F-2 Speedy

I think you need to get it out of the water and push pull side to side and up and down on the drives, there should be no movement

snapmorgan

Just thinking out loud here. When cone clutches fail, it causes a delay in shifting into forward or reverse. I have never seen one slip while under way. I would rule that out.
I don't believe the drives would be raising while under power. The force of forward thrust would push the drives down, not raise them. It this was happening, you would be able to see it on your drive indicators.
I don't think the engine couplers are stripped. When they fail, they fail. No movement at all.
I believe that you have some serious prop cavitation going on either from the drives not going down all of the way, some improperly installed drive showers or something of this nature. Another possibility would be the bottom of the hull is in worse condition than you think it is. Very dirty, algae or barnacles growing on it or something along those lines.
I am surprised that it has 3 blade props on it. What are others running on this hull?
Boat could be waterlogged and so heavy that it just can't go.

TomZ

Did I read correctly that you had a Formula 311 on plane, with one engine? A 330 at that?