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Greg Maier
08-04-2002, 09:16 PM
I was using the prop calculator today and I figured that I have 20% prop slip. Is that possible, from what I've read on the board, it seems very high. Here is the data that I'm using:
5000 rpm
1.5 ratio
72 mph
29 pitch Blackhawk Props

Can anybody help me out with this?

Ralph Savarese
08-04-2002, 09:34 PM
www.boatpropellers.com/propcalc.html (http://www.boatpropellers.com/propcalc.html)

AVickers
08-04-2002, 10:10 PM
Since about 1/3 of your props are out of the water when at speed -- Blackhawk surface piercing drive -- 20% sounds like a possibility.

Are you hitting the rev limiter at 5000 RPM? Or is that all she'll do?

I understand that there are steeper Blackhawk props available -- up to 34??? What pitch are the other guys w/ Blackhawks turning to get their 80+ MPH?

Greg Maier
08-05-2002, 06:19 AM
I'm hitting the rev limiter at WOT. Thanks for the help.

Woodsy
08-05-2002, 06:24 AM
Greg...

Most of the factory BlackHawks came with 31P props... those are the biggest you can get. I would try out a set, it should drop your RPM to 4800 or so, and your speed should go up 2-3mph...

Woodsy :D :D

GEOO
08-05-2002, 06:33 AM
Greg,
20% slip seams high. I would think you should between 10-15%. How much trim are you using? How much slip do the other BlackHawk guys run?
My surface drive run's 12-13% @ WOT..

Bad-Tat
08-05-2002, 06:34 AM
Tomahawk and I got 72 in Bad Tat yesterday at 5000. Not fully up on top as was a bit rough so was still dragging some tabs. Have not hit the rev limiter yet. I have the 31" props on it.

Greg Maier
08-05-2002, 08:21 AM
I hit 72 yesterday with the 29 pitch props, but it was 95+ degrees out and 3 big guys in the boat, so that could have something to do with it. Does anybody know where I could get a set of 31 props. I've looked out on the net and couldn't find any. Can 29 props be repitched to 31 or is that too much?

blackhawk
08-05-2002, 09:25 AM
Greg, since the blackhawk props are surface piercing the slip is a lot higher. I believe I calculated mine to be 22-23%. When you think about it a 31 pitch prop is HUGE with a stock 502. The only reason we can pull it is because of the surface design. Your boat should have 31's on it.

Greg Maier
08-05-2002, 09:49 AM
Thanks, does anybody know of a source for Blackhawk Props? Can a 29 inch be reworked to a 31 inch?

Woodsy
08-05-2002, 09:57 AM
Greg...

I have been looking for some for awhile... I know where there is a set for $1400.... YIKES! You can rework your props, and it would be cheaper... you could also regear your BlackHawk... ie spin your 29's faster without bouncing off the rev limiter... that would be cheaper too...

Woodsy Von Donzi...

searching for the ever elusive BlackHawk propset.....

blackhawk
08-05-2002, 10:01 AM
5000 rpm x 29 pitch = 145000
1.5 ratio x 1056 = 1584

145000/1584 = 91.54 theoretical speed

72 mph actual speed/91.54 theo. speed = .7865

This would give you 21.35% slip

Going to 31 pitch props would give you a theo speed of 97.85mph x .7865 = 76.96 mph actual speed.

blackhawk
08-05-2002, 10:07 AM
Woodsy, the only problem with regearing is the next ratio is 1.36. I don't know if you could pull 29's with a 1.36. Even if you could I think you would give up acceleration. Not to mention idle speeds would go even higher. I already get yelled at in no wake zones! :rolleyes:

Woodsy
08-05-2002, 11:13 AM
Hola Compadres...

Had a little fun with the prop calculator over at Chad Hill Propellers, this is pretty much what I found out...

Greg's current setup, 5000RPM, 1.5 ratio, 29P props @ 72mph has a slip of 21%... pretty much in line with what we are seeing from everyone else... if you use .21 a constant slip factor (real world its variable, but not by a whole lot) these are the calculations you get...

RPM/Gear Ratio/Prop Pitch/Slip % = MPH

4700RPM/1.5/31P/.21 = 72.7MPH
5000RPM/1.5/31P/.21 = 77.3MPH

4700RPM/1.36/29P/.21 = 75.0MPH
5000RPM/1.36/29P/.21 = 79.8MPH

Now we know that the 502MAG MPI setup CANNOT pull 31P props to the rev limiter (unless you are airborne), most people see around 4700-4800RPM with this setup and 1.5 ratio gears at WOT. Regearing your drive nets anywhere from 2-3mph at the same RPM. Then you can tweak the existing 29P set to perfect, a little more cup get the slip factor down, a little less pitch, maybe get the RPM a little higher, lab finish etc...

I don't think you would give up anything in the way of acceleration, and as for the no wake speeds, according to the prop calculator this was the data returned...

600RPM/1.36/29P/.01 = 12.0MPH
600RPM/1.50/31P/.01 = 11.6MPH

No real difference... and by re-gearing the drive, you get the opportunity to put in the newer high strength gearsets... an added bonus!!

Prop Calculator Website...
http://www.chadhillpropellers.com/propellercalculator.html

Woodsy Von Donzi :D :D

BigGrizzly
08-05-2002, 11:39 AM
Blackhawk is correct. Surface props have 20+% prop slip. The higher X dimention is higher because less of the biting area is in the water

Randy

blackhawk
08-05-2002, 01:14 PM
I don't think you could pull a 1.36 with a 29's. I think you would be low on the rpm's. Like 45-4600 and that would put you at the same speeds as a 1.5 with 31's and less acceleration. Just my opinion. However, if you planned on doing some motor mods in the future the 1.36 would be the way to go. I tried a 1.65 balckhawk on my boat and the acceleration was UNBELIEVABLE. I could not believe the difference it made. Unfortunately, I gave up about 5mph top end. So I am sure going from a 1.5 with 31's to a 1.36 with 29's would make it accelerate a little slower.

The idle speeds would be pretty close so I guess that's no big deal.