[Brett]

13:38 gear ratio on your outboard, standard is 13:27.
Hi-thrust designed to push a hull at displacement speeds.

 

[Brett]

Let's see here...

5500 rpm is middle of max rpm range for the 8 hp...13 x 5500 / 27 = 2648 13 x 5500 / 38 = 1881
so for the 13:27 the prop turns 2648 rpm
on the 13:38 the prop turns 1881 rpm
so a 9 inch pitch blade on the normal motor has a theoretical travel distance 1986 feet per minute.
To equal that the hi-thrust would need a 12-1/2" pitch prop.