'93 Merc 3.0l LX with 1.84 Gearing Prop Choices

[thormx11]

So I have a 1.84 gear ratio outdrive with my 3.0l merc engine. The boat is a 20' cuddy. I am getting about 4200 RPM at WOT, while running about 33-35 mph (according to the speedometer) The outdrive has a hydrofoil on it as well, and has no problem getting up on plane.

I have read that WOT should be around 4400-4800 for this engine. Do these numbers sound close to what they should be or should I try a different prop to make up for the different ratio? The boat cruised at about 28 mph or so while running 3700-3800 rpms.

It currently has a 14.5x19 prop on it right now.

 

[Scott Danforth]

drop down to a 17p

if you want more speed, loose the hydrofoil

 

[thormx11]

Thanks for the tips. How much does the diameter affect it? If I do decide to go with a different prop, should I stay with the same diameter and only change the pitch?

I am new when it comes to props (never had to change one on any of my previous boats)

 

[alldodge]

If your tach is correct and your normal cruise rpm is the 3700-3800, dropping to 17 will get your cruise in the 3900 to 4100 range. The motor will run easier but the peak torque may start falling off. Reason I bring this up, is current cruise rpm looks good, but running at the higher range for long periods needs to be thought thru. The 17 could be a good fit if its cupped to make it more of a 18 by reducing slip

Diameter makes a difference but so does everything else. Pitch makes more of a difference then diameter.

 

[thormx11]

So should I try to find a 14.5x17 prop? Or should I change the diameter as well?

Or, do I even need to change the prop at all? It seems to plane out very well, even when pulling a tube, and I am not really concerned with getting more speed, I just want the engine to operate at its optimal performance. I am happy with the prop set up I have now, I was just a little concerned with the low WOT rpm

 

[alldodge]

If you could try a 17 then sure, but since the boat is preforming the way you like, I would not be to much in a hurry to change. The thing that gives me concern is the high rpm where your cruise is. If it was lower then I would have no issue with going a smaller pitch, but being that high, might leave it were it is.

 

[jimmbo]

A 17 should put you around 4600 at WOT, and your 28mph cruise will require about 400 more rpm. If the tach is accurate, @4200 the engine is being overloaded, and that will sooner than later, hasten its demise