Prop Slippage...What % Is Considerred Good

[Senderofan]

Good Day All:

I've done a search here and elsewhere on the internet. I'm trying to figure out what an ideal % of slippage is for a prop. I'm in the process of gathering data on my re-power and want to make sure my set-up is proper and that my data is correct.

I've run calculations on my data and come up with 6-7% slippage. This doesn't seem right with an aluminum prop ( I think the % is too low)...but I'm looking for advice? My assumption is that slippage calculations can help with proper set-ups ( Mounting height of the motor) as well as selecting the model of prop.

The data I have is...My i-Command gauges tell me I'm turning 5850 rpm's at WOT with a top speed of 47-47.5 mph..per gps with my normal load. My 2009 175 e-tec has a 1.86 gear ratio. The current prop is a 17" aluminum BRP prop.

We are going to try a 19" pitch Ballistic today...and gather some additional data. The aluminum is allowing the motor to rev. beyond what BRP says is their "Ideal" max. operating range rpm of 56-5700. I'd really like to make sure my data is correct and have a better understanding of what changes I need to make before I make an investment in a stainless prop.

Thanks in advance for any information / advice,

Wayne

 

[hwsiii]

Re: Prop Slippage...What % Is Considerred Good

Wayne, their website show a maximum WOT RPM to be between 4,850 and 5,850. In theory if you were changing to the exact same model prop you have and hanging to a 19" Pitch you would lose right at 500 RPM and stay right at the same 47 MPH.
But when changing from an aluminum zero rake stern lifting prop to a high rake bow lifting prop with a 2" higher pitch it is hard to say, especially not knowing what kind of a hull form you have.

H

 

[Senderofan]

Re: Prop Slippage...What % Is Considerred Good

H:

Yes, they publish the 5850 as the top rpm....but BRP has published "Ideal" WOT rpm's for their motors....mine being between 5600-5700 with ones normal load.

My boat is a Lund fisherman 1800. It's a deep hulled fish and ski type arrangement, aluminum. Weighs 1475 lbs...dry weight.

I'm thinking something is amiss with the information I'm getting........I'm thinking 6-7% slip is too low. I'm trying to make sure my tach is correct...It should be more accurate than my speedo. Both are i-Command...the speedo has its own gps antenna....and the tach connects into the EMM network.

We're going to head out to test the Ballistic prop in about 30 minutes...so hopefully that will give me some additional info.

Wayne

 

[hwsiii]

Re: Prop Slippage...What % Is Considerred Good

I agree, when you get those numbers place them here and I will help you make sense of it all.

H

 

[Senderofan]

Re: Prop Slippage...What % Is Considerred Good

I agree, when you get those numbers place them here and I will help you make sense of it all.

H

Thanks for the help...As fate would have it...we went to two different lakes and both the parking lots at the ramps were over filled with rigs. Guess I should have figured it would be that way...first decent weekend and a holiday and all. Guess we'll have to try during the week.

Thanks Again,

Wayne

 

[hwsiii]

Re: Prop Slippage...What % Is Considerred Good

Just post back here after you get the new numbers. I will tell you that in my database of over 85 boats running aluminum props at very close to maximum WOT RPM they average a Prop Slip Rate of 9 to 11% at WOT.

H

 

[Sandhammaren05]

Re: Prop Slippage...What % Is Considerred Good

The less 'slip' (thinking of a blade as a screw-it isn't), the less drag (eddy creation) from the leading edge. It sounds good, I would leave it alone. A prop is a hydrofoil and works exactly like an airplane wing (or sail) in a nonintuitive way, via creating circulation about the blade (see Dave Gerr's 'Propeller Handbook' for a very good discussion for laymen). The closer the pitch of the helical wake (prop pitch times ' screw efficiency'), the smaller the thickness of that helical wake and the less the drag created by the blades.

My rule of thumb in racing was that the pitch measured along a small chord (about a 1" chord with a Rundquist pitch gauge) at the leading edge of the blade should run close to 100% 'efficiency'. That means: less eddy creation/drag created as the leading edge slices though the water.

Good Day All:

I've done a search here and elsewhere on the internet. I'm trying to figure out what an ideal % of slippage is for a prop. I'm in the process of gathering data on my re-power and want to make sure my set-up is proper and that my data is correct.

I've run calculations on my data and come up with 6-7% slippage. This doesn't seem right with an aluminum prop ( I think the % is too low)...but I'm looking for advice? My assumption is that slippage calculations can help with proper set-ups ( Mounting height of the motor) as well as selecting the model of prop.

The data I have is...My i-Command gauges tell me I'm turning 5850 rpm's at WOT with a top speed of 47-47.5 mph..per gps with my normal load. My 2009 175 e-tec has a 1.86 gear ratio. The current prop is a 17" aluminum BRP prop.

We are going to try a 19" pitch Ballistic today...and gather some additional data. The aluminum is allowing the motor to rev. beyond what BRP says is their "Ideal" max. operating range rpm of 56-5700. I'd really like to make sure my data is correct and have a better understanding of what changes I need to make before I make an investment in a stainless prop.

Thanks in advance for any information / advice,

Wayne

 

[Tacklewasher]

Re: Prop Slippage...What % Is Considerred Good

I'm thinking something is amiss with the information I'm getting........I'm thinking 6-7% slip is too low.

I've got an 18' Starcraft and get 7-8% slip running an aluminum 17p prop.

I get around 3-4% running a SS Stiletto 15p. 5800 rpm at 39.5 (gps)

I've got a 1980 Evin 115 with a 2:1 gear ratio for those looking at the calc.

Now I've spent a good chunk of time setting my motor height and weight in the boat to get there, but I think it can be done on these lighter boats.

I think you may be chasing your tail a bit here.

 

[hwsiii]

Re: Prop Slippage...What % Is Considerred Good

I agree with you tacklewasher, trying to find a prop that will turn between 5,600 and 5,700 RPM is a VERYYYYY hard thing to do. You could spend manyyyyyy days trying to meet that specification, and if that is their IDEAL RPM they need to redesign the motor to have a wider IDEAL RPM.
With that being said, the aluminum prop you are running now is running at a very efficient Prop Slip and a very decent speed. You will be hard pressed to find a prop to run exactly between 5,600 and 5,700 RPM.

H