Prop Slip Questions

[45Auto]

Anybody have any ideas on how to accurately measure actual prop slip? My calculated slip works out to about 2% at WOT. I very seriously doubt if my boat is that efficient, figure something in the 12% range would be more realistic.

Drive is a Bravo 3 (twin counter-rotating props) with 28? pitch props. As you can see, standard prop slip calculations give me about 2% slip at WOT. If I assume (***-U-ME) that the props are effectively 31? pitch, it puts the slip up around 12% which I feel is more realistic.

I understand that cup on the props can be the equivalent of 1? of additional pitch. I?m wondering if the efficiency of the counter-rotating props could be responsible for the additional 2? of pitch I need to account for to get the calculations to work out.

Wish I could afford a Bravo 2 lower to stick on it and see what difference it would make!

Any thoughts or ideas from the prop gurus?

 

[mthieme]

Re: Prop Slip Questions

Never heard of way to measure it.
I have read that 5-6% is to be expected under the best conditions.

 

[wca_tim]

Re: Prop Slip Questions

I kinow it's a silly question based on who I'm asking... but have you checked the accuracy of your tach? adding 1" for cup... and a little off on the tach would make that much difference, no? I don't know about you, but I can't even read mine that accurately...

 

[walleyehed]

Re: Prop Slip Questions

Just for grins, why don't you post the actual stamped pitch, the max RPM and the actual GPS speed, and of course the gear ratio.....let's do it the old fashioned way and see just what you may have there.

 

[45Auto]

Re: Prop Slip Questions

Took me a week to get back to my work computer that had the actual numbers on it. I'd figured it was easier just to show it on the graphs, but here's the data:

Motor RPM........GPS mph

1000...................6.4
2200...................28
3000...................41
4000...................56
4400...................63

Drive is a 1.81:1 Bravo III, props are stamped 28P.

The tach is a digital Smartcraft tach reading from the PCM555 engine computer. It agrees with the analog tach that was in it before I replaced it with the Smartcraft instruments. That's never a silly question, Tim! I've learned to verify data the hard way.................

 

[Turbo1]

Re: Prop Slip Questions

I calculate 6% at WOT...
8% at 4000...
10% at 3000...
over 16% at 2200...
58% at 1000RPM

These are the figures I come up with using the 28P props....31P would prove un-realistic in that they will show a negative number.
Not sure where you came up with your numbers for slip/prop pitch but my calculations show realistic numbers as it is with 28P's.

Let's see if walleyehed comes up with any figures......

 

[45Auto]

Re: Prop Slip Questions

Not sure what you're using to figure slip, but all 3 of these work out to 2% at WOT. Don't have time to check all the other points right now but I used the same calculations for all of them.

http://www.go-fast.com/Prop_Slip_Calculator.htm

http://www.go-fast.com/prop_slip.htm

http://sites.mercurymarine.com/port...26_49291:126_49315&_dad=portal&_schema=PORTAL

 

[walleyehed]

Re: Prop Slip Questions

I come up with numbers within tenths of Turbo1...
Where are you figuring the 31P? or "why"? 31P comes out negative...not possible, so there's no reason to even consider they act as 31's.
The numbers T1 and I both come up with are, if anything, actually a bit low at WOT, yet realistic. 2% just isn't possible, so don't get hung-up on that number.
There is some variable you are using that is not representing what you actually have, and I "think" it's pitch.

This para is right off of the (one of) calc page you used....

"Slip is never a negative number, and will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent."

Just that phrase alone puts T1 and myself right smack in the middle of what you should be.

 

[45Auto]

Re: Prop Slip Questions

Maybe we need to figure out what prop slip is. All the definitions I'm familiar with define it as:

(Theoretical Speed - Actual Speed) / Actual Speed

Theoretical speed (MPH) in my case is:

Prop pitch x (engine RPM/gear ratio) x (1 foot/12 inches) x (1 mile/5280 feet) x (60 minutes/1 hour)

If you cancel out all the constants it works out to:

Theoretical Speed = (Pitch x Engine RPM) / (gear ratio x 1056)

Means my theoretical speed is (28 x 4400) / (1.81 x 1056) = 64.46 MPH

Slip = (64.46 - 63)/63 = 2.3%

The Mercury Online slip calculator here:

http://www.mercuryracing.com/propellers/propslipcalculator.php

gives the same result. Put in 28 for pitch, 1.81 for gear ratio, 4400 for rpm, and 63 for speed and it'll give you 2% just like the calculations.

So does the Go-Fast calculator here:

http://www.go-fast.com/Prop_Slip_Calculator.htm

Maybe I don't know what I'm doing, could you show me how you're getting 6%??? Having a hard time understanding how I'm screwing up the online calculators!!

 

[45Auto]

Re: Prop Slip Questions

You can also use the equation straight off the Mercury page:

http://sites.mercurymarine.com/port...26_49291:126_49315&_dad=portal&_schema=PORTAL

which is:

Slip(%) = 1 - ((actual MPH x 1056)/(prop rpm x prop pitch)) x 100

Gives us:

1 - ((63 x 1056)/((4400/1.81) x 28)) x 100

which equals 2.26%

What calcs are you and Turbo using, Walleye??

 

[Dhadley]

Re: Prop Slip Questions

What motor is it?

