Pete,
Do you have trim tabs?
For some reason I thought th Zukes would take a bigger prop, but biggest stuff I saw was 14" like you said. Your rpms are perfect, but it really sounds like to have a bow lifting prop.
Where were your buddies situated on take off? Until you get a 4 blade prop, better put 2 riders in front of the console and wrap'em up in Grundens
p.s. Nice work on the 'togs!