gellis11

Just tested a PowerTech OFX 4 blade, 22p., 15.25 d and 454 with an alpha SS on an 87 Donzi Z25. Only got 3800 rpm and 52 mph gps. Love the way it seems to lift the whole boat when trimmed out. What prop should I run to pick my rpms back up to 4600 without dropping mph so much?

A.O. Razor

I would try the Revolution 4 in a 21". I think you will gain both speed and rpm's. If your prop shaft centerline is deep, 6" or more below bottom, try a Mirage+ 21" as well. With a 454, I would be carefull, running that amount of power with a big 15"+dia. 4 blade prop thru the Alpha.

AIR TIME

I agree, you don't want to hurt your drive. the rev 4 is a great prop but , with your drive start with the 21 m.

A.O. Razor

Just noticed your numbers don't add up. What gear do you have in your drive? Your tach might be off. If you have a 1.5 gear, your slip is 1%, and that is not happening.
Anyway. Going down 1" in pitch and 3/4" in dia. will net you about 300 rpms and about 450 with the M+. If your rpm's are 3800 and your GPS is off, you'll need a 19" to get in your rpm range, maybe even less. If it is your tach that is off, and you run, let's say 11% and you're actually spinning the prop @ 4200 rpm's the 21" is right. Last option is, that the prop is wrong and is 24" of true pitch, then the numbers look ok again. Bottom line is, you'll need to figure this out. If you know the GPS and RPM's are right, your prop is off, you need the 19" as baseline prop. If the tach show 400 rpm's or so too low, try a 21" as baseline, and if GPS is off, it's the 19" again. Hope this makes sense, I'm a bit sleepy.

gellis11

I just had the alpha ss lower rebuilt and the upper is in great shape. I know it's a weak link but it has had 24 yrs and at one point ran about 360 hp thru it. Looks like it was pampered. I hit a rock and bent the prop and prop shaft causing the rebuild. I would love to put a bravo on her and then play with the 454 Mag. I will test the tach and gps in the am. Thanks for the info.

A.O. Razor

You're welcome. The prop you have now, has that been rebuild? If so, there is a possibillity that the pitch is different as I suggested. Why I'm reccomending the Rev 4 and M+ is because of this. You have a 22' boat with relatively mild power, so the boat is not difficult for a prop to carry and the motor will use some of it's power to spin the prop. A good bow lifting prop with smaller dia. like tha Rev4 that is 14.625" should be plenty for your size boat, so is the M+ if the prop sits deep. My point is, there is no need to waste speed and power on a big dia. wheel. Heck if you can get your hands on one to test. Try a Vensura just for the helI of it, might be enough. Again, how deep your prop shaft centerline?

Let us know how the tach and gps thing works out.

gellis11

The prop shaft runs 4.5 inches below the keel. The Mirage that was damaged was 14.5 d 23p. The alpha ss is 2.5 inches shorter than an alpha one.

gellis11

Z25
(aka 25 Firefox)
LOA: 24'7"
Beam: 8'6"
Deadrise: 22 degrees
Draft to Keel: 18"
Draft Outdrives: 34"
Approximate Weight: 4,200 lbs.
Fuel Capacity: 66 Gallons
Standard Power: Merc 330 HP 454 Mercruiser Magnum

gellis11

New prop and old

gellis11

Are these numbers close? RPM-3800, Prop Pitch 22, Gear Ratio 1.32, Speed GPS 52, Slip 13.3%. Is this prop slip number realistic?