Donzi Classic 18 Prop
 

Need some prop guidance. Here's where I'm at.....
Engine:
383 small block chebby
7 psi boost, roots
alpha drive, 1.63 ratio
Performance:
Mirage+ 25P 72 mph 5670 rpm
Mirage+ 27P labbed, 75 mph 5638 rpm
Mirage+ 27P labbed, 76 mph 5728 rpm
The speeds are GPS. The WOT rpm is an average of about 30 seconds at WOT from a datalog. Add 70 rpm for max rpm. The state of tune was the same for the 25P and first 27P run. The second 27P run was after tuning and raising the rev limiter 100 rpm. Basically it looks like all my runs are against the rev limiter.
I'd like to get my speed up and the rpm down (~500 rpm). Some time ago I ran a labbed 29P M+ with a slight drop in speed and rpm. The boat felt a little floaty and sluggish in the midrange. The 27 feels like a good all around prop and I'll proably go there when I stop chasing speed but that's not the goal right now.
Any suggestions? Going back to the 29 now that I have more horsepower seems like a logical next step but was wondering if anyone had a better idea.

i wuold go to a 136drive or 150 so you will trun less rpm. i run 136 on my boat i pick up midrange 10mph 3600 i run 57mphif i run 150 i have to run4200to do 57

29 M+ props are act like 31 pitch M+'s.

I like the gear ratio change or I go to a small diameter Bravo 1 28-30 prop Labbed small to get the best over all performance.

Brett

Probably too late in the year to do a gear change. Probably try the 29 and call it a year. How does my slip sound at 15%

I have a 18 Donzi. Your slip is terrible. You definitely have to change the gear ratio to a 1:50.
For props , I would consder a Hydro OT 4 blade. You'll be cookin.

So what is a reasonable slip number? My slip numbers have been consistent with different setups over several years. Perhaps I should make sure my tabs or something else isn't adding to my slip? If I can reduce the slip through "tuning" I shouldn't have to do any gear or prop changing right?

Yesterday I ran the 29 and it was a little floaty as I recalled but not awful. It is nice crusing at 55 or 60 without spinning up the motor.

Date Pitch Labbed RPM Actual Speed Slip
8/21/2010 25 N 5670 72 12.7%
8/22/2010 27 Y 5638 75 15.4%
9/6/2010 27 Y 5728 76 15.6%
9/18/2010 29 Y 5732 78 19.4%

I run a 383 Mag with a standard Bravo lower. Mine HATED a labbed and cutdown Bravo prop. I mean HATED it. Well ok, you could drive it but it did not hold a candle to a M+ or Turbo. More torque lean and chine walk with the Bravo.

I can easily run less than 10% slip with either M+ or a Turbo. I can run 71 (7% or so slip) on a good day and 69 any day. The Turbo (spare the bass boat talk) is the best all around. The M+ is not bad. These older model hulls with the deep X like 3 blades. My experience mirrors most of the new 18's with the 6.2 (377).

All changes if you run a shorty though.

I would do down in gear and try some more props. You should be able to break 80 easy with the right setup.

Remember, all Labbing isn't alike!!!! Lab recipes need to vary with each application depending on what the boat is doing.

Brett

Agreed.

I wanted to share what many of us know works on the same hull.

He should try a number of combos. Who knows, the added torque of a roots blower may not like a 3 blade down low. I am very happy with my setup but will still try more. Dialing it in is part of the fun. It is also a great way to learn your hull. At least as long as you can still have fun. I have yet to regret trying a prop and still have a lot to learn.

That slip is still WAY high though.

i have a 24baja 718hp 136drive 23mirage prop at 5600doing 80.6 mph gps my boat runs best with 3blade. i think the 163gears are slowing you down . i think you will get more speed if you chage your gears

Are you running a nose cone?
Do you have external steering ?
Any side to side play in your drive?
Any 18 foot Donzi running over 70 mph needs both of these.
I think you should be seeing 80 mph with that set up

Agreed on the steering. I used a new transom ass, drive and all on my X-18 when I did the build up. I planned to to the steering any but I ran without for a few weeks. MUCH better with the external steering. It really settled the boat down.

I have not heard the nose cone thing that much on an 18. Mine runs on rails without one.

My classic 18 likes a labbed 29m+ that's been put down some, it's prolly more like a 28 but with the larger diameter it works better than the small diameter 27 on my boat, I've tried over 16 props on the boat and it is the best, when I tried the 27m+ it was one of the worst props I ran, here's my set up, stock mx 6.2 bravo 1.65 gears with -2 imco full dual steering, also agree on way too much slip! Even with shorty and stock 3 blades I'm still around 8-9%. Mine does not like 4 blades that well unless labbed, causes too much stern lift, makes mine real loose, another decent prop to try would be a TXP 3 blade. I'd try a 29m+ or try a TXP if it was me, Steve

Thanks for the responses.

Tried a 28 B1, here's where I'm at...
DatePitchType LabbedRPMActual SpeedTheo Speed Slip
8/21/2010 25M+N 5670 72 82.5 12.7%
8/22/2010 27M+Y 5638 75 88.6 15.4%
9/6/2010 27M+Y 5728 76 90.0 15.6%
9/18/2010 29M+Y 5732 78 96.8 19.4%
9/25/2010 28B1? 5691 78 92.8 15.9%

Again, the rpm is an average at WOT and the first 3 entries are against the rev limiter. It looks like the 29 and 28 are starting to pull the rpm down. (rev limiter raised) The B1 chined walked a little but otherwise ran pretty smooth.

To answer some of the questions...
No hyd steering.
No nose cone.
No play in drive.

We're out of season up here in Maine so I'll pick this up in the spring. I'd really like to know what's causing the slip. Stared at the transom in the garage and don't see anything that would cause turbulance into the prop. Any suggestions? I do have tabs and they're above the bottom of the hull but the tips could be dragging when the bow is up.

Some times being over concerned with slip isn't the right thing to do. I have encountered many set ups that get their best speed with slips near the 20% range. When we have harnessed the slip at WOT, the speed has dropped in the 3-5mph range.

We all want 9% slip but it isn't always realistic. Low slip does typically increase cruise speeds and fuel economy.

I'm not saying you should have 19%, but I'm am suggesting you may not get much bigger numbers on the top if the slip drops and the pitch does too.

Are you trimming the drive excessively?

Brett

Brett, thanks for your input and time. I don't believe I'm overtriming as on occasion I've gone past peak speed and had to bring the drive back down. Today I pulled the drive and physically checked the gear ratio, it is definately a 1.6. We'll talk in the spring and decide which prop I should try. Hopefully you have a demo I can try before purchase.