What does prop diameter have to do with the overall formula?

[Gregs89Thomoson]

Hi everyone. I'm Greg and I'm new to the boating community. I'm interest in swapping out my 21p 13.5" diameter aluminum prop to a stainless prop for one reason. Overall performance. I have a '25 1989 Thompson Carrera, with. 5.7L Merc and alpha1 outdrive. I bought a Merc Mirage stainless prop that is 19p x 15.25 diameter. A friend said it's to big and I won't get up on plane, but my local marina says that diameter really shouldn't matter. Any input from all of you more experienced and well seasoned boaters is greatly appreciated. I did not try it yet. Any one with ideas or basic rules to follow when changing out props is also greatly appreciated. Thank you everyone.

 

[craze1cars]

Re: What does prop diameter have to do with the overall formula?

It will get up on plane just fine. Pitch trumps diameter every time.

If you're going to a lower pitch, in theory it will get up on plane EASIER, despite the larger diameter.

That said, you're changing from aluminum to stainless, which is like adding an inch-ish of pitch. And you are probably adding some cupping and more aggressive rake, which again is like adding an inch-ish of pitch. Yet you are compensating for the material and added aggressiveness by going down 2" in pitch. This is good.

Overall I suspect your RPMS will be very similar to your current prop, maybe a little bit lower, and your performance overall will be improved. But I'm only giving it an educated guess because you haven't shared any of your RPM data currently, so I have no idea if you are appropriately propped now or not. Try it out and please report back!

The only thing you might need to look out for is interference with the trim tab, if equipped. If your drive doesn't have a trim tab, then I believe this prop will probably work fine on your boat.

 

[SparkieBoat]

Re: What does prop diameter have to do with the overall formula?

diameter makes about 50-100 RPM difference per inch, but that is if all other things about the prop is exactly the same. your friend does not know what he is talking about. generally speaking the smaller diameter props will also give you slightly more top end while bigger dia props will give you slightly better hole shot. you are going from alum to SS and dropping 2 pitches, these 2 changes are what is going to make a big difference. with a motor that big I think most pros recommend SS only because you will get too much flex/slippage from a alum prop with that much torq/HP. my thoughts are thaT YOU SHOULD GET GREAT PERFORMANCE OUT OF YOUR NEW PROP, but you need to make sure that your WOT RPMs are in the recommended range for your motor, I think you are shooting for around 4500. you may have to get an offset anode if your prop hits it, you can temporarily remove it if you need to.

 

[Gregs89Thomoson]

Re: What does prop diameter have to do with the overall formula?

Thank you guys I appreciate the feedback. I'm going to try her out this Friday. I'm going run her WOT and get her to plane on the aluminum prop first to get some solid numbers for RPM range. Then Im going switch to the SS and hope fir the best. I will be reporting back by Monday.

 

[Rocky_Road]

Re: What does prop diameter have to do with the overall formula?

Thank you guys I appreciate the feedback. I'm going to try her out this Friday. I'm going run her WOT and get her to plane on the aluminum prop first to get some solid numbers for RPM range. Then Im going switch to the SS and hope fir the best. I will be reporting back by Monday.

I'm going to take a guess and say that you will come back and say that your new prop wouldn't clear your current trim tab.

If this were mine, I would install your new prop in your driveway to see if all is well...then take the trip to the water to run your tests!

The Mercruiser high performance trim tab will work with either of your props, and take this out of the equation.

 

[Gregs89Thomoson]

Re: What does prop diameter have to do with the overall formula?

Currently the new SS is on the boat and there is no interference. I would say I have an inch or more of clearance. That was my first thought when I first compared the SS to the aluminum.

 

[Rocky_Road]

Re: What does prop diameter have to do with the overall formula?

Currently the new SS is on the boat and there is no interference. I would say I have an inch or more of clearance. That was my first thought when I first compared the SS to the aluminum.

Do you have power steering? If so, then you probably don't have a finned trim tab to worry about.

Let us know how the test comparison turns out!

Happy boating!

 

[Gregs89Thomoson]

Re: What does prop diameter have to do with the overall formula?

Hello everyone! I'm a very happy boat owner today. She planed out at 43.4 mph (via GPS).
RPM range of 4200-4400 depending on conditions. What do you guys think?

 

[craze1cars]

Re: What does prop diameter have to do with the overall formula?

That's probably not too bad a speed for a boat of that size with a pre-vortec 5.7...it sounds like you're running a hair low on RPMS for my taste, but I don't know the specifics of your engine. Is there a sticker on top of the engine somewhere that gives you an RPM range? Usually they would say something like "4,200 - 4,600" If this is the case, then you are in the low end of the range....but being anywhere within the range is perfectly fine. Just be aware that if you were to prop for the high end of this range instead of low end, you would improve holeshot, and your top speed would probably be very similar.

But nobody can answer this question like you can. Your stated in your first post that your objective was to improve the overall performance of the original 21 pitch aluminum...

so....Did this prop improve the overall performance? Or did it not?? If yes, then mission accompolished. Run it and enjoy it....

 

[Gregs89Thomoson]

Re: What does prop diameter have to do with the overall formula?

Improve it overall..... I would say that's a big 10-4 goodbuddy. Before I would loose 10 to 15 mph turning somewhat aggressively. Now I loose 3mph. I'm very impressed.

 

[Texasmark]

Re: What does prop diameter have to do with the overall formula?

Not much. Many more things impact performance much more....main ingredient is pitch.

Mark