HELP, 89 DT 115 acts like lower unit is slipping when beyond trolling speeds.

Just put this 89 DT 115 on my boat and put it on the water. Prop does not appear to be slipping since I can turn the motor over while grabbing the prop and turning it when engine is in gear (and motor off obviously). The motor goes into gear fine and moves the boat at low speeds (no wake speeds), but when you give it gas, the engine revs and its like it looses traction to the water. The boat will not go fast enough to plane out. There is something slipping somewhere. Is there a clutch or something in the lower unit that would cause this? or some other issue?
Thanks for the help!

Likely it is your prop slipping !!-----Cheap to try / borrow another prop.

What did you determine it to be. My 40 hp Suzuki is doing the same thing.