Do these numbers look ok

[a70eliminator]

Thanks, This is a 20' chaparral open bow with an OMC 5.8 V8.
I was getting 49 MPH (GPS) at 4200 RPM's trimmed up.
Light load was myself wife and 12yr old daughter 400lb at the most and half tank of gas (20 gals.) Prop is aluminium 14-1/4 x 19.
When pulling a skier with a moderate load aboard I'm lucky to get 4000RPM's motor spec is 4200-4500 It seems maybe I need to drop the pitch to a 17, does that seem too small for this set-up, should I be looking somewhere else for an additional 500 RPM's to keep it in range? (Tune up already done).
Finally is it just expected that sometimes depending on conditions your just not going achieve the WOT specs.
And yes this daredevil kid wanted me to crank it up.

 

[Silvertip]

Re: Do these numbers look ok

You might just have one of those combinations that one prop can't serve both purposes well. Since you are a little short of revs with a light load, you might consider a stainless 18P for cruising and it may even help with the towing duties. I don't know what kind of towing you would want to do where you need max rpm since that would likely put the riders in the hospital. If hole shot is really bad when towing, then a 17P may be in order. What you end up doing depends on what the mix of cruising and towing you do. If you do a 50-50 mix, you might consider two props.

 

[a70eliminator]

Re: Do these numbers look ok

Thanks, just so I can understand this better, if I'm alone and can spin up to 4500 RPM's (top of the range) then with a boat load of riders I should get at least 4200 (bottom of the scale) if prop is efficient. To determine best prop I should go out alone and see what I get and go from there right?

 

[chris0061]

Re: Do these numbers look ok

I think you want to test your prop with the normal load you would hit the water with, full tank, equipment exc,exc. When I put on a different engine on my 17 foot runabout it came with a 22 pitch stainless raker prop and I could only gain 4800 rpm. My max is 5500 on a GT 150. I bought a 19 pitch alum and got to 5200, then got a 19 pitch stainless and got to 5500 with normal load and goes to about 5700 as the 30 gallon tank gets empty. I got a 18 pitch turbo prop and it pulled a little more rpm not much though and I didnt get my speed down till my GPS came in and have only tested speed with the Michigan 19 pitch I have on now. It seemed to perform better than the turbo so I have the turbo as a back up. I get 51 mph at 5500 rpm that's about a 4-8 percent slip and therefore get good hole-shot. I think I got lucky getting the right prop as it makes so much difference in design, cup, not cupped, exc. exc. I found out that certain props are for certain applications. example, the raker is for bass boats and high performance boats. I can tell you this though. If the boat is propped right the performance throughout the entire rpm range is so much better.

 

[a70eliminator]

Re: Do these numbers look ok

Thanks Chris, thats some good real world information there, and it helps. I'm running the prop that was on the boat when I got it, I've recovered from the initial cost of the boat and now I'm going to start playing, you know, if it aint broke fix it anyhow right, I just always think things can be better. I verified my tach is dead on and I should be turning 4600WOT.