It would seem to me that 2.6: 1 reduction would leave your shaft turning correspondingly slower at a given RPM than a
transmission having a lower reduction ratio, for example 2.1:1. Check that your tachometer is accurate, if so, the move normally you would add probably 2 inches of pith based on your mis propped description. Face area being less likely the problem but in the extreme maybe both. If
pitch is already too high or diameter is too low (the latter not being as likely) your prop is likely already cavitating due to excessive pitch and you wouldn't achieve speed. Thus, For many hull shapes the proper three blade prop will be a smaller diameter than a two blader would. Your problem is not due to drag. As for me I have 19 x 12 LH two blade prop on a boat is displacing about 18,000 lb, it reaches
hull speed at about 2300 RPM in flat
water. A smaller and lighter boat such as yours might do best with something like15 x 15 if it is a three blade prop or perhaps a 15 x 14. Do not exceed tip-hull clearance guidance as being somewhere between one fifth and one sixth of the prop diameter. There is an answer for why your boat isn't reaching hull speed, I don't claim to be a prop expert, just a boater that happens to be an
engineer with a lot of experience
boating. It is my opinion that if propped correctly, a
displacement hull should reach its hull speed at roughly 80-85% of the engines rated max RPM.