 

[45Auto]

Re: Prop Slip Questions

496HO - 425 Hp, in a Crownline 22' bowrider. It would keep going, probably up to 4800 RPM or so. It was still pulling pretty good at 4400 RPM. I've only ran it up over 4000 RPM for a few seconds twice. I typically cruise about 30 MPH, play around between 25-50 MPH. The acceleration in that range is impressive!!!

 

[Dhadley]

Re: Prop Slip Questions

I've never tried to calculate slip on a dual prop drive before but obviously it doesn't work the same as a single prop or even twin drives. Even working the formula backwards you end up with a prop in the 32-34" range. I'd guess there's some sort of rule about adding pitch for the second prop.

 

[Turbo1]

Re: Prop Slip Questions

On a duo-prop type drive, the props are in "sets"...these Sets are numbered in a fashion that reflects the "Set", not the pitch of 1 of the props...in reality, 1 actually has greater "bend" or twist than the other, but the "set" is given a value. That Value is 28P, and being of the design they are, you would punch in about 29.2-29.5 for "pitch". I used a conservative 29P for my calculations, of which is a business tool that I do not wish to share the formula.
The duo's are most always lower slip at WOT than a single prop.
Personally, unless you want to lower top-end RPM, I don't see any problems with your set-up.

 

[45Auto]

Re: Prop Slip Questions

On a duo-prop type drive, the props are in "sets"...these Sets are numbered in a fashion that reflects the "Set", not the pitch of 1 of the props...in reality, 1 actually has greater "bend" or twist than the other, but the "set" is given a value.

Got any documentation on that related to a Mercruiser Bravo 3? I know the Volvo DP's are set up with different pitch props (thus the A2, A3,etc, prop sets), but the prop shops I've talked to (2 of them) both claim that both B3 props are nominally the actual pitch as stamped from Mercury.

Edit: Read post #12 on this thread from Don. All the documentation I need about B3 prop pitch - the word of the Guru!:

http://forums.iboats.com/showthread.php?p=1840386

I like that "secret prop slip formulas" idea . Since slip is defined as (Theoretical Speed - Actual Speed) / Actual Speed, isn't that kind of like saying you have a secret multiplication formula that lets you say 2 X 2 = 5???

I'm happy with the performance of my boat. Not looking to change anything. Just trying to find out if anyone's aware of any commonly accepted industry values used for computing slip on a B3. I can always throw in an arbitrary "duo-prop" factor and make it come out to 6% or 10% or 12% or anything else anybody wants.

 

[walleyehed]

Re: Prop Slip Questions

doesn't matter how "we" figure it...the numbers are built into the design.
The slip is figured by the numbers given and the set together is equal to 28P...
Lets use a std old Johnson 115 as an example....let's put a duo type prop system on it. Let's say that normally we would turn a 19P with a single prop...OK, now, let's put 2 19P props on the duo-type drive. I think you can see where the RPM would go...we've just doubled the load on the engine. To make it work out, we need to drop the pitch about 4" on each prop for this to even come close to working, maybe more.
The props used on the B3 may be 28P as a set, but they are not 28P each.

Also, the B3 prop sets are NOT the same diameter or bld area-FACT!
the B3 sets are just as Volvo...front is larger than the rear.
I am in direct contact with Merc on this stuff-nothing against Don-he knows Mercruiser...I work with props, Don doesn't.
This is becoming way too complicated and yet it is SO simple.
I'm done.

 

[Turbo1]

Re: Prop Slip Questions

Give it a rest Wally...some get too hung on the graphs and numbers..
It's clear as tap-water to me.

 

[Tail_Gunner]

Re: Prop Slip Questions

While i am not a prop expert by any mean's, there is something called a drag coefficent. When dragging more mass @ high speed's and assuming greater effiency there is but one word. Frankly at any speed that create's resistance...

 

[wca_tim]

Re: Prop Slip Questions

I like that "secret prop slip formulas" idea . Since slip is defined as (Theoretical Speed - Actual Speed) / Actual Speed, isn't that kind of like saying you have a secret multiplication formula that lets you say 2 X 2 = 5???

ROTFLAMO!!!

 

[45Auto]

Re: Prop Slip Questions

doesn't matter how "we" figure it...the numbers are built into the design.

I think it does matter how you figure it. Only way to make valid comparisons is if everyone uses the same terms. If the entire industry calculates prop slip based on a common definition and gets 2%, and you calculate it some other way and get 6%, it pretty much makes all your other prop calculations questionable when compared to someone elses. Kind of like you saying you define 1 horsepower as 16,500 ft-lbs per minute, while everyone else uses 33,000 ft-lbs per minute. You can claim you have a 300 HP motor when to anyone else in the world it would be a 150 HP motor. No way to compare unless everyone uses the same definition.

From Walleye;

I come up with numbers within tenths of Turbo1

Turbo1 claims he uses a "proprietary business tool" and won't show his calculations to get his 6% prop slip numbers. Do you use a "proprietary" prop slip calculator also? I've shown the calculations where my numbers come from. Pretty useless discussion unless we're using the same terms. Can you show your calculations where you get 6%? Maybe we can figure out the differences and come up with something applicable to the B3, kind of like the "add 1" of pitch for a cupped prop" rule.