comparing 2 different size props, 1 alum and 1 is SS

[Cptkid570]

Hi everyone,
I'd like your input on 2 different sized props and comparing the performance each should give. Going on a 24' bowrider, 1996 Four Winns that weighs about 4000 pounds.. will be powered by a 5.7 mercruiser (260 hp)

The test prop used by the manufacturer is an aluminum 14 1/2 x 19 prop.

I have a 15 x 17 Stainless Steel prop that's been in my garage for a while now and I was planning on using it on the Four Winns.

What performance differences should I expect between the aluminum 14 1/2 x 19 and the SS 15 x 17?

Boat isn't ready for the water yet, so I can't do a water test.

 

[solar7647]

Re: comparing 2 different size props, 1 alum and 1 is SS

You will most likely see a lose in top end speed. With the SS prop. It will come out of the hole a little faster though. I would suggest that you sell it and get a new higher pitch prop only an inch or two though. The size and pitch of the prop changes your WOT RPM and you want to stay in the range that the moter is rated for. most of the time its between like 5000-5500 rpm. Thats the main thing you need to know. Also the higher the pitch the slower the Rpm get and the more top end speed you get. The lower the pitch the higher the Rpm get and the lower top speed gets. You should also stick with 14 1/2 inch prop. A lower pitch prop will also come out of the hole better then a higher pitch prop. Good Luck

 

[Silvertip]

Re: comparing 2 different size props, 1 alum and 1 is SS

Without a baseline test using your existing prop it does little good to buy a new prop as you stand the chance of making a bad situation worse. Read the sticky at the top of this forum titled "Read This First". Without a baseline to work from you have no idea if you have a prop that is even close to optimum for your setup. And just so you know, the prop that came on the boat is probably not the best one for the boat. I can also assure you wide open throttle rpm for the 5.7 is not 5500 rpm. Also be aware that just because you increase pitch it does not guarantee increased speed. It merely means rpm will less at the same speed as a prop you replaced. Only if the engine has enough power to twist a prop with higher pitch will you see an increase in speed.

 

[Cptkid570]

Re: comparing 2 different size props, 1 alum and 1 is SS

Let's assume I want to stick with a 17" pitch... how much difference could there be between a 14 1/2" and a 15" diameter?

I understand the whole WOT thing... Just wondering if you are looking at minor changes or major differences

 

[JJJ]

Re: comparing 2 different size props, 1 alum and 1 is SS

The 15" should (stress should, since we don't know the design), give a little more lift and more oompf out of the hole. I would try both before buying ANYTHING. You might already have close to the perfect prop there

 

[Silvertip]

Re: comparing 2 different size props, 1 alum and 1 is SS

I repeat -- you cannot predict what will happen with any degree of confidence unless you do some baseline testing. To complicate matters, two props from two different manufacturers but with the same pitch and material will very likely perform differently. It is called "prop design". Add a change in pitch as well as a change from AL to SS and you add another set of variables.

But since you asked: IF you have one AL prop of 19P and another prop of "the same design from the same manufacturer" but 17P, then and only then can you expect better hole shot and an increase of 150 - 200 RPM/inch of pitch at the same speed. Further, as prop pitch decreases (again within the same design) prop diameter will increase slightly. You can see this in the iBoats or any other prop listings. Conversely as prop pitch increases diameter decreases slightly. Prop diameter generally has little to do with performance and is a result of prop design for a given range of engines or outdrives.

As for the stainless prop, because they are generally more efficient than "an equivalent design" in AL, you will likely not see as much RPM increase as you would in a straight switch from 19P AL to 17P AL.

Finally, since we have no idea who the manufacturer or model of these props are, everything above is a quess and only testing can tell you which one is best. Without testing, prop selection is a crap shoot.

 

[Cptkid570]

Re: comparing 2 different size props, 1 alum and 1 is SS

I understand completely.. thank you for your responses..

 

[JJJ]

Re: comparing 2 different size props, 1 alum and 1 is SS

I don't know in my limited exp., prop diameter can have a significant impact ESPECIALLY IF the blade surface is increased with the corresponding diameter increase. (Maybe more of a function of increase blade area vs. diameter though)

For ex. the Solas Amita 3 (15 3/10") series is a powerhouse for an aluminum prop on heavier boats.

On my boat it outperformed both the smaller stock 13 3/4" Merc alum. prop AND the 14" (or so, I don't remember exactly) Powertech SS 4 Blade by a big margin.

THOUGH I admit not a fair comparison, due to different manufacturers, as you say!

 

[rthomas]

Re: comparing 2 different size props, 1 alum and 1 is SS

not that this is for everybody but i have a buddy that will change his prop to fit his plans for the day, he has a 17 for tubes and boards, a 19 cruising with a few people on board, and a 21 he uses when alone. so depending on what you do a little variety might be good