Where to go now 17p or 19p

[jwronkow]

I have a custom made 21' aluminum center console boat. I have a 1990 evinrude 150. The current prop is an OMC SS 4.25 dia x 21+ pitch prop. At WOT rpms are at 4700 ~ 44 Mph. From what I have read I should be closer to 5500. Then a mech told me between 4500 and 5500? Should I just leave it?

 

[Dhadley]

Re: Where to go now 17p or 19p

5800 is your target with an average load. Have you tried raising the motor to see if you can gain rpms with less drag?

 

[jwronkow]

Re: Where to go now 17p or 19p

DHadley, sorry for my ignorance but what is "saising the motor"? I did get my hands on a 19 pitch 14.5 dia prop and only noticed 200 increase in rpms. It is at 4900. I am going to get an external RPM test meter to see if the guage is correct.

 

[AguaSki]

Re: Where to go now 17p or 19p

I think DHadley ment "raising the motor? Just a typo.

 

[walleyehed]

Re: Where to go now 17p or 19p

excellent Idea on verifying the tach...that's a good start.

 

[jwronkow]

Re: Where to go now 17p or 19p

I have already adj engine hight. Looking for a rpm tester today. Not as common as a compression guage. :>

 

[jwronkow]

Re: Where to go now 17p or 19p

Ok got a car tach tester. It is a 12 pole so I had to divide by 2. Looks like the std tach is off by 50. Here is what I have with a 14.5 dia 19 Pitch OMC prop.
plainning 18 ~ 19 MPH
3000 RPMs 27 MPH
4000 RPMs 36.5 ~ 38.8 MPH
MAX RPMs at WOT 4750 - 4800 RPMs

 

[walleyehed]

Re: Where to go now 17p or 19p

I had a 20ft deck boat on Texoma with a 2004 150 Carbed johnson and it took a 15X17 to get 5700 in the third hole...don't know what the weight diff would be, but 21P sounds high to "me" for a non-pad hull of 21ft.