Prop Slip

[FreeBeeTony]

How much prop slip is acceptable? Think I am at 20%..........

I'm turning a 4 blade aluminum 15X16 prop with a 1.5:1 drive @ 3600RPM going 29MPH (GPS).

The boat is a 24' Grady White ~4000lbs. Bottom is clean.

Does this sound right?

 

[FreeBeeTony]

Re: Prop Slip

No replies?

 

[MikDee]

Re: Prop Slip

What engine? Is that top speed?

 

[MikDee]

Re: Prop Slip

My old 79' 24' SeaRay Cuddy (4000lbs) with a stock 350/260 Merc. would do 40 fully timmed out, with a 15X17" 3blade Merc Black Max aluminum prop, it didn't need a 4 blade?

 

[45Auto]

Re: Prop Slip

Slip is pretty much an irrelevent term (but fun to talk about) when it comes to props. You can easily find combos that show "negative slip" (meaning the boat is moving faster than theoretically possible) due to drive combos, prop cup, etc.

Your "20% slip" is due to the angle of attack relative to the water that's required for the prop to drive your boat at those conditions. You'll find that your slip varies from about 70% as your boat comes on plane to around 10% at top speed. 20% slip is about correct for the combination of pitch, RPM, and speed you posted. From the Mercury prop slip calculator:

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

"Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust."

Hopefully the 29 MPH at 3600 RPM is not top speed on your boat. If it is, you have a problem in that your RPM is about 1000 RPM too low. I would expect your boat to run around 40 MPH at WOT (wide open throttle) with your engine RPM between 4400 and 4800 RPM (real similar to what MikDee posted). At that point your slip will probably be 12% to 15%. If 29 MPH is your top speed, when is the last time you had a tune-up and compression check done on the motor? If 29 MPH isn't your top speed, post the conditions at top speed and let's see what your slip is.

 

[mthieme]

Re: Prop Slip

Sweet 'Ray !!!!!!!

 

[Tail_Gunner]

Re: Prop Slip

Tony could you post your top rpm and speed @ wot, i know its boring but it is how you get to a prop that will be the most efficent at crusing speed's.

As to prop type's there are very few options in a stern drive....3 blades for top speed and 4 blades for midrange performance and the latter is either bow lift or stern lift.

Just a personal opinion but sternlifting props on a I/O are probably gonna deliever the lowest planing speed and fuel econ. aka i use a 4 blade ss with a huge amount of stern lift...she will run 34 mph @ 3100rpm....that's the up side....On glassy smooth water i can get her to porpise with just a small jump in the bow area.... The prop has so much lift there is very little wetted surface on plane and a little weight imbalance... she falls off and begins to porpise.

 

[QC]

Re: Prop Slip

You can easily find combos that show "negative slip" (meaning the boat is moving faster than theoretically possible) due to drive combos, prop cup, etc.

You got some splainin' to do Lucy . . . I was going to go the PM route with this one, but this needs to be clarified here. What choo talkin' 'bout Willis? Me no understandy . . . I agree with everything else in your post in regards to Tony . . . But this one has me scratchin' me . . . well . . . my front brain cannot accept my thinker, see? If this is true I need to go edit about 1750 posts Say it ain't so, Joe . . .

Edit: I just fingered it out, you're talking about downhill . . . love you man, just want to get unscrunchified.

 

[MikDee]

Re: Prop Slip

Man, if I had a 350 Vortec motor, with those heads, a stock roller cam, & roller rockers, electronic ignition, 4" risers & thru-hull (instead of 3"), and my usual synthetic oil, plus a stainless 3 blade prop, and some 92 octane gas, I probably could've pushed 50! but who knew back then

I figured my slip to be around 15-20%

P.S. My affectionate name for the boat was "Tweety Bird", but I never got around to actually putting that on it.

 

[FreeBeeTony]

Re: Prop Slip

Forgot I posted this...............

My WOT is ~34MPH @ 4600RPM.

 

[MikDee]

Re: Prop Slip

I figure about 20% slip, but top speed should be more for a 350Mag with Vortec heads. Is that fully trimmed at WOT?
I was doing better then that with my old 79' 350/260 Mercruiser.

 

[FreeBeeTony]

Re: Prop Slip

Seems to be fully trimmed...........maybe it's the difference in the type of hull?

 

[bolo78910]

Re: Prop Slip

hey tony i see you are still fine tuning, i change from a 17 pitch to a 15 pitch and getting the same 4000 rpm so i think my tach is bad. with your speed in the thirties let me get some of that lol

good luck.

 

[Tail_Gunner]

Re: Prop Slip

Forgot I posted this...............

My WOT is ~34MPH @ 4600RPM.

GPS speed...How accurate is that tach...

 

[FreeBeeTony]

Re: Prop Slip

That is GPS speed...........I am assuming the tach is accurate within +/-100RPM.

 

[45Auto]

Re: Prop Slip

34 MPH at 4600 RPM (WOT) is probably about right. According to this article on Grady White Offshore 24's, http://www.boatus.com/jackhornor/power/24gradywhite.asp, a 225 HP Yamaha outboard would push it to just over 30 MPH. Since you're probably around 250-260 HP on your 350, mid 30's is where you would be.

Sounds like it's propped right, the slip number is irrelevent. Just means your prop has more angle of attack than a boat with lower slip numbers.

 

[FreeBeeTony]

Re: Prop Slip

Funny you posted that article............I read the same one today!!!

Guess I am about right........give or take...........it's not a speed boat.

 

[Tail_Gunner]

Re: Prop Slip

Forgot I posted this...............
My WOT is ~34MPH @ 4600RPM.

Two things i would do... first get your rpm up to 4900-5000, i know a lot of people believe that is over doing it..but your going to spend very little time there. The benefit is increased hole shot and increased touqre(higher rpm) at midrange operating speed's. If you go with a prop with a heavy cupping in will increase the bite and need the tourqe increasing its effeciency.

Something is wrong here, First a alpha one does not appear to have a 1.51 gearing..that's a bravo ratio..(i guess it could happen)

29@3600 and 34@4700 it will not even work in prop selector..

That motor sound like a 300-350 hp motor..4 barrel carb?? does it have a roller cam???.

Is your prop new....Is it slipping...aka bad hub??..you should be nailing 40

So i dabbled a bit and ran a slip calc with 1.81 gearing

%



I have no ideal if that correct...but it seems to align the stars correctly...

Damm it TG..it didnt paste ..

1.81 gears give you 14% slip

 

[FreeBeeTony]

Re: Prop Slip

Alpha's do come in 1.5:1 ratios........that's what is required for a V8.

It is probably 300+ HP.....350SBC/Vortec Heads/Roller Cam/4BBL.

 

[Tail_Gunner]

Re: Prop Slip

Alpha's do come in 1.5:1 ratios........that's what is required for a V8.

It is probably 300+ HP.....350SBC/Vortec Heads/Roller Cam/4BBL.

No Tony they dont go to the merc website......:redface: call them....there are people here who can tell you how turn your prop and determine your ratio...Bondo would be one of them i believe..

If not your facing gears that are far to tall for you setup and need to go to a high cup prop with a huge bite and small pitch...Ok that was a mouthfull...