Rpm's don't make sense with prop change

[Cortes165]

This is on a '91 Evinrude 100hp on a 16-1/2 ski boat.<br /><br />I thought things were running fine but after trying a different prop, just wondering if things really are.<br /><br />Using a 13-1/4 x 17 (391199) I can only turn 5100 but get app 43 mph. (this is the recomended prop by the manufacturer)<br />With a 13-3/4 x 15 (#391198). Rpm went right up to 5800 but speed dropped to 37-38 mph. <br /><br />With the 17 it does lug more.<br />With the 15, I can barely tell that there are 2 people dragging behind the boat. <br /><br />Something doesn't add up.<br />Should there be such a drastic difference in these? A gain of 700 rpm and a loss of speed.<br />Or does this sound normal??<br /><br />Besides the engine height, what else should I be checking????

 

[walleyehed]

Re: Rpm's don't make sense with prop change

The numbers you give for the 17P represent negative slip...Imposible.<br />The numbers of the 15P are showing about 5% slip which leads me to believe the tach, speedo or both are off..One of them is off for sure.<br />I've spent alot of time with the 2 props you list and 700RPM difference is not typical...300 is, maybe 350.

 

[Cortes165]

Re: Rpm's don't make sense with prop change

thanks for the info. Looks like a day of testing.<br />I'm not too worried about the speedo. I know they can be +/- but thought it would be consistent enough to see a difference in props.<br />So what's the easiest way to check the tach???

 

[Cortes165]

Re: Rpm's don't make sense with prop change

Can I hook up another tach off the same wire as my first tach???<br />Or do I wire off the engine.???<br /><br />That way I can read both while I'm cruising along.

 

[moderator1]

Re: Rpm's don't make sense with prop change

I would get a shop tach or buy another, maybe borrow one and test in place of the original....see what walleyehed says...