Prop Guru's


How much does prop diameter affect RPM?

I am running a 32 Baja with boxes and 2 1/2" raised 1.36 B1 drives.

Hydromotive Q4 gives the best overall rough water ride. I am spinning 23p Q4's at 4600 left and 4800 right (69 mph). The right hand prop had been reworked by Hydromotive. I measured the blade diameter of both props. The right hand prop has 1/8" less lenght per blade than the left. RH blades are 5 3/8", LH blades are 5 1/2".

Does 1/8" give 200 rpm? (1/4" total diameter)

Can I take these props down to a total of 15" diameter and get the RPM's up to 5000?

Will removing the diffuser ring help max RPM?

Should I lab finish them and cut diameter to get optimum performance?


Thanks for any help.

Your rpm difference is probably due to a pitch difference in the right and left prop, not the rpm difference. If the prop has been worked or repaired the pitch may have changed if they didn't have the other prop to match it to.

Removing the diffuser ring won't make much a difference in max rpm but may give you a little more speed on the top end depending on which flare ring you have.

What motors do you have? That will dictate what can be done to your props.

Matt

HP450's

I was told by a prop shop that a 1/4" diameter will equal the 200 rpm difference. Hydromotive cleaned up the ends of the reworked prop. I swapped the props left to right and the rpm difference followed the prop. They were not told to do anything with the pitch just clean up the rough ends.

I have run Bravo 1's spinning in on the boat and they seem to be the fastest top end. I am using a pair of mirages now but I believe they have been reworked so they are not a good baseline.

Does anyone know how much the hydromotive prop diameter affects RPM?

I've heard the same thing about taking off 1/4 inch diameter = about 200rpms.

Yes,

That is common to hear and is pretty close but not 100%. I would recommend measuring both of the 2 opposing blades on each propeller and see if one propeller has a larger diamter than the other. If not there are a few other things that can cause this.

Julie

I've never seen a 1/4" give 200 rpm on these props without doing other things to get the rpm's. You will see some rpm difference but not 200. Something else is going on.

The diameter on those props is inconsistent. I have found this over and over again. It isn't just one manufacturer either. Mass produced castings have so many variables in the process that total consistency is tough. The custom prop shop should handle that end of things.

In order to tell exactly what you have going on will require a complete measurement of the propellers.