What prop for 18.5' walleye boat?

[farginicehole]

I have a 1991 Alumacraft 185 CS Competitor with a 1989 Evinrude 150 XP. I bought the boat used a few years ago, and it has the stock(?) 15" x 17 pitch aluminum prop. At full throttle I get around 5000 rpm, at least according to my tach. The boat only tops out at around 40 mph (my GPS agrees pretty closely with the speedometer). I don't need to set any water speed records, but it seems like I should be able to get a lot more out of this rig. Compression checked out around 95-105 for all cylinders back when I bought it. Seemed kind of low, but I'm told that these particular OMC motors had lower compression. Assuming all is well with my motor (and I do plan to decarbonize it and replace plugs as well), does anyone think I should spend the money and try a stainless prop? I'm hoping someone out there has a similar setup and can give me some idea of what size/pitch prop to try. I'd hate to spend $300-400 and find out I was way off. I've been apprehensive about going with any more pitch, as I figured my rpm might drop even lower. Someone told me that a stainless would help get more of the bow out of the water though (by being able to trim the motor up even further) and that my rpm and top speed would actually increase substantially. Any thoughts? Thanks much!!!

 

[jomac]

Re: What prop for 18.5' walleye boat?

Not sure about compression,but 115-120 is the norm for any v4-6 OMC's as far as know, as to a SS prop don't waste the money if your expecting a major change, step up to a 19" pitch,but it will lower your RPM's, if it's a carb motor or fuel injected make sure trottle plates are opening up fully (cables do go bad)P.S. 40mph ain Bad

 

[farginicehole]

Re: What prop for 18.5' walleye boat?

If you're right about the compression, maybe I'll get more performance out of the motor after decarbonizing. Seems like I've read on here that the 90-deg(?) OMC V6s ran lower compression from the factory though. As for the prop question, I was also very skeptical of what this guy was telling me. The only reason I gave it ANY credibility is that he works in the service dept. at a major boat dealership in my area! One other thing that would make me question the prop size/pitch is that this aluminum one is 15" diameter, whereas most(?) SS props I see are 14.5" or smaller. I'm hoping that someone with a similar boat/motor combo will share their experience and opinion. Thanks!

 

[Spidybot]

Re: What prop for 18.5' walleye boat?

Depending on your load (boat and all), your trim and the the motor setup on the transom, there is some truth in lifting the boat to gain more speed.<br /><br />Also, the pitch determines the hole shot as well as the top end.<br /><br />If you decide to get a new prop, you should talk to a dealer that allows test runs with various designs to ensure you need only ONE purchase to get what you want.<br /><br />If you like to increase your knowledge to better valuate the advice you get, you can get the DIY prop estimation spreadsheet here. <br /><br />Focus is most often on pitch but the diameter is important as well - especially on a fast boat you have no need for large diameter.