can someone calculate slipage ?

[REELSPORT]

I have a 16 ft boat with a 1969 55 hp evinrude on it. the prop is 13.75 x 17 pitch I was running 32 mph at 5000 if my rpm gauge is right. If someone could calculate the slipage for me and let me know if it is good or bad and if bad what can I do about it. Thanks in advance for your help

 

[Paul Moir]

Re: can someone calculate slipage ?

Sure thing - that works out to a slip of 4%. Unless there's something you haven't told us like the boat is made of carbon fiber & titanium, that's a better number than you can reasonably expect. Something appears to be off a little but likely you're doing good now.<br /><br />I made up a little spreadsheet to calculate slip. Here's the calculation:<br />Gear ratio of a '69 55hp is 12:29 (from Old-omc.de), ie the propeller turns 0.414 rotations for each engine rotation.<br />Theoretical MPH = RPMS x 0.414 x Pitch x 0.000947<br />Theoretcial MPH = 5000 x 0.414 x 17 x 0.000947<br />Theoretical MPH = 33.3MPH<br />The 0.000947 is a conversion factor for inches per minute to miles per hour. Now that we have Theoretical MPH, we can calculate slip:<br />Slip = (1 - Theoretical/actual) x 100<br />Slip = (1 - 33.3/32) x 100<br />Slip = 4%<br /><br />Simple, huh?

 

[REELSPORT]

Re: can someone calculate slipage ?

Thanks Paul<br />simple no do I understand no will I take your word for it. Yes. I know the prop size is right and the pitch is right and the mph I got off my gps so I think that is right so that leaves the tach which I do have some question about if it is correct or not.

 

[JC1933]

Re: can someone calculate slipage ?

Hi Paul,<br /><br /> I have the same question, I would like to know the slipage of my prop, i cant make head or tails of these calculation,dumb i gues.<br />My boat is a 17.8ft,aluminum smoker craft my engin is a 130 hp johnson 1995 i think the gear ratio is 12:27 my prop is aluminom 15x17,my speed is 40mp on speedo and 39mp by gps. Hope this is not to mutch trouble for you since you just did one for Reelsport. thanks J C.

 

[Paul Moir]

Re: can someone calculate slipage ?

ReelSport - In my limited experience with this, I would think you're right. The tach is likely off a bit. If it's set correctly and such (it should be set to 6 poles), it probably isn't far off which would make your slip pretty good.<br /><br />jc1933 - Just plug the numbers in:<br />Take 12:27, divide 12 by 27 and you get .444, so that's the decimal equivalent of your gear ratio. Then:<br />Theoretical MPH = RPMS (you don't mention) x 0.444 x 17 x 0.000947<br />Then:<br />Slip = (1 - Theoretical MPH divided by 39) x 100<br />I would be happy to finish it off for you if you would provide your WOT RPMS.

 

[JC1933]

Re: can someone calculate slipage ?

sorry Paul,<br /> My wot rpm are 5900 rpm. thanks verry mutch. J,C,

 

[Paul Moir]

Re: can someone calculate slipage ?

Great, we can finish it off then.<br />Theoretical MPH = 5900 x 0.444 x 17 x 0.000947<br />Theoretical MPH = 42.2mph<br /><br />Now for the slip%:<br />Slip = (1 - 42.2 / 39) x 100<br />Slip = (1 - 1.08) x 100<br />Slip = -8%<br />Hmm, looks like I wrote out the wrong formula above there. Sorry guys - this is it correctly:<br />Slip = (Theoretical MPH/Actual - 1) x 100<br />Slip = (42.2/39 - 1) x 100<br />Slip = (1.08 - 1) x 100<br />Slip = 8%<br />Sounds like a good setup to me jc1933!