Prop help needed

[John Hawkins]

I have 79 merc 90hp motor, with a now pretty digned up prop. I don't know what size it is or what size I need to replace it with. Before it was damaged, I was doing 41mph at 4500rpm's. I know I should be doing about 5500 rpms WOT, not sure what speed I should be doing, but anyways.... Any body know what size prop I should use to reach 5500 rpm's?? I know is should go down 1 in. pitch for every 200rpm's up. But what is the size for this motor. It is on a 75 terry bass boat tri hull. Help, prop man

 

[Dhadley]

Re: Prop help needed

The pitch will be stamped somewhere on your prop depending on what make it is. I have a couple of Stiletto 17's at a real good price that are for your model motor. I think a 17 should be about right if the motor is running well.<br /><br />Let me know!

 

[John Hawkins]

Re: Prop help needed

I think the proper size prop is 13.25 x 17. Not for sure. I'll look on the prop again, didn't see last time, I'll post you, what price are we talking and it ss, this one is, but alum is good

 

[John Hawkins]

Re: Prop help needed

do you know the WOT for motor with that size prop???

 

[jamescody]

Re: Prop help needed

Hawk69...I think for that motor, you should run it at about 5000 RPM WOT.....these inline 6 90's have much smaller ports which increase the torque, but lower the max rpms.....It wont hurt them to turn at 5500, but you really arent gaining much..If you look at the owners manual, the spec is 4500-5000....well at least it is for my '82...and I'm pretty sure they are the same. A 13x15 on my 90 hp Mariner turned about 4800 rpm, I got the pitch beat down to a 14 and it turns about 5100-5200...<br /><br />Dhadley, what are the props you are selling, can you email me the particulars at jcody@telusplanet.net<br /><br />Thanks

 

[Dhadley]

Re: Prop help needed

The props are new Stiletto's and are polished stainless steel. I'll take 175.00 each for them. If anybody needs one let me know here or dhadley50@aol.com.<br /><br />Thanks!!

 

[Bob Kimber]

Re: Prop help needed

Hawk69,<br />Jamescody is correct when he said your motor's WOT is 4500 - 5000, I sent you a reply to your earlier post, I owned a 79 90hp for 21 years and ran a 15" pitch prop on a heavy 17ft glass boat. My experience with this motor and mechanics advice proved you were wasting your time over reving that model as you gained no worth while performance, unless you were propping down for skiing, and even then they reccommended backing off the rpm to be within WOT once planing.<br />Those motors can run lean on the middle carb particularly if you are over WOT<br />BOB

 

[sony2001]

Re: Prop help needed

Get your prop trimmed down in size, thus removing the nicks, and rebalanced.