Trying to understand Aluminum vs. Stainless performance

[buckeye_sean]

OK, I have a fairly good grasp on how pitch, diameter and rpms affect speed/performance. (I think, anyway.)

My Fisher 17' aluminum deep V is powered by a Mercury 90hp 2-stroke. (My motors max rpms should be right at 5500 based on the book.) The boat came with a 21 pitch 13" aluminum prop and the edges showed a little wear. The previous owner also threw in a brand new 13.25" prop with 19 pitch.

Boating last year I noticed with the 21 pitch prop, the boat was sluggish on the low-end and I could never get the max rpm's above 5200-5300. Top speed with that setup was right around 37-38 mph with 1/2 tank gas and a couple people and gear. (I'm guessing total weight was around 2400-2500 lbs.)

Last fall I switched out the 21p for the 19p. Bottom end was greatly improved and top end was 35-36 mph. However, according to my tac I can hit nearly 5700 rpm at WOT. *Probably not good, so I always hold back the rpms to 5500 on the tac.

So I'm thinking there is room for improvement. If I stick with aluminum, I'm guessing a 20 pitch prop sounds like the ticket. However, it seems that they are harder to come by, and pricier to boot. The other alternative is to switch to a stainless prop. What I'm not sure about is how much difference does a stainless vs. an aluminum prop make?

-I've heard that stainless props are harder for the motor to spin, but push more water. If I were to buy a stainless prop do I want to stick with something in the 19 pitch category, or possibly jump back up to a20 or 21 pitch prop?

-Another question is what would be the affects of going from a 3 blade vs. 4 blade on the stainless props? Does my 90hp warrant a 4 blade prop, or are they for bigger performance motors?

 

[buckeye_sean]

Re: Trying to understand Aluminum vs. Stainless performance

Anyone????

 

[Dhadley]

Re: Trying to understand Aluminum vs. Stainless performance

Are you willing to move the motor up when going to a prop with more technology or do you just want to leave it where it is?

 

[cbavier]

Re: Trying to understand Aluminum vs. Stainless performance

Sorry can't help you with your questions but I would not buy a SS prop. Brittle, High theft, can't repair or if you can find someone who can repair is very expensive. Aluminum works just fine for me. I run a 21 and it takes lower RPM's to move more water and my speed increased from 38 mph at 4800 rpm running a 19 to 42 mph at, if I remember correctly 4500 rpm with the 21. I'll stay with the 21 aluminum 3 blade thanks. I'm no speed demon anyway and I notice no difference in hole shot although I'm sure there is some. I have a Mercruise I/O 185 HP

 

[steelespike]

Re: Trying to understand Aluminum vs. Stainless performance

My feeling is if you go to a 19 stainless it will bring you closer to the right rpm and net a little more speed.
As far as a 4 blade generally they improve low end performace with a slight sacrifice of top end.They are also used for certain specific heavy duty applications.You can also improve performance by fine tuning your setup especially the vertical height of the motor.If you have little or no ventilation problems now there is room to get some more height.Starting with the antivent plate even with the bottom of transom raise it a hole at a time to a point just before venting is a proiblem.be sure it pumps water.
Depending on the result you might be able to go back to a 21.

 

[buckeye_sean]

Re: Trying to understand Aluminum vs. Stainless performance

My feeling is if you go to a 19 stainless it will bring you closer to the right rpm and net a little more speed.
As far as a 4 blade generally they improve low end performace with a slight sacrifice of top end.They are also used for certain specific heavy duty applications.You can also improve performance by fine tuning your setup especially the vertical height of the motor.If you have little or no ventilation problems now there is room to get some more height.Starting with the antivent plate even with the bottom of transom raise it a hole at a time to a point just before venting is a proiblem.be sure it pumps water.
Depending on the result you might be able to go back to a 21.

I'm not against moving the motor up. I'll have to check to see where everything falls right now - if my memory is correct, I think the plate was pretty much even with the lowest point of the transom. Will an inch or two make a major difference? (Excuse my ignorance here.) I do not currently have a problem with ventilation. Actually I feel I have to trim it up quite a bit before I experience any.

As it stands, I'm fairly happy with how the boat performs except for the max rpms being up in the 5700 range. This was my main concern for going up another pitch on the prop (from 19 to 20) -OR- going to a stainless prop. Just wasn't sure how a stainless prop would affect my rig. BTW - yes I know stainless props are prone to theft and more brittle. However, I would likely buy a locking nut if I invested in a SS prop and I don't really boat in much shallow water with my 90hp. I have a 9.9 kicker to handle those duties.

-IF- I could hit the magic 40 mph top end with a little work I would be very pleased, as long as I'm not lugging the motor in the bottom end to achieve that goal.

Thank you all so much for the replies so far.

 

[walleyehed]

Re: Trying to understand Aluminum vs. Stainless performance

Going to a 20P(if you could find one in 3-bld) isn't really the answer.
As Dhadley stated, and was leading into.....let's raise the engine and see what that prop will do....you very possibly could get around 5500 with that rig. 5700 is in no way hurting your engine...5200 would cause more problems in the long-run.
To add another note, Going to SS and ruining the lower unit is an old wive's tale with todays technology, and when you max your set-up with aluminum I would recommend you go to SS.

