1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 133Get this manual
POWER LOSS
Distribution of load on boat Improper load distribution, Overload Boat hull Fouled with seaweeds and shells Tilt angle Improper Transom height Low or high, Cavitation
LOWER UNIT
Propeller Bent, Broken, Loosely fit, ncorrect size Water inlet .Clogged Water pump .worn, Broken