Finally got it out with a tach. Just me in it it was 4800rpm with me and one other guy (both of us 200lb, two Rods a tackle box and 6gallon has tank) it was 4900rpm.
I have a 1975 evinrude 15hp on a 14ft semi v. Very lite boat. I had myself and 2 other people in it and it would only go 7.5mph. With just myself in it it would go 15mph. The prop is a 9 1/2 x 10. Is that all it has?