Finally got my Wellcraft put back together 21' 305 ci OMC Cobra ratio 1.59:1 Michigan wheel recommended a 14.5 x 21 prop. Here's the problem, terrible out of the hole, tach only shows 2100 rpm @32.9 mph by my calculations even figuring 15% slip I'd have to be turning 3100 to get to 32.9. wot...