Prop pitch change & 3-blade to 4-blade

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sh00ter

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I am reading stories on aluminum boats with 40-60hp that they went to the spitfire 4 blade and actually gained speed & RPM...However, I see some opposite reports...Generally, you are supposed to drop a pitch when you go 3-blade to 4-blade. But some of the accounts I've read reported people increasing the pitch by one increment when they went 3-blade to a 4-blade and then they not only got better holeshot, they also gained MPH. I am most concerned about gaining MPH. My boat right now runs about 36mph @6100rpm trimmed out with just me and the stock 3-blade on the 50hp merc. I want to gain speed and any improved holeshot is secondary, but i want to gain speed with a 4-blade. I am already up to the max RPM so I am not looking to increase RPM or lose it.

Would a single INCREASE in pitch and going to a 4-blade accomplish more speed? Again, most say go down a pitch but they also lose speed. I went 4-blade and dropped 3 pitch increments on my big boat and it increased RPM and decreased MPH so I have to assume the same for the littler boat which is why I want to go up instead of down as an offset.

Here is a guy with a similar boat (not exact same) who went up a pitch when he got a 4-blade and he got better speed as I am hoping.



Thanks
 

chuter

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I had a 3 blade aluminum on my Tracker 175 75hp and I replaced it with the 4 blade Mercury Spitfire X7, I think I gained about 3mph. I didn't really have a lot of time in the boat on the old prop to make a good comparison.
I got the same pitch for the 4 blade that I had with the 3 blade, I saw that recommended somewhere.
 

swampratt

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I have changed props many times in a few different boats and what I got was you must try and see.

Sometimes going down increased MPH sand then going up increased MPH.
Had some 3 blades without the cup at the edge of the blade that would slip really bad and not MPH as well as a cupped blade
with the same pitch but ran higher RPM.
Built a 355" inboard for a 21 foot ski boat.
Guy owned a boat shop and we tried many blades one day to see which one was best.
They were all stainless and the 3 blade was faster than a 4 blade same pitch and different pitches.
We tried from 19-27 and many in between.
Got 68 MPH GPS with 3 of us in the boat on slightly choppy water, Eufaula lake with small white caps.

You get 36 MPH with a 50 HP to me that is excellent.
My little Bayliner from 1975 with the 1975 50HP merc 4 cylinder 2 stroke will only go 28.

I have a 15 pitch to get that and tried a 13 but gained nothing but a hole shot. No way will it handle larger than 15 though on my boat I would never get the nose down with the fishing gear in it.

But like I say try one and see what you get.
You always need spare props in the boat anyway.
I do at least.
 

jakeman

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Long time boat owner here.

The ever present need for more speed is an affliction that doesn't go away easily.

This is what I know. 15 years ago it would have cost me in the neighborhood of $3700 to pick up about 5 GPS MPH thru engine modifications and additions. I decided that 55 GPS MPH in a 21' bowrider was plenty fast, because I wasn't going to spend that kinda cash.

So I started jacking with props. 3 blade. 4 blade. 5 blade. Pitches from 17-23. I don't think my actual MPH changed more than about 3 mph from top to bottom with the exception of the 5 blade. That was a speed stealing mf'er, but I could get on top quicker than a hiccup. It was crazy how fast that thing would throw that boat out of the water, but about 48 mph was as fast as she would run.

If you're looking for big changes from a prop or pitch change, you're going to be disappointed.
 

Cowbaby

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I traded a guy a 3 blade 17 pitch for a 3 blade 21Raker only because his was stainless and mine wasn't. I can go 38 with a 50hp Evinrude which is about 2 mph faster but hole shot now sucks bigly, takes forever to plane out.

Wish I had my 17 back and you can have that 2 miles and hour and running wide open throttle and half the gas mileage to get it.

That old 17 hit plane lickety split and could throw you out back in the seat. You could run twice as far by planing out and then backing it to about 3/4 throttle and go fishing twice on a 5 gallon tank instead of now once.

But, I like to fish more.

Easiest way to get all you can with what you got is ditch all the unnecassary weight you can.
 
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sh00ter

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I had a 3 blade aluminum on my Tracker 175 75hp and I replaced it with the 4 blade Mercury Spitfire X7, I think I gained about 3mph. I didn't really have a lot of time in the boat on the old prop to make a good comparison.
I got the same pitch for the 4 blade that I had with the 3 blade, I saw that recommended somewhere.

