Do I need a different prop?

[tonyjh63]

I have a 1999 Bryant 182 Limited (dry weight, 2500 lbs), Mercruiser 4.3 liter V6 with Apha 1 Gen 2 outdrive. Currently running an aluminum Turning Point prop, 4-blade, 19 pitch. The outdrive has 2.0 on it, indicating it has the 2:1 ratio, meaning it was originally sold in a high-altitude area (that's what that indicates, right?) Unfortunately, I can't confirm that it is actually a 2:1 ratio - the boat was a mess when I got it almost 4 years ago - kinda cobbled together, including throttle controls for an outboard modified to run the 4.3 Merc.
Anyway, finally was able to do some WOT runs yesterday, and wanted to get your feedback on the reults. At WOT, all it could muster was 42 mph (gps) at 4050 rpm (holeshot seemed fine, btw). Am I over-propped and bogging the engine, since I can"t reach 4400-4800 rpm at WOT? I figured I would need a higher pitch prop, since I'm running a 2:1 outrrive, but the results seem (to me) to indicate otherwise.
Any advice would be gretly appreciated. Thanks!

 

[WesNewell]

Theoretical max speed for 19P prop with 2.0 GR at 4050 rpm is 36 MPH, so something doesn't add up.

 

[porscheguy]

I was just going to say that the data provided results in a -15% prop slip which is not possible.

Recalculating the prop slip after assuming the tach is incorrect and basing it on 4900, and your prop slip is 4.5%. This is theoretically possible but extremely unlikely with a turning point proint.

So the next thing I did was assume that the tach was off and maybe you had proper gearing(1.82) packed into a gearcase marked 2:1. That comes out to 15% slip. That’s plausible.

Confirm that your tach is operating properly. You are not overpropped with a 19p and a 2:1 ratio.

 

[QBhoy]

Hi your speed is about spot on for a well set up unit like that and probably even a 21”...but your rpm is way out and almost certainly wrong. I’d bet you are actually doing at least 4800 rpm. Haven’t checked the slip but it will surely be ridiculous at those figures.

 

[tonyjh63]

Ok, so how do I check to make sure my tach is working properly? Thanks!

 

[WesNewell]

It's More likely you have a 1.62 GR and there's nothing wrong with you tach. AFAIK, No. 4.3L mercruiser has a 2.0 GR, and only 1, a 190 model, has a 1.81 GR. All the rest are 1.62. Now that would make sense, as theo speed with 19P prop @ 4050rpm is 45 mph. and 42 mph would be a 7% slip, which is very doable.