slip numbers of 10% and higher, why are the numbers high?

[mrcrsr]

my hydrostream virage running a 32 cleaver running 1.62 gears at 6200 rpm is running 102. when you input the numbers into the online mercury racing slip calculator it shows 11% slip, which seems pretty high. even running a 30 et if i input the info it shows 10% slip. is there something wrong w/ my setup, or is that the way it is? the whole rig comes in a little over 2k lbs w/ the 3 litre. thanks!

[stvhelm]

the more slip I had the faster mine went. I had more like 15%

[Jimboat]

my hydrostream virage running a 32 cleaver running 1.62 gears at 6200 rpm is running 102. when you input the numbers into the online mercury racing slip calculator it shows 11% slip... is there something wrong w/ my setup, or is that the way it is?

There's a certain amount of "slip" that is required in order for a propeller to do it's work. If slip were zero, then velocity must also be zero, since the prop would not be able to move any of the water out of the way.

So propeller design and prop selection is always a trade-off. When there is "more slip" it is easier for the propeller to move water out of the way, but when there is "more" slip we move a slightly shorter distance for each revolution of the prop.

A smaller prop will usually display a bit lower calculated "slip" ratio, but may or may not be a sufficient advantage to generate the result of higher measured top speed.

It's not uncommon at all, especially with high performance boat setups, to achieve the fastest "top speed" condition while also measuring higher calculated "slip" ratios.

A really efficient performance hull could shoot for 9% slip. It's quite common to see the best setup performance hulls with 10%-15% slip.

Listen to Helmut's experience, for example. I've never seen a better setup or better performing STV than Helmut's was, and he was getting 15% slip!

[JWTjr.]

Slip is always viewed as a negative. It is not. As Jim notes above, some slip is necessary for the assembly to even move. It doesn't matter how much calculated slip there is unless the boat is not performing well. If there is a problem...i.e. not enough lift, too much wetted surface, engine running at an exaggerated positive trim angle to obtain best performance, etc.--then it's time to solve the problem *(which often shows up in the calculated slip %). If your setup runs faster and handles better with a propeller that, when running the numbers, calculates to 13% slip (for example) as opposed to running slower and/or handling worse with a propeller that, for example, calculates out to 8% slip...then the prop with 13% slip is the better prop!!

Don't forget that in order to calculate slip accurately, you need:

(1) an accurate tach--too many say "I know my tach is right" when it's very possible to be off 100, 200, 300 rpm--which will dramatically affect your slip % calculation

(2) an accurate speed reading...true, GPS is the best (i.e. cheapest and easiest relative to accuracy) currently available way of recording speed, it's not always right. Many things can affect GPS reading reliability. This is why I still use a calibrated and tested Stalker radar gun when testing for the magazines. This is why the UIM and APBA do not use GPS for speed record recording.

(3) an accurate pitch number for your propeller. How many own and are using (know how to use) pitch/rake/diameter measuring tools (i.e. Rundquist, Combi, etc.) to check what their props actually measure as opposed to just taking the number stamped on the hub as "gospel"? I bet not many. Propeller pitch as measured can vary widely from what is stamped on the hub. If your prop says 30 on the hub and actually measures 29" across the blade faces, and you are using 30 in your calculations, it will obviously throw your results off dramatically.

(4) an accurate gear ratio number. This is actually the easiest and most reliable constant in the slip equation, as there are only a few factory stock gear ratios and it is easy to check which one you have.

So, it's not as easy or cut-and-dried as most think. Obtaining the most accurate data and plugging it into the equation to solve for slip will give you more accurate results.

John

[baja200merk]

Cleavers always slipped a bunch on my heavy mirage and wouldnt cruise worth a chit :nonod:

Seemed like the longer i strung it out slip would go down a little but I'm sure its got somthing to do wit the fact that the most expensive prop I own cost $200 new :p

[Jimboat]

Agree with John on all counts. "Slip" is ONLY a "calculated" ratio (compares theoretical distance travelled with actual distance travelled) that can be interesting to consider when doing comparisons. But it's ONLY a calculated number.

John has outlined the inputs that MUST be accurate and correct, in order to even compare two slip ratio's (%). Of particular difficulty is obtaining the "correct" pitch value. Most performance propellers are made with some amount of pitch variation throughout the propeller. The pitch marked on the hub can only be the "average" pitch of the propeller - and can only be an estimate since it's hard to measure the "real" value of a varying pitch. So unfortunately, the accuracy of the value that you would use in your "slip" calculation will vary from prop to prop - and this really affects the results of your "prediction".