 

[Dhadley]

Re: Trying to understand Aluminum vs. Stainless performance

Exactly. Since you're willing to move the motor I'd suggest something like a 21 3 blade Stiletto. My guess is that by fine tuning the set up you'll end up at more rpm and speed with a better holeshot than you have now.

Many folks are not willing to move the motor, they simply want to change props, period. And yes, an inch can make a huge difference. In some cases an inch is considered a drastic change.

 

[jamieat]

Re: Trying to understand Aluminum vs. Stainless performance

Just a suggestion, but you can get a Hustler Turning Poing 3 blade prop in just about any pitch for about 129.00. This is the ONLY aluminum vented prop on the market which is great to help you get out of the hole quicker without having to sacrifice topend. I currently have two SS props, and two of these aluminum props. The aluminum is great and since it is lighter, I can turn the 21p about 150 RPM's faster than the 21p SS! You should look into this prop, it could help you out in whatever pitch you decide to go with.

 

[walleyehed]

Re: Trying to understand Aluminum vs. Stainless performance

The reason you can spin the alumn. prop faster than the SS is NOT because it is lighter...it's because it's a less efficient prop.
Your slip ratio is higher, therefor your RPM is higher...you've lost performance, not gained any.
Venting any aluminum prop is a 3 minute process...anyone that can read a tape and run a drill can vent any prop.

 

[Dhadley]

Re: Trying to understand Aluminum vs. Stainless performance

Now Admiral, everyone knows an aluminum prop is way more efficient than stainless steel. Aluminum is more efficient, lighter, easier to shape, easier to rework, hold their shape, have more complex shapes, less expensive etc. That's why all race props are.........wait.......oops! Never mind.......

 

[Ed R]

Re: Trying to understand Aluminum vs. Stainless performance

Hey buddy,
1. Aluminum prop is half the price of stainless and is efficient, but the aluminum blades tend to flex under high RPM. However, the stainless prop is much more efficent due to its non-flexibility, thereby causing less slip. The stainless prop actually has built in cup to blade angle, which causes more bow lift and efficiency. My recommendation would be a stainless 19 inch pitch prop. (stainless prop would bring RPMs down 200 to 300 due to built in cup!)

ER

 

[walleyehed]

Re: Trying to understand Aluminum vs. Stainless performance

Hey buddy,
1. Aluminum prop is half the price of stainless and is efficient, but the aluminum blades tend to flex under high RPM. However, the stainless prop is much more efficent due to its non-flexibility, thereby causing less slip. The stainless prop actually has built in cup to blade angle, which causes more bow lift and efficiency. My recommendation would be a stainless 19 inch pitch prop. (stainless prop would bring RPMs down 200 to 300 due to built in cup!)

ER

Efficiency is blade design..not flex.
"Built-in cup to angle"....it's called parabolic rake. Not "All" SS props have this...not all are bow-lifters.
With SS we can add cup/rake in areas we can't with aluminum.
Cup can be in many forms and when we start talking brands of SS, not all are created equally...for example, in my test data I find that you can run almost any 17P 3-bld SS, then slap a Turbo1 in a 15P on and carry the same RPM, enjoy an increase in speed yet have better hole-shot.
Pitch means very little if not staying brand-specific.
"My recommendation would be a 19 inch pitch prop".........of what brand??? If we installed a 19P Turbo, it would be like a 20 or 21P in most other brands....it's just not that simple.

 

[Tail_Gunner]

Re: Trying to understand Aluminum vs. Stainless performance

Now Admiral, everyone knows an aluminum prop is way more efficient than stainless steel. Aluminum is more efficient, lighter, easier to shape, easier to rework, hold their shape, have more complex shapes, less expensive etc. That's why all race props are.........wait.......oops! Never mind.......

Damm sounds like a liberal some place here...opp's i retract that statement......

 

[gotboostedvr6]

Re: Trying to understand Aluminum vs. Stainless performance

Use the prop that offers you the lower rpm and start to raise the mount location of the engine.

Do this until you get the speed your looking for or until it starts to cavatate or lose speed/ holeshot then tell us your rpm and speed.
We then can make a good recommendation.

Watch your water PSI... Yes you will need a pressure gauge to do this properly.

If you dont watch the PSI you will need a new PH in short order.

 

[jamieat]

Re: Trying to understand Aluminum vs. Stainless performance

Okay, since this seems to be such a contraversial issue, and everyone on here has their own opinion, my suggestion would be to do what I did. Call 1-800-718-1233. This is the toll free technical support line for I boats. Let the "experts" who do nothing but deal with props for a living give you the advice you need to find which prop is going to best suit your needs. These guys are very knowledgeable and can also educate you on the formula used to determine the specific prop you need and explain why. Good luck!

 

[jamieat]

Re: Trying to understand Aluminum vs. Stainless performance

Also, it would be a bit naive to say that "All" Stainless props are more efficient than "All" Aluminum props. You cant say that the worst SS prop is more efficient than the best built Aluminum. That is an uneducated assumption.

 

[walleyehed]

Re: Trying to understand Aluminum vs. Stainless performance

jamieat, I test props as part of my living....
I have also set down with iboats staff members in person and have given training to the people answering the phones.
When questions get out of the "general" area, they refer these questions to either Dhadley or myself.
You're getting the best information available, one-on-one, right here on the forum.