I recently bought a used boat, I had the clutch dog replaced in the lower unit and resealed. When the boat is out of the water I can put it in forward and reverse with the engine off and turn the prop. I can't get it to slip, as soon as I put the boat in water the prop slowly turns but won't engage completely. The motor rev's up fine. I toook it out of the water and again the prop was solid with the engine off. I started it and put it in forward and the prop turned but I can put my foot on it and stop it with some force. I don't have any problems in reverse. I've checked the shift linkage the height is correct and seems to be okay. What could it be.