What year 175? what is the stock pitch on the prop that came with it? What is your top speed now with 1 in the boat?

On my boat i put on a hydrofoil and it allowed me to gain speed by not porpoising at higher trim. I am pretty sure I picked up a couple MPH that way but i never GPS'd it before the foil.

I am now looking at a jackplate for shallow water fishing but I think if the motor is moved up I could gain MPH that way too with no prop changes. The foil did also increase holeshot, but not as dramatic as expected with the 3-blade still on it, but it can be on plane at a lower speed.

Long time boat owner here.

The ever present need for more speed is an affliction that doesn't go away easily.

This is what I know. 15 years ago it would have cost me in the neighborhood of $3700 to pick up about 5 GPS MPH thru engine modifications and additions. I decided that 55 GPS MPH in a 21' bowrider was plenty fast, because I wasn't going to spend that kinda cash.

So I started jacking with props. 3 blade. 4 blade. 5 blade. Pitches from 17-23. I don't think my actual MPH changed more than about 3 mph from top to bottom with the exception of the 5 blade. That was a speed stealing mf'er, but I could get on top quicker than a hiccup. It was crazy how fast that thing would throw that boat out of the water, but about 48 mph was as fast as she would run.

If you're looking for big changes from a prop or pitch change, you're going to be disappointed.

Well speed not everything for my larger boat..I lost speed going from a 23pitch 3-blade to a 20pitch 4-blade but it is much better for a loaded down boat and pulling tubers. It prob only runs low 40's now, but for what I'm doing it is fine. But on the fishing boat, I wanted the ability to fish skinny water but it would be nice if I could still get to a spot a little faster. I can remove the restrictor ring and flash or replace the ECU but that is a lot of money for 10hp. But then again, so is a jackplate but at least I could run better in shallower water.
 

dlbleak

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When I was looking at getting a 4 blade for my inboard V6, IIRC, the opinion was to go up or down one pitch angle? I don’t remember which way was suggested
Edit- I ‘think’ I borrowed my neighbors 4 blade 21 pitch in place of my 3 blade 18 pitch. Lost the holeshot but my middle and top end was awesome. It pushed over the redline though and I had to be careful.
 

magna19

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Long time boat owner here.

The ever present need for more speed is an affliction that doesn't go away easily.

This is what I know. 15 years ago it would have cost me in the neighborhood of $3700 to pick up about 5 GPS MPH thru engine modifications and additions. I decided that 55 GPS MPH in a 21' bowrider was plenty fast, because I wasn't going to spend that kinda cash.

So I started jacking with props. 3 blade. 4 blade. 5 blade. Pitches from 17-23. I don't think my actual MPH changed more than about 3 mph from top to bottom with the exception of the 5 blade. That was a speed stealing mf'er, but I could get on top quicker than a hiccup. It was crazy how fast that thing would throw that boat out of the water, but about 48 mph was as fast as she would run.

If you're looking for big changes from a prop or pitch change, you're going to be disappointed.
That pretty much is right!!!! Ive played with props/pitch/light /heavy/fiberglass/aluminum boats since 1976 and every time wished I had the prop that ran in the upper rpm range of the motor. Reason being sometimes my load increased for numerous reason people/gear/water/rough conditions. The mph chase is not worth it unless its just what you want to do. No problem Ive been there. Just remember 3 mph just feels alot faster
 

jakeman

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That pretty much is right!!!! Ive played with props/pitch/light /heavy/fiberglass/aluminum boats since 1976 and every time wished I had the prop that ran in the upper rpm range of the motor. Reason being sometimes my load increased for numerous reason people/gear/water/rough conditions. The mph chase is not worth it unless its just what you want to do. No problem Ive been there. Just remember 3 mph just feels alot faster


I gave up the speed chase and settled on a 21 3 blade ballistic. The 5.7 250 hp alpha turned it pretty nicely and on a calm morning with me & the wife and a full Yeti 65 & 42 gal of gas it would run 53-55 GPS MPH (depending on temp) pretty easily. Hole shot was okay, but not great. Was hard to cavitate, but you could do it if you weren't careful. That was fast enough for me. The hull would have taken more speed, but alas, my wallet could not.
 

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