Prop slip %

[caper_45ca]

Can someone please help me out....I have a 1995 90 hp Johnson v4 with an almost new 13.75 x15 aluminum prop..top speed is 36 mph at 5200 rpm....what is my prop slip?

 

[JB]

Re: Prop slip %

Let's try this in the Prop forum, caper.<br /><br />Good luck.

 

[Dhadley]

Re: Prop slip %

That figures out to about 3.5% which is too good to be true. Is that GPS speed? What tach do you have?

 

[caper_45ca]

Re: Prop slip %

Its the factory tach on the boat....1985 sportcraft....not gps on my boat...gps on the boat beside me....

 

[Dhadley]

Re: Prop slip %

OK, describe "factory tach". OMC? Teleflex? Faria?

 

[caper_45ca]

Re: Prop slip %

I cant b certain but I know the other gauges r Faria....boat is at dealer getting new piston n rings....just trying to do a little preventative maintenance b4 I get it bk.....or gain a little bit of knowledge ...if thet is possible in this old head...lol

 

[Realgun]

Re: Prop slip %

3% slip is really not possile on the hull you have.<br /><br />I figure 34 mph as a speed with a 9% slip which is what I get.<br /><br />If I believe the 36 mph then the tach should be @ 5600 rpm.<br /><br />Its possible you have a very efficient hull and have a cupped prop. so 5200rpm gives 36 mph and 8% slip.

 

[walleyehed]

Re: Prop slip %

Why are we installing new pistons????

 

[Dhadley]

Re: Prop slip %

Let me guess....the rings on the starboard side broke.<br /><br />Hummmm....maybe the tach is correct????

 

[caper_45ca]

Re: Prop slip %

Yes rings on starboard lower cylinder did break ....opted to rebore one cylinder and re-ring other 3 and install new piston....how did you know cracked rings and what side>>>>engine had very low hours and what hours were on it were at very low rpms ..until I got it and ran it wide open 5200....unless the guy using gps wanted me to believe his boat is faster than it actually is...he tells me he can get 40mph...but never ever got it with me in boat...his best was 34....??????? cant see a boater lying about his speed....lol

 

[MrBill]

Re: Prop slip %

You have not provided enough information...unless I missed something or I'm not familiar enough with this out board application. In order to get an accurate answer, you have to provide the gear ratio...I believe some have assumed engine RPM is the same as prop RPM. I will run the numbers through my engine rpm/prop pitch/speed calculator if you provide the gear ratio.

 

[Dhadley]

Re: Prop slip %

The gear ratio is 2:1 MrBill. At 5200 the prop is turning 2600.<br /><br />At 5200 with todays fuel the combustion temps are higher than what the motor was designed to handle. If the fuel is old (more than 30-45 days) that will add combustion temp. If the wrong spark plug is used or the wrong octane that will add temp too.<br /><br />These higher combustion temps cause a black sticky substance to form on the piston and ring groove. Thats known as "coking". The ring starts to stick in on the back side. Eventually the piston looses enough support that it rocks far enough to shear off the ring at the end gap. Usually to the right of the locating pin first. The startboard side runs slightly hotter so it usually starts there. <br /><br />If you dont get the rpm up, youll have to decarb very often. Probably every 25-30 hours of run time. If you get the top rpm up to 5800-6000 you may never have to decarb no matter what rpm you run at.

 

[caper_45ca]

Re: Prop slip %

says in my manual gear ratio is 13:26....unless Im looking in wrong place

 

[Realgun]

Re: Prop slip %

The 2:1 Ratio is what I plugged in and 13:26 is------- Drum roll please <br />----2:1

 

[MrBill]

Re: Prop slip %

Realgun...13:26 is 1:2, but I'm assuming 2:1 is correct. That would mean at 5200 engine rpm with speed of 36 mph prop slip is 2.5%.<br /><br />5200 ENG RPM / 2 Gear Ratio = 2600 Prop RPM<br />2600 X 60 = 15600 Revs per Hour<br />156,000 X 15 pitch (inches) = 2,340,000 inches/hour<br />OR 36.932 MPH with zero slip (2,340,000/12inches per foot/5280 feet per mile)<br />36.932 MPH - 36 MPH actual = = .932 MPH Slip<br />.932/36.932 = 2.52% Slip (2.5%)<br /><br />Seems low, weight of boat and accuracy of speedo weigh heavily in calculation......

 

[Realgun]

Re: Prop slip %

Try it with 16 pitch, Cupping adds about 1 inch to prop figures.

 

[caper_45ca]

Re: Prop slip %

would the slip% change much if top speed was 34mph?????I am thinking that guy who was on other boat with gps may have boosted the #'s a little to have me believe his boat is a little faster than it actually is....and i know he can get 34 cus I was with him when he did....<br />Ill have to wait till mine comes back from shop and I get it broke in and then ill do a gps reading myself

 

[MrBill]

Re: Prop slip %

If actual speed was 34 the slip would be almost 8%, a much more reasonable number. <br /><br />Theoretical (zero slip speed) 36.932 MPH - 34 MPH actual = 2.932 MPH Slip<br />2.932/36.932 = 7.94% Slip (8%)<br /><br />In most instances slip averages 10-12% at WOT, assuming optimal trim and good conditions.

 

[f_inscreenname]

Re: Prop slip %

Try this thing out. You put in your numbers and it will tell you slip or MPH.<br /> http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html