Try this,it’s easy to use and best of all it’s free!
http://download.com.com/3302-2192_4-10203247.html?pn=1&fb=1

I have a 1850 Crestliner Sportfish w/115 Johnson 4 Stroke. It’s the best all around I’ve ever had.

I am going by my depth finder speedometer which is pretty close to the radar gun(well within 1 mph). GPS reaings on the water isn’t “truely” accurate do to the contour on the bottom vs. the flat water level or so everyone tells me.It makes sense if one ponders about it.

I have a 2004 Sportfish w/115 Johnson 4 stroke w/19 pitch prop and I run 42mph.I just picked up a 21 pitch to see how much it will tach.It should take me up around 45+ and hopefully tach around 5500-5800 rpms.I love this boat!