The high thrust yamaha's are great engines with a super low gear ratio and are generally propped with larger diameter lower pitched props
2.92 ratio on 8T hp High Thrust model
2.08 ratio on regualr 8hp four stroke model
Here's a good article on the 8t from Continous wave.
22 outrage with 8t => 6mph
http://continuouswave.com/whaler/ref...aT8Kicker.html
My Dad had a 15hp Honda HT . . . and it pushed his 25ft pretty well 6 - 7 mph.