Tried My New Prop Today and Got High RPM's

[01 cummins]

I just got the boat a few weeks ago and it had a 14 1/2 X 23 stainless prop on it. It has the OMC King Cobra drive in it with a 5.7 350 motor and the manual says that the ratio is .71 / 1.41:1 . The gauges said I was going 45 MPH @ 4000 RPM's so I read a lot on the prop selection and it said I would gain about 200 RPM's per pitch so I went to 14 1/4 X 19 alumium prop and tried that out today. I tried the new one today and the RPM's went to 5200 @ 45 MPH. The specs say to run it at 4200-4600 but the engine didn't sound like it was reving that high to me. It was just me (230 pounds) and my 1 1/2 year old with 3/4 tank of fuel. NADA says the net weight is 2886 on the boat. The bottom was painted before and I know that takes some speed away also.

 

[hwsiii]

Re: Tried My New Prop Today and Got High RPM's

01, you have to know the differenes between the propellers and how aggresive they are as well as blade surfae area differenes and other things I won't even get into. What exat make and model of SS and Aluminum props do you have.


H

 

[01 cummins]

Re: Tried My New Prop Today and Got High RPM's

The new prop I put on is the Vortex XHS and SS is 14 1/4 not 14 1/2 and that one says Viper on the side of it. I hope that helps

 

[hwsiii]

Re: Tried My New Prop Today and Got High RPM's

Will they let you return the aluminum prop.


H

 

[01 cummins]

Re: Tried My New Prop Today and Got High RPM's

HAHA that doesnt sound good on my part. I just bought it from here and used it today plus I had a little mishap if you read my other thread today which took off some paint on the one blade.

I'm guessing I shouldn't have listened to my buddy then?

Should I have gone with a 21 pitch?

 

[hwsiii]

Re: Tried My New Prop Today and Got High RPM's

Theoretially speaking he evidently used the 200 RPM per inh theory and didn't take into aount the differene between the Viper and the Vortex.
A 21" would have put you muh loser. I will get bak with you shortly with an answer.


H

 

[01 cummins]

Re: Tried My New Prop Today and Got High RPM's

So should I bother buying a 21 pitch Vortex or will the 19 be good hauling more people because I plan on having at least my wife and 3 kids total which is only about 150 more pounds?

By the way I do appreciate you (hwsiii) taking the time to help me and others out when you don't get anything in return besides a thank you.

 

[hwsiii]

Re: Tried My New Prop Today and Got High RPM's

Take the Vortex to a prop shop and ask them to put cup in the trailing edge, up to a theoretical 2" of pitch, and that will bring your RPM bak down to a reasonable RPM range and you will have a prop that will suit your needs. Also cup keeps your prop slip down and will add theoretical pitch to the prop.
And you are very welome.

H

 

[Dhadley]

Re: Tried My New Prop Today and Got High RPM's

When you have them add cup tell them if you need more bow lift, stern lift or overall lift. They can then place the cup to assist with what you need.

 

[01 cummins]

Re: Tried My New Prop Today and Got High RPM's

I will see if anyone in the area does props and if they do I will get it done. If not I might just pick up another Vortex since I already have the hub for it and the prop is only 80.

I'm still surprised that the new prop added 1200 RPM's and a little less MPH's.

 

[Dhadley]

Re: Tried My New Prop Today and Got High RPM's

Just remember, the 200 rpm per inch rule applies to props of the same make and model. You cannot apply that rule when comparing different designs (apples to oranges deal). Even comparing the same make props of the same pitch but different model won't necessairly mean the same rpm / speed / slip.