at a good rpm right now!!

i would want to go with LESS rpms????

set your motor up to rym max rpms!!! is it a looper or a crossflow? the oceanrunners and ocean pros can be run up to 6000K easily...you wont over work the motor. just watch the temps...thats your sign of impending trouble.....make sure the carbs dont lean out....

most of the bad stuff starts with temperature rises!!!

is your prop SS or aluminum now?

bob

 

[Chas0218]

Re: Will I see a speed and rpm improvement?

its aluminum and I'm not sure but I think its a looper motor. It is just a normal 150 vro.

 

[dazk14]

Re: Will I see a speed and rpm improvement?

What boat is this running on? Do you carry a lot of fuel and gear.

What brand/model props have you tried. Stainless or aluminum?

Where is the A/V plate relative to the bottom of the boat.

 

[steelespike]

Re: Will I see a speed and rpm improvement?

answer the Qs above then::
You need to get us a full report boat size weight,remind us of the year of the motor, prop size and the wot rpm and speed.
We need to know if your present setup is working.

 

[Chas0218]

Re: Will I see a speed and rpm improvement?

What boat is this running on? Do you carry a lot of fuel and gear.

What brand/model props have you tried. Stainless or aluminum?

Where is the A/V plate relative to the bottom of the boat.

It is on a 1981 23' Proline W/A 3700lbs. loaded
Motor is a 1992 150hp johnson not sure on model # but can get that this weekend.
Current prop: 15.5"x15P 5300rpm at WOT 32mph

Previous props:
14.25"x21P 4300rpm WOT
14.5"x22P 3800rpm WOT

All props are aluminum.

Cav. Plate is just below the hull ~1" when trimmed all the way down.

 

[Chas0218]

Re: Will I see a speed and rpm improvement?

Anyone?

 

[Philster]

Re: Will I see a speed and rpm improvement?

Motor is a little low.

 

[sublauxation]

Re: Will I see a speed and rpm improvement?

My motor doesn't like smaller diameter props I have tried a 14"x22p and only pulled 4000 rpms and with a 14"x 19p i pulled 4500rpms.

Thanks,
Chas

What you're saying here is that your boat doesn't like props with a larger pitch. In general RPMs go down with more Pitch.

If your Johnson is supposed to have max RPMs of 5500, then I'd say you are pretty close and the best way to get more RPMs is either drop to a 14 pitch aluminum prop of the same model you have now or start looking into stainless props. From my experience with a Turning Point Hustler I'd guess a 17 pitch will drop your RPMs by about 400, but as you'll find with props, that's only a guess. The only way to really know if you'll get more speed with any different prop is to hang it on there and test it out. That's why I now own 3 and am still not set up quite right.

 

[Philster]

Re: Will I see a speed and rpm improvement?

You can put on half the HP, get the prop buried too low and find a prop that gets max RPM's out of the engine. Don't mean you done good.

So... anyone else contributing to the thread: Ain't the prop too low, given to position of the antiventilation plate in relation to the keel? Isn't there an opportunity to move the motor height up, which will give back some RPM (which might allow some more prop pitch, which equals more top speed?)

 

[steelespike]

Re: Will I see a speed and rpm improvement?

As suggested if you have no venting issues you could raise the motor.Start at 1" above the bottom if it vents lower it a hole.
if no vent raise a hole.You may gain some speed and rpm.Right now speed seems a little low though the boats probably pretty beefy so slip is probably a little high.Is the speed by gps?

 

[Outsider]

Re: Will I see a speed and rpm improvement?

Cav. Plate is just below the hull ~1" when trimmed all the way down.

But where is it in relation to water level when at WOT? If it's anywhere except very slightly above water level, then you know what to do with the motor. If you're buried now, any affect of prop change would be false magnification ...

 

[topglock]

Re: Will I see a speed and rpm improvement?

Motor is a little low.

Agreed. Trim the motor 'til the A/V plate is parallel to the bottom of the hull. Then, using a straight edge as a reference point, lift the motor until it is about 1" higher that the bottom of the hull. Put it in the water and see what you get. Adjust as necessary using the readings you obtain...

 

[Chas0218]

Re: Will I see a speed and rpm improvement?

Agreed. Trim the motor 'til the A/V plate is parallel to the bottom of the hull. Then, using a straight edge as a reference point, lift the motor until it is about 1" higher that the bottom of the hull. Put it in the water and see what you get. Adjust as necessary using the readings you obtain...

When I go home this weekend I will throw a straight edge on the A/V plate and see how far under the boat it is. Thanks for the replies!