prop producing neagative slip

[Hawaiian]

How common is this? <br /><br />My 15.25 X 17 Powertech OFS stainless prop produces the following numbers on a 19 foot Boston Whaler Nantucket with a Honda BF135 motor mounted 2 holes up:<br /><br />RPM SPEED SLIP %<br />4000 31 mph -3<br />5000 38 mph -1<br />6000 45.4 mph -.5<br />6300(wot) 46.7 mph +1.5<br /><br />I called Powertech yesterday and a technician told me that the prop privides good lift and when mounted high, the cupped tips in effect add more pitch to the prop. Is this true? I think the prop may be mis-stamped and be a 18 pitch prop. This 17 pitch prop would produce single digit slip numbers for an 18 pitch prop.

 

[timmathis]

Re: prop producing neagative slip

Could be true. Also A 17 ss will pull like A 19 alum.

 

[Perfidiajoe]

Re: prop producing neagative slip

On the prop slip calculator I use they tell you to add 1 inch of pitch for cupped props. I don't believe it is posible to have - slip. Joe

 

[Perfidiajoe]

Re: prop producing neagative slip

Do you have the gear ratio correct in your calculations?

 

[umblecumbuz]

Re: prop producing neagative slip

If you decide to manufacture props like that, I'll buy one.

 

[ErikDC]

Re: prop producing neagative slip

That's true, cupping the blades adds almost 1" to the effective pitch.

 

[pwiseman]

Re: prop producing neagative slip

Sounds like your boat is performing well. This is the most important thing to keep in mind. If you use the same number in your calculations when you make your tweaks, it will give you valid numbers in terms of how much you are improving performance. Ultimantly you just want to make sure you have things setup well. It really does not matter if your slip is 9% or 11% if you know you have it set as good as you can get it.<br /><br />These calculators are meant as guidlines, and two factors you need to consider are cupping and stainless material. Stainless blades are thinner material and will decrease friction. Cupping will grab more water and behave like a higher pitch. <br /><br />I would change your pitch number in your calcs until you get somewhere around a 10% number at WOT. This is going to be a good ballpark at where your prop is performing in terms of pitch. I find it somewhat interesting that changes in material and design that increase performance are not reflected in the pitch marked on the prop. I suppose it makes some sense when you look at hole shot. A 17 that behaves like 18+ in terms of speed will probably not provide a hole shot that behaves like an 18+ pitch.

 

[timmathis]

Re: prop producing neagative slip

As Elmer Thud said twu twu twu.

 

[Hawaiian]

Re: prop producing neagative slip

Perfidiajoe, I used the 2.14 ratio per Honda specs.<br /><br />Paul, you make some good points. I too find it interesting that changes in material and design that increase performance are not reflected in the pitch marked on the prop. When I plugged in 10% slip, I get close to 19" of pitch not the 17" marked on the prop. And yes, the hole shot is not nearly as good as my other 17 pitch prop that I believe is a true 17 pitch prop.

 

[walleyehed]

Re: prop producing neagative slip

Not sure how you are figuring your slip, but I get 2% positive figuring a 17.....which is unlikely, but figure an 18P-typical of Turbo and Powertech and it shows about 7% slip...very likely.<br />If your tach is off 50 RPM it can change what you are getting in figures as well. "IF" it's correct, I figure 7% slip.

 

[Hawaiian]

Re: prop producing neagative slip

Figured slip by using this prop calculator:<br /><br /> http://continuouswave.com/cgi-bin/propcalc.pl <br /><br />I get 1.5% slip at 6300 rpm and 46.7 mph but the motor is trimmed WAY out and probably not that efficient. At 6000 rpm it gets 45.4 mph at a more normal trim and achieves minus .6% slip.

 

[walleyehed]

Re: prop producing neagative slip

beings a negative is impossible, that indicates to me the calc. you are using is incorrect.<br />With my formula, I show about 5% slip at 6000RPM at 45.4mph.<br />You trim up until RPM rises without a speed rise, then back in a touch, and, if you can trim out "that" far, your engine is still too low on the transom.

 

[Hawaiian]

Re: prop producing neagative slip

I think the calculations are correct because I tried 3 different propeller calculators in addition to the link above and still got negative slip which leads me to my original statement that I believe the prop is not really 17 pitch. Are you using 2.14 gear ratio in your calculations?<br /><br /> http://www.rbbi.com/folders/prop/propcalc.htm <br /> http://www.admboatworks.com/pitch.htm <br /> http://www.csgnetwork.com/marinepropcalc.html

 

[walleyehed]

Re: prop producing neagative slip

Yes, I'm using 2.14.<br />I've tested a few props over the last few years (well over 100), and I've developed my own system based on a combination of other calc. systems, and I find it to be quite accurate. I also know the technical aspects of the powertech props, and I know what figures to punch in for their props.<br />The fact you show a negative should tell you something...1 of the variables you are using is incorrect... usually it's pitch, but unless you have compared that prop to 30-40 others, you wouldn't know that it may pull harder over-all than a "normal" 17, if there is such a thing.<br />Other-wise, we "could" have a tach issue, but we don't know that...<br />Several of powertechs props are anywhere from 2 to 4 inches greater than stamped due to the way they cup them. Some of there props for example....say we normally run a 19 in a std SS...several of the powertechs will give the exact same numbers (for the most part) in a 15 pitch, side-by-side with your 19.<br />PT uses some strange figures on stamping pitch, and I don't care for the way they do it, because you have no comparison pitch-wise, to what you actually need.<br />Unless it is an "OEM equivilant", it's got more pitch than the number stamped.<br />If you can give me the prop series, I can get an exact slip ratio for you.

 

[Hawaiian]

Re: prop producing neagative slip

Good info Kenny. I'll look at the prop when I get home tomorrow and post the serial numbers. It is an OFS "offshore" series prop but it was made for Honda. It has Honda stamped on it as well as "Made by Powertech".

 

[BillP]

Re: prop producing neagative slip

Originally posted by Stillfishing:<br /> If you decide to manufacture props like that, I'll buy one.

Put me on the buy one list too! I will use it on my motor that "makes oil". Just think how economical it will be to go boating with a rig like that.<br />

 

[walleyehed]

Re: prop producing neagative slip

Bill, eventually, you can barrel the oil, and if it pushes more water than the prop is capable of, you should start seeing some gas money back...

 

[BillP]

Re: prop producing neagative slip

 

[jhebert]

Re: prop producing neagative slip

Re: the Propeller Calculator at CONTINUOUSWAVE.COM (and apparently others) which calculate values for SLIP which are negative numbers being incorrect calculators.<br /><br />Yes, it is true that there cannot be a negative number for actual slip by the definition of slip. <br /><br />If this type of result is obtained, the value entered for PITCH is understated. <br /><br />The text which accompanies the PROPELLER CALCULATOR at CONTINUOUSWAVE was recently revised to include this notice:<br /><br />"If a negative value for SLIP is calculated, the propeller PITCH was understated. Add one-inch to the PITCH for any propeller with cupped blades."

 

[umblecumbuz]

Re: prop producing neagative slip

Quoted from Continuouswave site:<br />Add one-inch to the PITCH for any propeller with cupped blades."

Cupping is a variable, not a constant. And exactly where a prop is cupped on the blade just adds to this variable. So to state that ANY cupping will add an exact degree of pitch is to oversimplify. This needs to be used with caution when trying to arrive at a final figure.<br /><br />Kenny will no doubt correct me if I'm wrong, or add to this comment.