Prop size suggestions

[Sandjob]

i'm usually good at propping but can't quite figure this one out. Here are the numbers.

1-pro-line 23 ft walkaround 3,960 pound hull 500 pounds motor 800 average load. total-5,250
2-200 hp optimax 25 inch shaft mounted with cavitation plate about 1 1/2 inches above keel.
3-My use is primarily cruising and fishing.
I have run 3 props

15 1/4 x 19 Mirage 5,000 rpms Wot 42 mph
14 3/4 x 17 ballistic 5100 rpms Wot 41 Mph
14 1/4 x 17 stilletto 5450 rpms Wot 39-40

I'm lost my 225 efi was much easier

 

[hwsiii]

Re: Prop size suggestions

Sandjob, my information says that the Optimax should be able to turn 5,750 RPM when propped right and it has a 1.75 gear ratio, if this wrong everything I say after this is wrong, please verify this.

I ran prop slip numbers for you, maybe this will help you see better what is happening.

Prop Slip

H

 

[Sandjob]

Re: Prop size suggestions

H,
Thanks for the numbers. I see the 14 3/4 by 17 has the best slip percentage. I just can't figure ou how to get my rpms up without going down in diameter. I really can't imagine going under a 17 in pitch. I have had a few different engines on this boat and have never benn below a 17p.

 

[hwsiii]

Re: Prop size suggestions

Sand, you have three options in my mind,
1. Use the stiletto as is, because it brings your RPM up to a suitable level
2. Buy a mirage in a 17" pitch and that will bring your RPM up to about 5,600 RPM and you should not lose but about 1 MPH and may not even lose that
3. Try raising the motor one more hole up, a good indication of how high you can move it above where it is now is by seeing how high you can tilt it before it starts ventilating. If you can move it up one more hole, theoretically you will gain another 100 or so RPM


H

 

[Sandjob]

Re: Prop size suggestions

I think I will try to get my hands on a 17 p mirage. The diameter might keep me from getting those extra rpm's though. I think most of the mirages are 15 1/4 diameter. I had the motor on a jackplate until a few weeks ago. I tried several heights. I even had it about 2 1/2 inches higher than it is now. No change in RPM's. It baffles me. I had to remove jackplate because I could not trim my motor out of the water without hitting my steering cylinder on the transom. I keep it in a wet slip so it is a must to get lower end up. I'll keep trying.

 

[hwsiii]

Re: Prop size suggestions

Check the bottom as well, if you use the wrong kind of bottom paint it builds up in layers and gets very rough as it sloughs off the paint, and that certainly slows a boat down.



H