I think the A4 makes 30 hp at 3,000 RPM.
If it won't push your boat over 5 knots, then something else is wrong, for example a mis-matched propeller.
You're likely to see significantly LESS horsepower on a diesel drop-in for the A4.
1987 CS 36 Merlin "Kyrie"
"They drove a dump truck full of money up to my house. I'm not made of stone!" -Krusty the Clown