170 pops up ok but doesn't want to put the nose (Bow) down

[akbridge]

What is the max rpm of your engine?   I am guessing 5500 rpm.  Since you have a 2:1 gear ratio, assuming you reach 38 mph and have a prop slip of about 10%.  That prop would give you a rpm of around 7000.  Way too much.  Shoot for a 15" pitch.  Here is a useful website: http://www.rbbi.com/folders/prop/propcalc.htm