1983 Yamaha 30EN Outboards Service Manual, Page 169Get 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, Air draw, Cavitation
I LOWER UNIT
Propeller Bent, BrokenLoosely fit, ncorrect size Water inlet .Clogged Water pump .Worn, Broken