Prop question ... should I go with a 24 pitch prop?

[gwhjr98]

New engine replacement. Alpha 1 gen 2 outdrive. 300hp 5.7 engine. range is supposed to be 4400-4800 RPMs. I am currently at wide open with 4 folks in boat and full tank hitting 5000-5200 rpms with 23 pitch stainless. My question is should I be concerned and look to go with a 24 pitch prop instead? I don't run WOT all the time but don't want to mess up the outdrive or engine. thanks.

 

[Maclin]

Is it a stock 300hp? Or did you hop it up and are estimating? I was thinkin' the 300hp Mercs were closer to the 5000-5200 range for top RPM. In any event, I would verify the tach reading. If it romps out of the hole now and the tach reading is accurate then it would be good to go to the next pitch up since you are concerned about it. If it is just ok out of the hole then I would leave it as is and watch the throttle.

 

[steelespike]

300 that sure seems like a lot for an alpha one.We need your gear ratio , wot rpm and gps speed
To evaluate your setup.

 

[gwhjr98]

This was a replacement engine for a 1998 305 setup. The book on the motor says 290 hp. 5.7 Vortec complete. GPS speed is 54 and RPMs at WOT is 5200 with 2 in boat. 23 pitch prop.
So.. I assume the drive ratio is this.

5.0 Liter

230

5.0/305

8

1.61

24-24

1991-2005

 

[Maclin]

Did you run a 23p with the 305?

 

[gwhjr98]

No. it came with a 19 with a 21 as a option.

 

[dazk14]

Well, without knowing more about the boat and the prop, you could add some cupping to pull the rpms down a tad.

 

[gwhjr98]

One question that came to light when I looked up the ratio.. If the 5.7 uses an alpha drive it comes with a 1.47 the 5.0 motor comes with a 1.61. What is the difference in output in regards to RPMs or drive. Trying to understand if that also comes into play with my setup. thanks

 

[Maclin]

That drive ratio is definitely affecting your top RPM's now. The 5.0/305 does not have as much power in the lower RPM band as a 5.7 so hence the higher numerical ratio installed by Merc for more torque multiplication sooner. Your new engine does not really need the help that the current ratio provides, hence you need higher pitched prop(s) than before to keep the top RPM's down.

 

[gwhjr98]

Ok, I know the mix of gears and engine are not optimal however, it is what it is.. so I spend some time at the prop shop and the marina this weekend talking to them on what to do. Right now the path I am going to take is getting a stainless 14.5 x 25p. To lower my WOT RPMs. so in review of available props (Iocal did not have any).. I have two options.. 1). Stiletto Advantage 14.5 x 25P and 2) Lexor 14.5 x25p. Before I order.. any comments on which is a better prop?

thanks..

 

[edthearcher]

Ipossted the same question here a few months ago, never got any real answers. so i called mercury marine here in wis. where there made. i got the answer from them, even if its not a factory engine