Upgrading prop

[gim]

I've come across some winnings in my fantasy football league and decided to upgrade props for my outboard.  I've been using the factory prop that came with the boat when I bought it in 2015.  Its a 4-blade aluminum prop.  Its still in good shape, has a couple scratches and missing some paint but nothing more.  I've been happy with it.  I have a Ranger RT178 with a Merc 75 hp 4-stroke outboard.  I can get about 34 mph out of my current setup with gear and another person in the boat.  I also have a 12 foot stern mounted Talon that adds weight and possible wind resistance.

 

I'm considering a stainless steel prop.  Not just because I think I would get a higher top end speed, but overall performance would also be improved.  I don't fish big lakes more than about 5 or 6 times per season, and all of the accesses I use are concrete.  Some can be shallow during a low-water season (like last year).  Do you think its worth trying a SS prop or should I try a 3-blade aluminum one?  The cost is not an issue but I'm just wondering if in my current situation would it be worth it.  Thanks for the advice in advance.

[A-Jay]

Rarely ever hear of anyone complaining about a drop in performance when going from aluminum to Stainless steel.

Usually the other way around.

Also, and there will no doubt be some different opinions on this,

however, "still in good shape, has a couple scratches and missing some paint"

actually means that at some point you hit something with your prop, probably more than once.

While it may not be 'obvious' to you, your prop is most likely damaged enough that it's performance is suffering.

I'd get a new one just for that reason alone.

IMO, SS is the superior performer and you may like a three blade better.

Contacting your Ranger/Mercury dealer for accurate prop application information is a solid plan.

Good Luck.

A-Jay

[NHBull]

^^^^^This

[GaryH]

What AJ said.  SS will out perform aluminum all day long. A 4 blade will usually give you a better hole shot and slightly less top end.  But as said above call mercury or a reliable prop shop and tell them what your looking for as far as performance.

[gim]

2 hours ago, A-Jay said:

however, "still in good shape, has a couple scratches and missing some paint"

actually means that at some point you hit something with your prop, probably more than once.

While it may not be 'obvious' to you, your prop is most likely damaged enough that it's performance is suffering.

 

This is interesting.  There is no visible damage to the shape of the prop itself.  Black paint has worn off on a couple of blades in 2 spots and there is a scratch that I believe I caused when walking past it carrying something in the garage.  You think that missing paint and a scratch is enough damage to cause performance issues?

 

There is a prop shop not far from me and I intend to take the current prop in there when the time comes.  Their inventory shows they have several models of SS and aluminum props in stock and they are factory authorized to service the main motor brands.

[A-Jay]

Whenever this topic comes up, my response will always be the same, including now.

If your prop does not look and feel new, it is not performing like new. So scratches, dings, dents, bends, missing paint and or blemishes of any kind often indicate this.

My advice is to save yourself some time / money.

Get with your dealer and purchase a new ss prop.

Don't bother testing the old one, just retain it for an emergency spare. 

Also this is a perfect time to  check and ensure that your prop shaft is clean and running true.

A-Jay 

[slonezp]

3 blade SS. No need for a 4 blade unless you're fishing rough conditions or pulling skiers. 

An aluminum prop will flex under load and even though you don't see anything jumping out at you, you may have lost some performance. 

 

Don't just go and buy a prop. See if you can find a dealer that can loan you different props to try until you find the one that hits the sweet spot.

 

[K1500]

I’ve got a 20’ bay boat with a 115 pro XS CT and I went from a 4 blade 17 pitch aluminum to the same blade in a 19 pitch and gained around 6-7 MPH. I did lose a little bit of hole shot. Although I haven’t pulled a tube with the 19 pitch I can tell I will be way better off with the 17 pitch for that application.
 

There are charts listing prop recommendations. You will do best if you know your boat motor max RPM and your speed when running at max RPM (or max the motor will go). Mercury has a pretty good calculator and they were spot on in the RPM drop and MPH gain I saw when switching props. 

[WRB]

Think of a 4 blade prop as a lower gear ratio a 3 blade the next higher shift. 3 blade you loose torque under load in boating that is the lower rpm hole shot, it takes longer to up on plane with a load. 3 blades gain higher speed like shifting up a gear everything being equal. 

I would take AJays advice and consult a dealer.

75 hp power should be about 17 Pitch SST 3 blade.

Tom

[Way2slow]

Comparing props is like comparing paints, they all can look similar but the performance of each is totally different.

Unless you are totally familiar with prop blade designs, pitch, rake, cup, geometry, material etc, there is no way to can say whether a 3 blade or a 4 blade is better. 

A high performance SS four blade, designed to suit your hull and engine combination will out perform any aluminum prop, three or four blade, and most generic three blade SS props.  

Now, when you are comparing similar blade designs of the same material, a three blade might be one or two mph faster simply because of the extra friction the fourth blade has, but I have run custom tuned four blades that were just as fast as a tuned three blade.

[gim]

On 1/12/2021 at 2:03 PM, slonezp said:

See if you can find a dealer that can loan you different props to try until you find the one that hits the sweet spot.

Update on the prop dilemma this spring.

 

I took my used 4 blade factory aluminum prop in for reconditioning. While it was in there, I bought two 3-blade props with the intention of trying both and returning one when I picked up my 4 blade being worked on.

