Am I running the right prop?

[Beefer]

I'll start off with my info:

1) Performance issue you are trying to correct.
increase rpm's at WOT

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
Here's what I do know - It's a SS Quicksilver with a 19" pitch. I'm not sure of the diameter, but I believe it to be 14"

3) Current prop diameter and pitch (required).
See above

4) Wide open throttle RPM and speed with an average load (very helpful)
At WOT, I get up to 4000 rpm, 27mph (gps). It's my understanding that I should be at 4400-4800 rpm at WOT

5) Engine/drive make, model, year, and HP
1984 Mercruiser 488R, 1984 Alpha I (Gen 1). 188hp

6) Boat make model, year, length and weight
1984 Sport-Craft Offshore Family Fisherman (Walkaround), 22'2", approx weight 3800lbs dry (from NADA)


I'm trying to figure out if;

1) 19" pitch is the correct pitch. It is not the original prop, and I'm wondering if pitch was increased.

2) if pitch is correct, what's the next step in figuring out why I'm not getting the recommended rpms at WOT?

3) if pitch is not correct, what should I go to? 17"p? What diameter?

At 3000rpm, I'm running at 22mph. I'm on plane, but it just seems like she should be going faster. If the boat is underpowered with the original engine, I will just deal with it, but if there is a way to improve this, feel free to chime in, your thoughts are appreciated.

Off the line, I think it could also be quicker. I know this boat was not intended to be a speed demon, but I just think she's a little slower then it should be.

Thanks!

 

[45Auto]

Re: Am I running the right prop?

What's the gear ratio on the outdrive? If it's ever been replaced, it could easily be geared wrong for your motor.

Doesn't make sense that you would have to drop so far below a 19" pitch prop to get your RPM into the correct range. When's the last time the motor had a complete tune-up and a compression check? I would look into engine problems and outdrive ratio before going to something like the 15" pitch prop it would take to get your RPM right.

 

[steelespike]

Re: Am I running the right prop?

It does appear to me you have a little too much prop.
Don't worry about diameter,it simply follows the design.
A 17" prop same brand/model ss should add "about" 300 to 400 rpm.
A 15 should add 600-800rpm but a 15 seems undersize to me.
Going with same brand/material makes the results a little more predictable.
Are you sure of the weight? I found 4100 lbs.??
Boat does seem slow. Very roughly I figure 800 rpm might get you to 32
assuming about 5mph for about 1,000 rpm.
I would think you would be closer to 40.
Is the bottom clean and smooth? Does the boat respond when you trim up,
or does it lay down flat like its glued to the water?
Is the motor in excellent tune,timing set and advancing as it should?
Throttle plates opening all the way?