RPM vs MPH

[SweeperForce]

Hello,
I had my boat out for the fist time this season. She ran great. At WOT I was hitting 39 mph. However my rpms were only at 4600. I am using 21 pitch 3 bladed prop. If I drop to a 19 pitch my rpms will raise about 800, putting right where I want to be, 5500 approx. Will this make any difference on my speed?

Thanks,
Tom

 

[Tail_Gunner]

Re: RPM vs MPH

When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct...............
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful).......average load
5) Engine/drive make, model, year, and HP......
6) Boat make model, year, length and weight.....

 

[SweeperForce]

Re: RPM vs MPH

Sorry about the vagueness, so here are some details
1)No performance issues to be corrected
2) Aluminum 3 blade re-maned.
3) 14 1/2 inches 21 pitch
4) 3 adults, 1 dog and a cooler 650 lbs max speed 32// just me and dog 230 lbs max speed 39mph.
The rest in the info is either in my post or my signature

Excuse mu confusion.
Tom

 

[Bondo]

Re: RPM vs MPH

3) 14 1/2 inches 21 pitch
4) 3 adults, 1 dog and a cooler 650 lbs max speed 32// just me and dog 230 lbs max speed 39mph.

Ayuh,... Either lighten the load, or drop some pitch to get the rpms up...
Yer speed will be wherever it goes too...

 

[Silvertip]

Re: RPM vs MPH

The relationship between prop pitch and rpm is "theoretically" direct -- meaning "X" amount of pitch, with "X" amount of gear ratio, and "X" amount of rpm results in "X" amount of speed. Altering any one of those numbers "theoretically" does not increase or decrease speed. How can that be you ask. Well, in the current configuration you said 4600 rpm nets 39 MPH with whatever your load is. If you raise engine rpm by decreasing pitch, that means the engine must run faster to get the same amount of forward motion from LESS PITCH. Pitch remember is the "theoretical" forward movement of the boat in inches with each revolution of the prop. Since you want to decrease pitch by 2 inches, that means the boat also moves forward 2 inches LESS with each rev. Therefore it takes MORE revs to move the same distance as before. IF and I repeat IF the engine was lugging at 4600 rpm then one can see the rpm rise to 5000 (not 5500 with a 2-inch pitch change) and perhaps even a bit more. It is the "bit more" that results in any speed increase. But then if the engine is not making full power because it is either "tired" or in need of a tune up, then messing with props is an exercise in frustration. And then the tach may be incorrect as well. So don't buy props until you have this other stuff verified.

 

[rallyart]

Re: RPM vs MPH

If you change from a 21 to a 19 you have about a 10% change in pitch and should experience about a 10% increase in RPM. (unless there is a difference in the efficiency of the prop) When you change if you also go to a stainless prop you might pick up a bit more speed.
Silvertips comments are very good.

 

[Tail_Gunner]

Re: RPM vs MPH

OK lets start here: http://www.piranha.com/motor_specs.php?id=3085

Manufacturer: Force
Model: "Dual Exhaust Model"
Year: 1989
Horsepower: 150.0
Cylinders: 5



Type: 2 stroke
Gear Ratio: 1.93:1
Min WOT RPM: 5000
Max WOT RPM: 5500
Spline Count: 15
Drive Type: Outboard

21p @ 4600 @39 mph yeilds 17% slip....should be more like 21p @ 5500 @ 50mph yeidls 12% slip

Thats not good but the pitch sounds right.... Is that speed GPS verified.....has your tach been verified....IS the bottom of the boat clean with no growth.....Is your engine in a good state of tune.... Do you have the motor trimmed right.....How far above the keel does your motor cavitation ride

Before you do anything id check the motor for a tune...Verify speed with a gps and tach being verified

Looks like im backing up Silvertip here

 

[metriccrescentwrench]

Re: RPM vs MPH

I vote that he'll see a modest speed increase once the proper pitched prop is found that lets the engine run up to it's maximum rpm. At 4600 rpm the engine likely isn't making rated power, in fact probably a fair amount less. By going to a lower pitched prop you'll be allowing your engine to get into the rpm range where it can make more power, and more power equals more speed. (Of course this assumes a properly running engine.)