Prop slip calculator...?

[roozter2550]

14/19 Mirage Plus on my Mariner 225 EFI with 1.87 gear ratio lower
25.5' ProSports 2550CC, 25 degree deadrise, molded bracket style transom...

The water was like glass this past weekend and I got to open her up and play with trim settings

60 gals fuel, 2 people on board, 100lbs ice in the fish boxes, full 48qt cooler, rods and tackle,

trim tabs fully retracted up,
WOT 5200rpm,
47.8mph on the gps

ran the numbers on Hering Perf Props website calculator...showing a 4% prop slip.

I'd like to run the numbers on some other calculators...I figured the slip would be higher.


any body got some good sites?

 

[Silvertip]

Re: Prop slip calculator...?

Try this one.

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

 

[roozter2550]

Re: Prop slip calculator...?

came up the same....4%

 

[180shabah]

Re: Prop slip calculator...?

came up the same....4%

...and it should be the same. Your gear ratio, rpm, pitch and actual speed didn't change so neither will the results.

 

[roozter2550]

Re: Prop slip calculator...?

...and it should be the same. Your gear ratio, rpm, pitch and actual speed didn't change so neither will the results.

what...you've never looked for a 2nd opinon?...lol

I expected same/close results but since prop slip is not an exact, 2 different calcs might run diff numbers.



next question...

is 4% prop slip a decent value?
I read that 6-15% should be the target

prop slip is a confusing and largely misunderstood subject, including by myself.

you would think that 0 prop slip would be best as all rotational force is turned into forward motion, but from what I've been reading, prop slip provides thrust.

common sense tells us that slip is wasted energy...



am I reading this right?

 

[tdrudd87]

Re: Prop slip calculator...?

Nope, prop slip is bad under most conditions, but it is impossible to get rid of. It is mechanical loss which cannot be eliminated, only reduced.

It IS used to allow the rpms to rise on holeshot on ventilating type props. Still not a good thing when on cruise.

Terry

 

[180shabah]

Re: Prop slip calculator...?

...I expected same/close results but since prop slip is not an exact, 2 different calcs might run diff numbers.

Your prop pitch is the distance it would travel forward in one revolution with ZERO loss. Factor together RPM, gear ratio and pitch and you get a theoretical speed. It WILL be different from your actual speed. The difference is slip. The math is pretty simple, and all calculators should return the same result.

is 4% prop slip a decent value?
I read that 6-15% should be the target

Anything under 15% is decent and single digits = great. 4% is pretty rare, but given the margin of error when collecting speed data........

prop slip is a confusing and largely misunderstood subject, including by myself.

you would think that 0 prop slip would be best as all rotational force is turned into forward motion, but from what I've been reading, prop slip provides thrust.

common sense tells us that slip is wasted energy...

Remember that your slip value is all over the place and not fixed at your measured 4%. 100% slip when you first put it into gear, but at least it doesn't stall the engine....

Slip typically remains very high as you climb onto plane and drops as your speed increases. However, opperating efficiency doesn't automatically follow low prop slip - too many other variables.