[afr]

jim i came up with a formula myself for pitch varences and cup
if i have a 26 with a 150 rpm cup
i use 26.125 double cup would be 26.250 and if i have a 4 blade verses 3 i add inch of pitch so everthing goes up to 27 .?
but like you guys said its only as accurate as to how accurate the data is you put into it

and i was trained not only how to read the pitchometer but how to make them as well and from the years and years of prop repairs i became able to see pitch changes by eye so in other words without checking if it is a 26 or not i would see if it had more rake then a standard 26 or more tip pitch then a normal 26 pitch profile as well and
i almost always just at a glance even can guess what pitch a prop is and if its straight up or molested or bent

[Dave Strong]

As John said ACCURATE pitch, rpm, and speed are are needed. This rarely happens in the real world. Most of us would be depressed with the end numbers.

Dave

[David]

How do you get a tach calibrated? My SS slip numbers are improbably low with most props, which makes me think the tach reads a bit low. GPS speeds consistently yields low slip. Not that correcting the tach would make me change anything. 102 is pretty fast. You must have something right.

[pyro]

My Vegas XT ran a 30 cleaver at 95 mph, 7200 rpm. That's also a lot of slip, but like Helm said about his, it's also the fastest it has ever ran...

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[mrcrsr]

speed numbers are w/ gps, 4 runs total, same every time against the rev limit. tach was checked against the ddt, and i built the gearcase, the actual ratio is like 1.617, and the prop is a labbed 32 cleaver(same w/ the et being labbed), i just thought those numbers were high, but thanks for the help! this boat(even w/ the 2.5 at 110 mph) has shown numbers around 10%, and i have the old mercury slide rule horsepower calculator/slip calculator, it says 10-15% slip is good for runabouts???? the cleaver runs great w/ no weight in the boat, but w/ 4 passengers and coolers it was not the rite choice, the boat just drags thru the water until 70 mph, the et tends to throw the boat around over 95(it seems to induce something like chine walk), funny thing is it never did that until i put the 3 litre on(extra weight)

[mrcrsr]

How do you get a tach calibrated? My SS slip numbers are improbably low with most props, which makes me think the tach reads a bit low. GPS speeds consistently yields low slip. Not that correcting the tach would make me change anything. 102 is pretty fast. You must have something right.
david, my ally xr was showing slip numbers of 6%, it leads me to think that slip has something to do w/ the weight of the boat and the resistance of the hull going thru the air and over the water

[terry taylor]

david, my ally xr was showing slip numbers of 6%, it leads me to think that slip has something to do w/ the weight of the boat and the resistance of the hull going thru the air and over the water

Absolutely correct thanks.

[baja200merk]

david, my ally xr was showing slip numbers of 6%, it leads me to think that slip has something to do w/ the weight of the boat and the resistance of the hull going thru the air and over the water

weight has a huge affect on slip.. It takes longer for a heavy boat to air out and reduce drag as you do the boat goes faster at the same RPM as the prop slips less. Of course slip will never go completely away and about 90% of the time people have super low slip numbers its cause the props pitch is more then the number stamped on the side. All these et's etc... if you lay say a 30"p prop on a 30" pitch block it would not lay flat due to cup and progressive pitch, blah blah blah. Theres a few good threads on that subject you get the point :)

[keyssr]

wtf my boat has like 3% i think theres sometning wrong idk

[kellytheaker]

Prop slip conversation 2010

Bringing this forward - I thought it was a good quick thread - a further discussion would improve my understanding- and 10 has passed

[transomstand]

a further discussion would improve my understanding-

Here's what I know. Boats are REALLY stupid and don't get math. If I run a high number on the GPS, my slip is "good", if I run slow my slip is "bad".

I could care less if I had 20% slip if I put up a big number. Too many people are trying to be scientists and make this stuff more complex than it is.

Read John Tiger's post and Helmut's from page one. That's the whole deal right there.

[powerabout]

how are you measuring the pitch on teh prop, thats the variable when the slide rules gives you weird data IMHO?

[transomstand]

how are you measuring the pitch on teh prop, thats the variable when the slide rules gives you weird data IMHO?

That's the thing. If the prop hasn't been blueprinted dead nuts perfect to it's advertised pitch, it's just a guess anyway.

[powerabout]

That's the thing. If the prop hasn't been blueprinted dead nuts perfect to it's advertised pitch, it's just a guess anyway.

Unless the blades are flat like cutting a tin can the pitch varies across the blade so which formula do you use to get a pitch number?
Then there is some cup just to add to the confusion.

[XstreamVking]

It all happens at the blade tips. This is where a prop can "cheat" at the slip numbers game.

[Dave S]

Wow….. just mo......jo.... try.any any prop.....doo not be deciver buy its looks or polish...…..

[JPEROG]

The guy driving the boat can also blow up the slip numbers. When you lay into a fast boat with some patience and finesse you can tell that its hooked up and pulling well. If you just stab the throttle and point its usually a disappointing pass.

Joe

[Dave S]

Yes ….JPEROG....