 

The 2 props I tried are a 3 blade SS and a 3 blade aluminum. After using both more than once, I have decided to go back to my old 4 blade aluminum. Both 3 blades took way too long for my boat to get on plane and neither one resulted in a faster top end speed. The 3 blade aluminum goes about 4 mph slower and the 3 blade SS goes about 2 mph slower.

 

I purchased an exact new 4 blade aluminum to my original when I returned the two 3 blade props. I kinda chewed out the guy there too because he was adamant I’d get what I was looking for out of a 3 blade.

 

The lesson to be learned here is that the only way to find that “sweet spot” is to actually try it. I found it. It’s a 4 blade aluminum prop and I’m sticking with it.

[Michigander]

I would call Mercury, you can usually talk with one of their engineers. If you are specific about what you want ( better hole shot, higher top speed, etc) they can recommend a prop with the pitch and blade count for your hull and motor.

[K1500]

The worn spots of paint on your old prop may be cavitation burn, which can be caused by a variety of factors. It’s never a bad idea to have a spare prop, and the aluminum props are cheap enough that it isn’t a big deal to keep a spare. As my earlier post indicated, I use a 19p 4 blade for fishing and switch to a 17p 4 blade for summer fishing and tubing. 

[gim]

7 hours ago, K1500 said:

It’s never a bad idea to have a spare prop, and the aluminum props are cheap enough that it isn’t a big deal to keep a spare.

I agree.  I now have two of them that are exactly the same.  I was just quite surprised that neither of the 3 blade options gave me better results.

[K1500]

It could have been a pitch issue (I assume they are the same diameter). My understanding is a 4 blade provides more ‘grip’ for a better hole shot and for better performance in rough water and during turns. A 3 blade is more efficient. I have heard folks say that moving from a 4 blade that performs well to a 3 blade you should step down one pitch (I.e. from a 4 blade 19 to a 3 blade 17). It all really depends on your motor and whether you can hit max rpm at wot. I have 4 blade props and they seem to work well for me, but they also seem to be less common than the 3 blade (which I haven’t used). I’m glad you got it sorted out, it can be frustrating. 

[gim]

1 minute ago, K1500 said:

It could have been a pitch issue

The two 3-blade props I tried were 21 pitch.  My 4-blade prop is 19 pitch.  Is there such a thing as 20 pitch, right in between?  The guy at the prop shop made it sound like going from a 19 to a 21 pitch was the next step and that there was nothing in between.  After I explained to him that I got worse performance out of the 3-blade props, he really didn't have much for an answer.  Almost like he didn't believe me.

[K1500]

My understanding is they typically jump by 2. I would have thought if a 4 blade 19 was working to go to a 3 blade 17 or 19. If a 4 blade 19 was not giving enough of a hole shot and you wanted a 3 blade to go to a 17 or even 15. It sounds like you got the opposite advice. 
 

Disclaimer: I could have it backwards, so please verify before you take my advice. 

[gim]

10 minutes ago, K1500 said:

Disclaimer: I could have it backwards, so please verify before you take my advice. 

I may have it backwards too lol.  My understanding is that the higher the pitch, the slower the hole shot, but higher top end speed.  Maybe someone else will chime in and confirm this.

[K1500]

That is true, but if you have too high of a pitch the boat will be really sluggish and never hit top speed. You should be able to hit redline for your motor with the correct prop. If you cannot do that, you have too much pitch and will never hit top speed, plus it will be really sluggish to get on plane. 

You were sluggish to plane and slower with your 21p three blades. To me that says you were quite a bit over propped. I’m guessing that dropping 1 or 2 pitches (down to a 19 or 17) would have perked the boat up nicely. 

[E-rude dude]

Mercury Spitfire X7 was made specifically for your size motor. It’s a 4 blade SS prop with 13” diameter and extra hardened SS blades. It also can be bought in 1” pitch increments to really tune in your overall performance. You should be easily topping out at 38-40 mph 

[gim]

7 minutes ago, E-rude dude said:

Mercury Spitfire X7 was made specifically for your size motor. It’s a 4 blade SS prop with 13” diameter and extra hardened SS blades. It also can be bought in 1” pitch increments to really tune in your overall performance. You should be easily topping out at 38-40 mph 

Do you know what pitch? I use a 19 pitch on the 4 blade aluminum and the 3 blades in 21 pitch were worse. Never thought about trying another 4 blade in SS.

[E-rude dude]

3 minutes ago, gimruis said:

Do you know what pitch? I use a 19 pitch on the 4 blade aluminum and the 3 blades in 21 pitch were worse. Never thought about trying another 4 blade in SS.

What is your rpms at wide open throttle and what is the range of your motor?

[gim]

I guess I am not completely sure on the rpms. And I don’t know what you mean by “range.”

 

The rpm on the 3 blade was about 4000 rpms so the 4 blade must be higher.

[E-rude dude]

You need to start by finding out what your motors operating range at Wide Open Throttle is. 
Wide open throttle is how fast of rpms your boat motor goes with the throttle at max power for at least a minute.

 

With those 2 pieces of information you can make a better decision on what’s the best pitch prop for your boat. Not all boats are the same. Even the same make will not be the same. Close but not the same.

[TnRiver46]

Testing them is the best sure fire way to see which runs better,  sounds like you already did that. My boat I think runs somewhere around 5500 rpm wide open (50 merc 4 stroke)