Speed Vs Rpm which is better

cub48

Seaman
Joined
Dec 17, 2009
Messages
54
I have a 1998 Stratos with a 150 Faststrike 18' long My boat runs within 100 rpm's of the speed does not appear to lug comes out of the hole good. Don't know what wide open is as don't want to run that fast but have ran at 50 mph and it runs 5000 rpm's at 50 mph. What got me on this is my Son-law has a 2006 Stratos with a 150 Johnson 19' long his boat runs 40mph at 5000 rpm's. I don't know the pitches but both are four blade stainless. Just wanting to know the correct ratio or rule of thumb as to RPM to Mph. I have heard of over pitch and under pitch. I know boat weight will affect this. I know some want to come out the hole faster and some want the top end. I am just wanting to know the best ratio is as per Mph vs Rpm. Cub48
 

wilde1j

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Apr 15, 2002
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5,964
Re: Speed Vs Rpm which is better

Motor should be propped to get 5800RPM @ WOT. 5000RPM IS lugging the motor, regardless. The downside of too low RPM is reduced engine life. The choice is yours. Forget the speed part of the deal. A lower pitch prop will get you more rpm and you will need to find out what your current prop is to proceed. If you can't find the pitch & diameter stamped on it, you may have to take the prop to a prop shop, who can readily tell you.
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
Messages
45,907
Re: Speed Vs Rpm which is better

You need to talk to some propsperts, cub.

I will move this to the prop forum.
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: Speed Vs Rpm which is better

As allready stated you prop for the correct rpm then speed.
Usually best speed will follow the correct rpm.
If I understand you right you don't usually run wot(wide open throttle)
And the 50 @ 5,000 is a partial throttle figure.
We do need your rpm at max speed and your prop size.(usually on the barrel or under the prop nut)Speed by a speedo is ok though a gps is best; speedo can be off by +/-10%.
 

cub48

Seaman
Joined
Dec 17, 2009
Messages
54
Re: Speed Vs Rpm which is better

Wide open throttle is this from a take off or running down the river. My prop said 13X25 it's a 4 blade Renagade prop. What i was talking about at 20 MPH I run about 2000 RPM 30 MPH - 3000 RPM 40MPH - 4000 RPM. I don't run wot I open it up enought to plane which it does fast enough. What has got me thinking is I rode in a boat just like mine only it is a foot longer and his rpm's are like at 20 MPH - 3000 RPM at 40 MPH - 5000 RPM got me thinking is mine right or is his wrong ??? Thanks Cub48
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: Speed Vs Rpm which is better

His is different thats all we can say without that max rpm and speed figure for both boats.Neither can be termed right or wrong without enough info.
They could both be wrong depending on the final figures.Your 4 blade would generally perform better in low to mid range than a similar 3 blade.But a 3 blade
may perform better at the higher end.This could all vary greatly depending on the brand,design, material and condition of the props.
Prop selection is based on the typical load,weight of the boat hull configuration
activities etc. Depending on the configureation a 1 foot longer boat could be several hundred pounds heavier.
 

180shabah

Rear Admiral
Joined
Mar 26, 2005
Messages
4,995
Re: Speed Vs Rpm which is better

Cub - you are putting way too much thought into this. Unless two boats are identicle in EVERY way, you really can't compare them. Different hull design, different prop, weight, gear ratio, etc....

For your boat, if you want to to know if the prop is correct, you will need to do a WOT run. That is balls to the wall, trimmed properly as fast as it will go. Then we need to know how fast it went, and at what RPM. Form there we work backwards.
 

cub48

Seaman
Joined
Dec 17, 2009
Messages
54
Re: Speed Vs Rpm which is better

Thanks Guys The prop is a 13X25 four blade Renagade. Saturday I opened the boat up I was not at wot but was trimed and running at 50 MPH and at 5000 RPM that is about as fast as i want to go I still had pedal left so i am sure it will run the 5800 RPM at wot with this prop. Went fishing today and did a couple of wot starts the rpm was around 4200 on take off wind was blowing so did not want to run very fast. The other boat I was talking about has the same prop as mine he was fishing today too we were talking he got on some smooth water in a cove his speedometer matched his rpm must of been the water the other day. He tried a 3 blade prop said it came out of the hole good but would blow out on the plane quicker than the 4 blade. Thanks Cub48
 

kenmyfam

Supreme Mariner
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Aug 10, 2006
Messages
14,385
Re: Speed Vs Rpm which is better

Cub, we really need to know your WOT RPM to proceed.
Just my 2 cents though.
 

180shabah

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Mar 26, 2005
Messages
4,995
Re: Speed Vs Rpm which is better

Part throttle numbers don't help. Forget the other boat it is completely irrelevent. Of course if your are happy with your current performance, you may not care if it can be improved.
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: Speed Vs Rpm which is better

  • Manufacturer:Johnson
  • Horsepower: 150 HP
  • Years: 1991 - 2005
  • Model: V6 Outboard
  • Specs: Thru Hub Exhaust & 15 Tooth Spline
  • Max RPM: 4500-5500 RPM
I take it that is your engine...If so acknowledge it....from there you may want to go to the outboard section of the forum and find the sweet spot or rpm that works well for that engine...Maybe Walleyehed of Dhadley will pop in and illuminate things...Anyways when you find the the range post it and then post your current performance..and you will get plenty of advise...another varible is engine height much can be done with outboards and that combo you have there should do about 60 or better i believe..lots of room for improvement..;)

By the way first it RPM...and then how much speed you can acheive with a prop that will balance that sled of yours to its maxium effecientcy
 

cub48

Seaman
Joined
Dec 17, 2009
Messages
54
Re: Speed Vs Rpm which is better

Thanks Tail Gunner this helps my train of thought as I know my boat will run in this range as Saturday i ran 5000 at 50 mph and still had pedal left i have a hot foot. I am not looking to run fast. Just wanted someone to tell me if your RPM matches Mph if your prop is right. Example 4000 RPM at 40 MPH
Not 4000 RPM at 20 MPH or 4000 RPM at 60 mph while you are running down the river. Cub48
 

wilde1j

Vice Admiral
Joined
Apr 15, 2002
Messages
5,964
Re: Speed Vs Rpm which is better

The only relationship between RPM and hull speed is normally higher RPM yields greater hull speed as long as nothing else is changed. The relationship is not linear, however, and typically it takes more RPM at higher speeds to achieve relatively less of a speed increase.

Another way of looking at the relationship is if you were to replace your motor with one having 10% more HP, you won't get anywhere near 10% more speed.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Speed Vs Rpm which is better

cub,
I am just wanting to know the best ratio is as per Mph vs Rpm.

There is no such thing as ratio of MPH versus RPM, as is it is different with every boat and motor, some can have as low as 5,500 RPM at 15 MPH and some can have a ratio of 5,500 RPM at 75 MPH.

What you need to be concerned about is that your motor needs to be able to turn about 5,800 RPM with todays gas.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057



H
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Speed Vs Rpm which is better

Cub,

What everyone is trying to tell you is that the prop pitch determines the WOT RPM which directly determines the LOAD placed on the motor at ANY RPM.

A motor that cannot turn up to the rated max RPM has TOO MUCH LOAD and will not live near as long as it would with the correct pitch prop. No matter what RPM you 'usually' run the motor, overloaded is overloaded. PERIOD.

It is CRITICAL to know the WOT RPM for a prop.

Boat speed comes later, first determine if you are killing your motor because of too much pitch.
 
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