Blackhawk Propeller Slippage Info.

[DonCig]

For those that run a Blackhawk outdrive, what type of prop slippage numbers are your experiencing/seeing? 6%, 8%, 10%?

Thanks,

DonCig

[blackhawk]

You will see much higher numbers because of the surface drive. Even though you are running true 31s they don't act like 31's because run at the surface when trimmed out.

[mattyboy]

plus true slip numbers are times 2 or divided by 2 depends on how ya look at it
;)

[MOP]

Being the nit picking guy that I am I have delved into this more then many true BH owners (me trying to run a fake BH) from what I hve gleaned from this and that sources the BH props do not hook up until over 38-4000 RPM. Real BH owners are running a specialized package "not to be compared to most boats) Your slip numbers deminish at the higher RPM's, the true BH shines at the top! I am hoping to get close to the real BH mid range numbers with my mouse motor fake BH

Phil

[Footloose]

what drives are you running? HP and prop details?

[blackhawk]

What I meant was a Blackhawk prop is "measured" the same way all other props are. One revolution of the prop=31" of forward movement. However, even though they are measured the same they do not perform the same because the are surface piercing. The only time they act the same is when the prop is fully submersed. But, when running trimmed out it does not act like a 31 even though it is considered a 31. That's what I meant. Hope it makes sense.

[mattyboy]

in theory your spinning 2 props so shouldn't you go 62 inches in one rev???
so a BH is gonna have a much higher slip %
does the slip formula take into account number of props??

[Woodsy]

Matty...

The props are counter-rotating.... 31P is the theoritical movement forward without any slip. When the BH props are fully submerged, say idling forward, there is almost no slip, hence the 6-8mph idle speed.

At 5000 rpm, with a 1.5 ratio drive, and a guess of about 10% slip, the 31P BlackHawk Props are acting more like a 28P stantard prop.

Hope that helps!

Woodsy

[mattyboy]

Woodsy ,
thanks I think I have it now, stop me if I'm wrong
on a BH you're not spinning 2 31 inch props you're spinning 2 props that add up to 31 inches total???


how did I do :)

;)

[Woodsy]

Matty...

Think RH thread and LH thread... both spinning on the same shaft. In theory, both props are the same 31P. One spins forward in a clockwise rotation 31", while the other spins forward counter-clockwise 31". I say "in theory" because the rumour is that the props on BH were in fact 2 different pitches.... I never took mine to a prop shop to have that confirmed though.

try this link to a pretty cool prop calc and play with the numbers....

http://www.rbbi.com/folders/prop/propcalc.htm

Woodsy Von Outboard

[mattyboy]

woodsy,
it's like hooking batteries up in series or parrallel that always gets me too ;)

I think I have it

maybe I just stick with my old volvo :) nice link that will keep me busy for awhile

:)

[blackhawk]

Scott, you busting my ballz? :D

Woodsy explained it very well. A "set" of 31s will act more like a 28 or so single prop. But, you'll see much higher slip numbers with the BH props, more like 16-18%. So, a set of 31s slipping 18% will act like a 28 single prop slipping 10%.

I believe one of the props is a 32 and the other a 30 for an average of 31.

[blackhawk]

That's where I was heading...


Now, what if you mix, say a 32 and a 28 to get a "30"???

You can't mix more than a 2" spread. If you do try a 32 and a 28 they will fight each other and the rear prop will shear off and hit the boat behind you! :D

Actually, I did try a radical set of cleaver props(the same ones Bryant tried) and if I remember right they were 3" or 4" different and one was a 4 blade and the other a 5. Looked radical as hell! Too bad they didn't perform as well as they looked!

[MOP]

I have been told by quite a few the front prop has less pitch then the rear prop, the front feeds the smaller back and that the pitch is an average of the two.

I posted the link below awhile back it has a good over view of surface piercing, it is only on single prop apps, the section Why Surface Piercing explains the theory nicely.

http://www.simplicity-marine.com/surfprop.htm

Phil

[blackhawk]

Here's a pic of the cleavers. I can't remember what pitch they were but they had different pitch, different diameter and different number of blades. It did reduce the slip!!! :D

[MOP]

Sure does look wicked, what boat ran them before.

Phil

[blackhawk]

They were tested on a 20' Cig and I was told they worked great. However, they didn't work so well on the BH.

[gero1]

i ran 81.7 on gps late last fall in a all stock bh, can someone tell what the slip numbers are?

[Woodsy]

Gero,

What rpm? Gp to the prop calc link above and plug in the numbers....

Woodsy Von Outboard

[gero1]

about 5100- never hit the rev limeter yet,i am looking for this calulater thing but cant find it, also if its gonea ask a bunch of dumb ? like those kids at auto zone- 2 or 4 wheel drive- ac or not- 2 or 4 doors- what coler is it i am gonea screem!!!!!!!!!!!!!!!!!

[Woodsy]

approximately 18%... thats based on 5100RPM, 1.5:1 drive at 81.7mph

here is the link

http://www.rbbi.com/folders/prop/propcalc.htm

[gero1]

thats not good, not good at all. i think i"ll go sit in my rocking chair and have drink...18 slipage, little gimbals, short drive, no rams, h%$& i give up

[blackhawk]

16-20% is normal for a blackhawk.

[gero1]

thanks, i feel better

[blackhawk]

thanks, i feel better
Dude, you're running 80+ with a stock 502! You have nothing to feel bad about. :biggrin:

[thriller]

Dude, you're running 80+ with a stock 502! You have nothing to feel bad about. :biggrin:

Yah,, Until he hits a wave without the 280 tabs:redface: Then your air-borne staring at the Stars ...Bateman and Road-trip have been there:yes:

Was that 80+ GPS turning 29's or 31's??

MM

[blackhawk]

Yah,, Until he hits a wave without the 280 tabs:redface: Then your air-borne staring at the Stars ...Bateman and Road-trip have been there:yes:

Was that 80+ GPS turning 29's or 31's??

MM

What, blackhawks will do that? :rolleyes: :biggrin.: