[lecomte38]

that is about right. you travel 6" per rpm of the prop. 6 x engine rpm divided by transmission ratio = inches per minute divided by 12= feet per minute x 60=feet per hour divided by feet in a nautical mile = speed in kts

6 x 2500/2.6(? tran ratio) /12 x 60 / 6076 = 4.75 kts per hour

you loose some in in efficiency. (prop slip) My 14 x 14 prop pushes the boat 6 kts at 2000 rpm, 7.3 kts at 2800 rpm