Get me in the ballpark please.

[ckraker]

1989 19' four winns Horizon. 4.3 Mercriuser, bored . 030 over with 4bbl carb, est 220hp. Got alpha one out drive with 1.61 gears. Just want to be able have all around decent performance.

I started with a 14.5" 3 blade aluminum at 19p. Revs easily climbed well past 5500. Then i tried a stainless 3 blade 13.75" with a 21p. Not much change in performance. Anyone with a similar setup? What are you running?
Thanks

 

[porscheguy]

Sounds like you need to verify the tach accuracy. Even with a modestly built 4.3, going through a 1.61 ratio, seems like it would have a hard time with a 21p prop. Do you know what kind of prop it is?

what kind of speeds are you seeing?

 

[ckraker]

Funny you say that, i actually just bought a digital tachometer to check things out. It didn't sound like it was over reving. But i wanted to trust the tach over my ears. The boat is new to me so i can't go off what it sounded like in the past. I also blew the head gasket which made me think i may have ran her to fast. But that could be a completely separate issue. Thanks for your thoughts

 

[QBhoy]

You’d easily turn a 21” if not a 23” with this.

 

[ckraker]

What diameter?

 

[jimmbo]

A 19 should be about right for that unit, I too question the accuracy of your tach

 

[Watermann]

Diameter means little other than it will fit without hitting, it's all about pitch. Your gear ratio should be 1.84. Going from a 19p to a 21p should've dropped RPMs by 400. What was speed by GPS?

 

[ckraker]

From what I've read diameter makes a big difference. I increased pitch, but dropped diameter. So my rpms didn't change much at all. 40mph was top speed with a gps reading.

 

[ckraker]

Also, the out drive isn't original to this boat but it is off of another 4.3. i turned the input shaft one turn and the propellar spun just over half a turn. So i did some digging and found that 1.61 was an available option for that motor/year.

 

[WesNewell]

A half turn would be a 2.0 ratio. A 3/4 turn would be a 1.5 ratio iirc, so just over a half turn should put it somewhere between 1.5 and 2.0, depending of what you consider just over a half turn. A 1.61 would put it closer to 3/4 turn than just over half. At least I think that's right.

 

[Watermann]

From what I've read diameter makes a big difference. I increased pitch, but dropped diameter. So my rpms didn't change much at all. 40mph was top speed with a gps reading.

Ok, then buy your new prop by diameter if that's the right way to do it. Good luck

 

[ckraker]

Ok, then buy your new prop by diameter if that's the right way to do it. Good luck

Never said that diameter was the "right way"--- Just saying that it matters. You can't just disregard the diameter.

 

[00bullitt]

Never said that diameter was the "right way"--- Just saying that it matters. You can't just disregard the diameter.

I very much agree that diameter and pitch go hand in hand. It can't be ALL about pitch. Diameter is directly related to the props ability to carry....or lift a load. With all things being equal, a 14" and 15" prop will lift the hull differently. RPM's will also differ. Diameter is very dependent on the hull design as well.

A 14" x 19p prop may only slightly lift the bow at 4800 rpms at a speed of lets say 45mph.

A 15" x 19p prop may lift the bow and stern at 4600rpms but 50mph. The reduced RPM's are two fold, the prop is moving more water via the increased surface area and working harder to lift the boat and its turning more mass. The increased speed is from the hull producing less slip and being more efficient in the water making it easier for the prop to do its job and propel the craft forward.

 

[Watermann]

There's prop features that are more important than diameter, cupping and rake mainly. Prop surface area is a factor yes when talking 4 blade or a high 5 but it means little when talking about 3 blade 1/4" - 3/4"

 

[00bullitt]

All sorts of prop features mean more than diameter, cupping and rake mainly. Prop surface area is a factor yes when talking 4 blade or a high 5 but it means little when talking about 3 blade 1/4" - 3/4"

Blade design certainly is a factor as much as diameter. I certainly have found duameter to be the case on certain hull designs. I recently helped a buddy prop his Skeeter Fish & Ski with a 150 Yamaha outboard.
A 14.25" diameter would not lift the boat. It was a Yamaha 23m that had some dings in it. The prop shop reduced diameter and returned the blades to factory spec. Boat wouldnt get out of the water; barely lifted the bow and wanted to porpoise.
Got a new factory 23M in its full 14.5" diameter and the boat became magical; bow and stern both got up on top of the water. Went from 53mph at 5600 rpms to 59mph at 5400. Same props, same pitch; .25" diameter change. Ive seen the same with other boats and props. The diameter almost always had a correlation to the props ability to lift.

We can throw all kinds of variables out there, but to make a statement that its all about pitch is misleading, don't ya think?

 

[Watermann]

Blade design certainly is a factor as much as diameter. I certainly have found duameter to be the case on certain hull designs. I recently helped a buddy prop his Skeeter Fish & Ski with a 150 Yamaha outboard.
A 14.25" diameter would not lift the boat. It was a Yamaha 23m that had some dings in it. The prop shop reduced diameter and returned the blades to factory spec. Boat wouldnt get out of the water; barely lifted the bow and wanted to porpoise.
Got a new factory 23M in its full 14.5" diameter and the boat became magical; bow and stern both got up on top of the water. Went from 53mph at 5600 rpms to 59mph at 5400. Same props, same pitch; .25" diameter change. Ive seen the same with other boats and props. The diameter almost always had a correlation to the props ability to lift.

We can throw all kinds of variables out there, but to make a statement that its all about pitch is misleading, don't ya think?

A 1/4" diameter increase is magical? Alright good to know. more surface area lowers RPMs and increases speed too. I guess it's wrong saying all things being equal it's all about pitch to increase or decrease RPMs.

 

[porscheguy]

FWIW, you can’t return a prop to factory spec while removing 1/4” from the diameter.

As far as diameter goes, yes it does matter, but it doesn’t matter like pitch.

 

[00bullitt]

Yes, good point. Let me clarify.....they made sure it was 23p across the geometry minus the ground off diameter. My point was mostly that the small reduction in diameter made a massive difference in the performance of that specific hull.

I have propped a many of bass boats in the past 30 years. I have always seen a correlation to performance vs. diameter. I know a bass boat is a different animal than most planing hulls, but much of the same theory holds true.

I have my first stern drive boat and am already validating the above. I just picked up a 19p Rev4 and 20p Enertia.....very much look forward to running them next week to see the contrast to what is on the boat now.