blackboat

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

ZXXX Donzi

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.

osur866

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

Traveler

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.

bbladesprops

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

Traveler

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.

IRONMAN