same speed with different props

[jonny rotten]

mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?
All things were equal. Weight gas ETC
GPS checked in car and is dead on

 

[flashback]

Off hand, I say you lost 400 rpm and ran out of motor because of the higher pitched prop..If your engine could still generate the 4800 rpm you would go faster..

 

[Chris1956]

Flashback has one cause. Here is another. Some boats have hulls that cannot be driven faster, due to their design. Straight vee hulls are an example of those. They simply do not trim very much.

As you may know, Trimming reduces the amount of hull in the water, lessening resistance.

Not knowing what kind of hull you have, there is no way to tell if you should have gained speed. Did you trim to max in both trial runs? Max trim is where the trim is as high as it will go short of ventilation or porpoising.

 

[Scott Danforth]

mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?
All things were equal. Weight gas ETC
GPS checked in car and is dead on

your hull wont let you go any faster without a lot more motor than what you have
your 19p prop sounds like a much better match to the motor than the 21p which is lugging the motor

as you load the motor up, you back up the torque curve.

 

[racerone]

Test I did years ago.-----Same boat , same afternoon ,same style stainless props, same motor 115 HP Evinrude.-------19 "--5800 RPM --54 MPH.---21"--5400 RPM --54 MPH.----23" ---4900 RPM --54 MPH.-------Speed of the boat did not change.

 

[Scott06]

Mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?
All things were equal. Weight gas ETC
GPS checked in car and is dead on

to go faster you need more power, your engine makes the same power from 4400-4800 rpm.

I ran into the same issue played with different props and you are hitting the wall in terms of what that engine and hull combo can do, engine doesn't have enough power to turn the higher pitch prop any faster...takes about 10 more hp to go 1 mph faster.

The only way you may be able to get more speed is if a different prop will allow you to trim up more without blowing out /loosing grip. I found a 21" 4 blade Revolution 4 gave better top speed than a 22" Enertia because I could use all the trim available before you hit the trim limit. Hull came out of the water more and got a few more mph...

Also if you are comparing fairly pedestrian aluminum props might eek a bit more with a Stainle$$ Steel prop

 

[QBhoy]

Welcome to the magical world of props.
Could write chapter and verse about the reasons you find what you have. But short version might be that different props behave differently. But also as relevant is that it’s not always the case that more pitch means more speed. Especially when you are seeing the best from the rpms with less pitch like you were. More rpm on some props of lesser pitch than others…can mean more lift from the hull with the prop doing what it’s designed for. Loads of other things I could mention…but that’s the two main reasons to explain what you’ve seen there.

 

[Bondo]

your engine makes the same power from 4400-4800 rpm.

Ayuh,..... I agree with 'bout everything said in this thread,.... except this sentence,.....

With our marine motors, with decent induction, peak horse power is pretty much dead on at 4800 rpms,....

'n I'd rather bump 5 grand now, 'n then, than to run at any speed if it could only pull 4400 rpms,.....

 

[Scott06]

Ayuh,..... I agree with 'bout everything said in this thread,.... except this sentence,.....

With our marine motors, with decent induction, peak horse power is pretty much dead on at 4800 rpms,....

'n I'd rather bump 5 grand now, 'n then, than to run at any speed if it could only pull 4400 rpms,.....

I thought that the reason they give a wot rpm range is the power in that rpm range is basically the same. The dyno curve on the vortec flattens out in that rpm range Some engines have tighter wot range hence the one inch pitch increments in props now.

and i completely agree prop to the top of the rpm range or just above it

 

[Bondo]

I thought that the reason they give a wot rpm range is the power in that rpm range is basically the same. The dyno curve on the vortec flattens out in that rpm range Some engines have tighter wot range hence the one inch pitch increments in props now.

and i completely agree prop to the top of the rpm range or just above it

Ayuh,.... Every dyno chart I've seen show peak, at the peak,.....

Not arguin', just sayin',......

 

[QBhoy]

Ayuh,.... Every dyno chart I've seen show peak, at the peak,.....

Not arguin', just sayin',......

Agree. My vortec defo at its best above 4700…closer to 4900-5000 the better for mine

 

[tpenfield]

mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?

It's called horsepower (HP) and you are at the maximum horsepower that the engine can put out . . . and the fact that 2 different props give you the same resulting speed demonstrates it.

 

[QBhoy]

mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?
All things were equal. Weight gas ETC
GPS checked in car and is dead on

Hi. What make and model were each of the props ? But a really think the main reason is that most boats with these engines, especially the 4.3, like to run right at the top of the rpms. Like I say…most good props are at their best with more rpms…more rpms let’s then do what they were designed to do. Good chance you’re getting more lift with more rpms.

 

[tpenfield]

More often than not, folks find that propeller swapping does not make a world of difference unless the initial prop was a bad match for the boat/engine.

For those who believe in what I call "propeller magic", have at it trying to find what isn't there. The rest of us will be on out on our boats

What boat is this - BTW?

 

[jonny rotten]

More often than not, folks find that propeller swapping does not make a world of difference unless the initial prop was a bad match for the boat/engine.

For those who believe in what I call "propeller magic", have at it trying to find what isn't there. The rest of us will be on out on our boats

What boat is this - BTW?

1990 19 ft Glastron

 

[airshot]

That co pilot seat sure looks comfy !!!

 

[Texasmark]

Test I did years ago.-----Same boat , same afternoon ,same style stainless props, same motor 115 HP Evinrude.-------19 "--5800 RPM --54 MPH.---21"--5400 RPM --54 MPH.----23" ---4900 RPM --54 MPH.-------Speed of the boat did not change.

This right here verifies the other replies. Good answer. Sooooo get your 19P back on and your engine will like you better. Also not lugging, your engine may get you a little more MPG.

 

[jimmbo]

Test I did years ago.-----Same boat , same afternoon ,same style stainless props, same motor 115 HP Evinrude.-------19 "--5800 RPM --54 MPH.---21"--5400 RPM --54 MPH.----23" ---4900 RPM --54 MPH.-------Speed of the boat did not change.

I did the same in my teens with my dad's boats, and later with my own, pretty much the same results

 

[jimmbo]

mercruiser 4.3 Alpha 1 gen 1
With a 19 pitch I hit 39 mph WOT.
RPM around 4800
Switched to a 21 pitch and hit the same 39 MPH WOT
With the 21 it's a bit more work getting out of the hole and the RPM dropped to around 4400 so the prop is making a difference correctly as far as that goes;
Can't understand why I'm not gaining any speed. Any ideas?
All things were equal. Weight gas ETC
GPS checked in car and is dead on

Try a 23, which is more of a "Speed Prop" than the 21, the result more impressive